REVIEW: Potato virus Y (PVY) and Potato leaf roll virus

University of Pretoria Faculty of Natural and Agricultural SciencesDepartment of Zoology and Entomology Pretoria 0002 Tel (012) 420-3233 Fax (012) 362-5242

Potato Virus Y (PVY) and Potato Leafroll Virus (PLRV):

L i t e ra tu re Rev iew fo r

Po ta toes South A f r i ca

Prepared by

M. Warren, K. Krüger & A.S. Schoeman

April 2005

M. Warren, K. Krüger & A.S. Schoeman, Potato Virus Y (PVY) and Potato Leafroll Virus (PLRV): A South African Perspective

Table of Contents

1. Introduction ..................................................................................................................3 2. Potato virus Y (PVY) ...................................................................................................4

2.1. The disease and virus ............................................................................................4 2.2. Geographic distribution ........................................................................................4 2.3. Host range .............................................................................................................5 2.4. Virus transmission ................................................................................................6 2.5. Disease management .............................................................................................8

2.5.1. Crop management and tuber storage .........................................................8 2.5.2. Chemical control .....................................................................................11 2.5.3. Host plant resistance to the virus ............................................................13 2.5.4. Host plant resistance to the vector ..........................................................14

2.6. Recommendations for seed producing growers: PVY ........................................14 2.6.1. Reduction of virus inoculum within the crop .........................................14 2.6.2. Reduction of virus inoculum outside the crop ........................................15 2.6.3. Monitoring aphid flight to minimize transmission .................................15 2.6.4. Isolation ...................................................................................................15 2.6.5. Barriers ....................................................................................................15 2.6.6. Insecticides ..............................................................................................16

3. Potato leaf roll virus (PLRV) ....................................................................................16 3.1. The disease and virus ..........................................................................................16 3.2. Geographic distribution ......................................................................................16 3.3. Host range ...........................................................................................................17 3.4. Virus transmission ..............................................................................................18 3.5. Disease management ...........................................................................................19

3.5.1. Crop management and tuber storage .......................................................19 3.5.2. Chemical control of vectors ....................................................................21 3.5.3. Host plant resistance to the virus ............................................................22 3.5.4. Host plant resistance to the vector ..........................................................22

3.6. Recommendations for seed producing growers: PLRV .....................................22 3.6.1. Reduction of virus inoculum within the crop .........................................22 3.6.2. Reduction of virus inoculum outside the crop ........................................22 3.6.3. Monitoring aphid flight to minimize transmission .................................23 3.6.4. Isolation ...................................................................................................23 3.6.5. Barriers ....................................................................................................23 3.6.6. Insecticides ..............................................................................................23 3.6.7. Cultivars ..................................................................................................23

4. References ...................................................................................................................28

2


1. Introduction

Aphid-transmitted potato viruses, such as potato virus Y (PVY) and potato leaf roll virus

(PLRV), have long been recognized as a major problem in the profitable production of

seed potatoes. Planting of PVY- and PLRV-infected seed may result in total yield loss

(Radcliffe, 1982; Thomas et al., 1997).

Potato virus Y (PVY) and potato leaf roll virus (PLRV) are the two most important

viruses of potato globally (Radcliffe, 1982). Primary infection (current season inoculation

with virus) of tubers by PVY increases the number of undersized tubers, while infection

with PLRV induces net necrosis in tubers (darkening of the vascular bundle) (Radcliffe &

Ragsdale, 2002). This results in losses with respect to yield and quality for commercial

producers (Ragsdale et al., 1994). Infection with these viruses also leads to a downgrading

of seed lots because of the low tolerances (0-1 %) required by seed certification

programmes for high quality seed (Radcliffe & Ragsdale, 2002; South African Seed

Certification Programme). In South Africa, Daiber (1965) showed that the incidence of

PLRV increased from 0.4 % to 97 % after four plantings. Although tubers with higher

virus infection levels (1-4%) are unsuitable as seed potatoes, such tubers may still be used

to plant the commercial crop (Ragsdale et al., 2001).

Potato virus management strategies are preventative (reduction of virus inoculum) or

therapeutic (reduction of vector (aphid) numbers) (Radcliffe & Ragsdale, 2002). These

include spatial and temporal isolation of seed potato crops, mechanical barriers and weed

control (Ragsdale et al., 2001). The effective control of potato viruses requires a

combination of both management strategies and risk assessment with regard to other pests

to establish the best option in a region-specific manner (see Mowry, 1994; Caldiz et al.,

2002). For example, the need for virus-free seed potatoes requires a much higher level of

management than would be required for processed potatoes (Mowry, 1994).

This review focuses on potato virus Y and potato leaf roll virus, their insect vectors and

existing management strategies to control the spread of the viruses in potato crops. Local

and international research findings are reviewed and evaluated with specific reference to

potato production in a South African context. Areas requiring further research to

understand the epidemiology of PVY and PLRV in South Africa are identified.

3


2. Potato virus Y (PVY)

Potato virus Y (PVY) causes serious damage in potato (10-100 % yield losses) and other

solanaceous crops worldwide. The severity of the disease depends on the PVY strain

involved, host tolerance, time of infection and environmental factors. In potato, PVY is

transmitted to the new crop via seed tubers or through aphid vectors. Primary symptoms

caused by PVY may be mild or hardly detectable, which causes problems particularly in

potato seed production.

2.1. The disease and virus

Potato virus Y is a Potyvirus (family Potyviridae), and thus belongs to a taxon that

comprises the largest and economically most important group of plant viruses (de Bokx &

Huttinga, 1981; Büchen-Osmond, 1987). PVY is a single stranded RNA virus. Strains of

PVY include PVYO (common strain), PVYN (tobacco veinal necrosis strain), and PVYC

(stipple-streak strain, including potato virus C). The main diseases caused by PVY include

mild to severe leaf mottling, streak or ‘leaf-drop streak’ (PVYO) with necrosis along the

veins of the underside of leaflets (PVYN) and ‘stipple-streak’ (PVYC) (de Bokx &

Huttinga, 1981; Harrison, 1984).

2.2. Geographic distribution

The distribution of PVY is global, although some virus strains are restricted to certain

continents (de Bokx & Huttinga, 1981; Harrison, 1984). PVYO strains occur worldwide,

PVYN strains occur in Europe, parts of Africa and South America, and PVYC strains have

been reported from Australia, India, and Europe. The two major strains identified in South

Africa are PVYO (common strain) and PVYN (tobacco veinal necrosis strain) (Thompson,

1997). PVY-81 (a strain similar to PVYO) appears to be common in potatoes grown in the

Northern Cape region (Thompson et al., 1987), while other strains have been identified in

tobacco (Vorster et al., 1990).

4


2.3. Host range

The host range of PVY is broad, comprising many crop species in the Solanaceae

(including tomato, pepper, and tobacco) and some species in the families, Asteraceae,

Brassicaceae, Chenopodiaceae (Amaranthaceae), Commelinaceae, and Fabaceae (Tables

1, 2; de Bokx & Huttinga, 1981; Büchen-Osmond, 1987, Fletcher, 2001). Weed species

that are suitable hosts of PVY may be important virus reservoirs (Latorre, 1983).

Table 1. Examples of host plants of potato virus Y (PVY)

Family Species Plant recorded from South Africa

Asteraceae Cotula australis (Sieber ex Spreng.) Hook. F. X

Asteraceae Senecio vulgaris L. X

Brassicaceae Capsella bursa-pastoris (L.) Medik. X

Chenopodiaceae (Amaranthaceae)

Chenopodium quinoa Willd.

Chenopodiaceae (Amaranthaceae)

Chenopodium giganteum D. Don. (tree spinach) X

Commelinaceae Tinantia erecta (Jacq.) Schltdl.

Solanaceae Capsicum annuum L.

Solanaceae Capsicum frutescens L. (hot pepper, chili pepper)

X

Solanaceae Lycium sp.

Solanaceae Lycopersicon esculentum Mill. (tomato) X

Solanaceae Nicotiana glutinosa L.

Solanaceae Nicotiana tabacum L. (tobacco)

Solanaceae Physalis floridana L. (strawberry-tomato)

Solanaceae Solanum atropurpureum Schrank

Solanaceae Solanum chacoense Bitter

Solanaceae Solanum demissum Lindl.

Solanaceae Solanum tuberosum L. (potato) X

5

http://www.ars-grin.gov/cgi-bin/npgs/html/family.pl?110

http://www.ars-grin.gov/cgi-bin/npgs/html/family.pl?166


Table 2. Insusceptible host of potato virus Y (PVY)

Family Species

Solanaceae Datura stramonium L.

2.4. Virus transmission

The virus can be transmitted by sap inoculation, grafting and aphids in a non-persistent

manner (stylet-borne). Although PVY can be transmitted mechanically, this mode of

transmission is usually of minor importance in the field (Ragsdale et al., 2001).

Non-persistent viruses have a loose association with vectors, i.e. virus specificity is not

marked and many vectors can transmit the virus. In the case of PVY more than 50 aphid

species have been shown to be able to transmit the virus, although some species are more

efficient than others (Table 5; Büchen-Osmond, 1987; Ragsdale et al., 2001). The most

efficient vector appears to be the cosmopolitan peach-potato aphid Myzus persicae (Sulz.)

(Hemiptera: Aphididae) (de Bokx & Huttinga, 1981), which is also known as the green

peach or peach-potato aphid.

Viruses that are non-persistently transmitted can be acquired and transferred during very

short acquisition and inoculation feeding times. Infective aphids are capable of acquiring

PVY within seconds to minutes and able to transmit PVY within seconds of probing an

uninfected plant (Harrison, 1984). There is no latent period involved, i.e. the virus can be

transmitted immediately after acquisition. As the name suggests, non-persistent viruses do

not persist or replicate in the vector, and aphids lose the ability to transmit PVY after

having probed one or two uninfected plants following acquisition, unless they feed again

on an infected plant (Bradley & Rideout, 1953).

Table 5 lists the aphid species that have been identified as being present near potato fields

by researchers of the ARC-Vegetable and Ornamental Plant Institute (ARC-VOPI) in a

survey conducted across South Africa (1994-1997) (Thompson, 1997), as well as the

aphid species that are known to feed on potato and occur in South Africa. Of the known

PVY aphid vectors, 25 species have been recorded in South Africa (Thompson, 1997), 12

6


of which have been reported to feed on potato (I. Millar, pers. comm.). The relative

efficiencies with which aphid vectors transmit viruses have been determined for some of

the species (Table 5; see Ragsdale et al., 2001).

Aphids recorded can be divided into colonizing (reproduce on potato) and non-

colonizingpotato (reproduce on plants other than, and do not establish on, potato) species.

Usually colonizing aphid species are more efficient vectors that non-colonizing species.

Most of the species listed in Table 5 do not colonize potato. However, when searching for

a host, aphids take sap samples from the epidermis cells of plants. These sap sampling

probes are sufficient to transmit PVY. Although non-colonizing aphid species may be not

efficient vectors, the population density of these aphid species in a potato field can be

high, thus compensating for their lower transmission efficiency (Radcliffe, 1982; Robert

et al., 2000). Therefore, all species that occur throughout the potato-growing season and

that occur consistently across years may be important virus vectors and require

monitoring to apply the correct control measures (see DiFonzo et al., 1997).

A recent review by Radcliffe & Ragsdale (2002) highlighted the importance of vector

biology when attempting to understand and control the spread of potato viruses. For both

PVY and PLRV, the virus is moved from an inoculum source outside the potato field into

the field almost exclusively by winged aphids (Boiteau, 1997). Apterae (wingless aphids)

have not been shown to be significant in within-field spread of PVY (Ragsdale et al.,

1994).

In cooler, temperate climates most aphids of potato crops overwinter either as eggs on a

primary host or as viviparae (wingless aphids bearing living young instead of eggs) in

protected sites (Radcliffe, 1982; Radcliffe & Ragsdale, 2002). Primary hosts are usually

fruit trees (e.g. Prunus spp.). Secondary hosts, mainly weed species, allow aphid numbers

to increase before potato plants are available for colonization and provide a source of

migrants to crops (Radcliffe & Ragsdale, 2002). In milder climates, as is the case in South

Africa, parthenogenic (asexual) reproduction is likely to be year-round on weeds,

available crops (e.g. cabbage) and garden plants, providing an immediate aphid source

when potatoes are planted (Daiber & Schöll, 1959; Radcliffe, 1982; Annecke & Moran,

1982).

7


Although large populations of aphids are able to damage potato plants directly, the

economic importance of aphids results mainly from their role as virus vectors (Radcliffe

& Ragsdale, 2002; Radcliffe, 1982). To actively manage the economic impact of the

aphid vectors, it is essential to know which aphid species occur in a region, the efficiency

with which they are likely to transmit the viruses to potato plants and to understand vector

biology (Hanafi et al., 1995; Radcliffe & Ragsdale, 2002; Radcliffe & Ragsdale, 1998).

For example, in South Africa, peak M. persicae flight activity occurs in October -

November and January - February but fluctuates for each locality and across years (see

Daiber, 1965).

2.5. Disease management

Many aphid species that have been reported as vectors of PVY are incapable of

establishing on potato crops (i.e. they are non-colonising aphid species). However, both

colonizing and non-colonizing aphid species play an important role as vectors of PVY, the

former because they are usually efficient vectors, the latter because they may occur in

high numbers, thus compensating for their usually lower PVY transmission efficiency.

Therefore, all species that occur throughout the potato-growing season and that occur

consistently across years may be important virus vectors and require monitoring for

gaining an understanding of the spread of PVY in potato fields (Radcliffe, 1982) and to

apply the correct control measures (see DiFonzo et al., 1997). The importance of insect

pests also depends on climate, plant age (young plants are more susceptible to net

necrosis) and cropping practices (Caldiz et al., 2002; Radcliffe & Ragsdale, 2002). Thus,

an understanding of all the factors likely to affect potato production is required to

formulate an adequate integrated pest management (IPM) plan for potato seed producers

(see e.g. Marsh et al., 1998; Jones, 2004).

2.5.1. Crop management and tuber storage a) Adjustment of planting date and avoidance of vector (spatial and temporal

segregation)

Spatial and temporal separation of seed stocks from other crops (e.g. potatoes) that are

likely sources of virus inoculum remains one of the most effective preventative

8


management strategies (Radcliffe & Ragsdale, 2002). The same authors state that a

minimum separation distance should be established according to an acceptable level of

risk of spread from virus sources to potato fields. Separation distances of 400 m to 5 km

have been effective under different management scenarios for seed potato cultivation

(Radcliffe & Ragsdale, 2002). However, although maiden flights of Myzus persicae alatae

seldom exceed 100 m, wind-supported flight may cause aphids to disperse up to several

hundred kilometers (Radcliffe & Ragsdale, 2002). Therefore, viruliferous aphids from

distant localities may still occasionally colonize isolated potato fields.

Windswept areas along the coast may be almost aphid-free and provide good sites for the

location of seed potato fields. Growing all or most seed in a single region, or a group of

regions, spatially separated from other crops is practiced in many countries, e.g. Golan

Heights in Israel.

Isolation may also include modified planting and harvesting dates (Ragsdale et al., 2001).

Planting early if vector species colonise the crop late is a useful strategy. Older plants are

less likely to become infected than young plants (mature plant resistance) (Roosen et al.,

1997). Although early planting of seed potatoes is an effective strategy in cooler climates

where aphids overwinter or abundances are substantially reduced during the winter, the

strategy is unlikely to be effective across South Africa. Warmer winters in, for example,

the highveld are unlikely to lead to a reduction in aphid numbers. Year-round crops also

provide the aphids with a continual food supply so that abundances remain high. Delayed

(late) planting is another strategy that has been shown to reduce virus spread by 50 %

compared to early plantings (Ragsdale et al., 2001). Tuber yield will, however, be

compromised (Boiteau, 1984). Late plantings reduce colonization of the crop by M.

persicae (early colonizer) but has no effect on Macrosiphum euphorbiae (potato aphid)

abundances in South Africa.

Growing crops in periods in which aphid vectors are absent or aphid population density is

low is another effective strategy to reduce the spread of virus inoculum (de Bokx &

Huttinga, 1981). Early planting together with ‘vine kill’ (roguing/haulm destruction) has

been effective in The Netherlands; however, such strategies cannot be applied where the

growing season is short (Radcliffe & Ragsdale, 2002).

9


b) Roguing

During rouging, haulms of seed-potato crops are destroyed before maturity so as to

restrict virus spread at the end of the growing season. Roguing is effective only when

virus incidence is low (~1 %) and when fields are small as each plant requires multiple

inspections throughout the growing season (Radcliffe & Ragsdale, 2002). A disadvantage

of roguing is that it creates gaps, making the plants more apparent to the aphids (Ragsdale

at al., 2001). To be more effective at finding all infected plants, seed pieces cut from the

same tuber should be planted together but this is labour intensive and extends the time

taken to complete the planting of a field (Ragsdale et al., 2001). Because roguing requires

identifying infected plants through symptom expression, cultivars expressing symptoms

should preferably be grown instead of symptom-free carriers (Ragsdale et al., 2001). All

infective plants need to be removed prior to the arrival of winged aphids (Radcliffe &

Ragsdale, 2002).

c) Crop mulching, covers and barriers

Although reflective mulches and sticky yellow sheets may reduce PVY spread (51-80 %

in peppers), they are expensive, effective for a short time only (before the canopy closes)

and require disposal after use (W. Pett, pers. comm.; Ragsdale et al., 2001). Crop covers

may be relatively effective at reducing PVY spread but increased temperatures under the

covers negatively impact plant and tuber development, especially where daytime

temperatures are high (Ragsdale et al., 2001). Also, infestations under crop covers may go

undetected for extended periods of time (Ragsdale et al., 2001). Barrier crops are more

effective at reducing PVY spread than insecticides (Radcliffe & Ragsdale, 2002; DiFonzo

et al., 1996). If PVY-infected aphids feed on the barrier first, the probability of retaining

virus inoculum for transmission to potatoes is low (DiFonzo et al., 1996). Soybean or

sorghum barriers (~ 1 m wide) are just as effective at reducing PVY spread as taller crops

and should be planted with no gap between the barrier and potato crop (Radcliffe &

Ragsdale, 2002; DiFonzo et al., 1996). Barrier crops should have a fallow border to the

outside to be apparent to winged aphids (DiFonzo et al., 1996). Soybean is not a host for

the virus and it is therefore particularly suitable as a barrier crop, although potato barriers

have also been used. These need to be discarded from the seed lots on harvesting.

Standard barrier widths have not been established but it appears that barriers of a few

10


metres wide are effective (DiFonzo et al., 1996). Spraying the barriers with insecticides

has no influence on reducing the spread of PVY and should be avoided as the natural

enemies of the aphids will be negatively affected.

d) Elimination of virus sources

Weeds (e.g. other solanaceous plants) and volunteer potatoes may be sources of virus

inoculum and viruliferous aphids (Thomas & Richards, 2004). Virus inoculum must be

reduced by eliminating weeds and volunteer potatoes (Hanafi et al., 1995). Many South

African plant species that occur naturally may be sources of virus inoculum. However,

when commercial (ware) potato growers plant seed with acceptable levels of virus (1-5 %

according to international certification standards) in neighbouring areas, the potato crop

itself becomes a greater source of inoculum in the area than weeds (Ragsdale et al., 2001).

Virus levels vary depending on the generation and the end use of the crop. In South

Africa, these values are zero PVY infection levels for the first two generations and both

inspections used for the production of seed potatoes (South African Seed Certification

Programme). As is the international standard, much higher levels of virus are acceptable

for commercial production (South African Seed Certification Programme). Therefore,

potato fields are likely to be the greatest source of PVY inoculum in South Africa.

Spraying in wind breaks where aphids numbers build up has also been suggested as an

additional management strategy to reduce sources of virus inoculum.

2.5.2. Chemical control

Chemical control of aphid species such as M. persicae on overwintering host plants,

weeds and within potato and other crops has been shown to be largely ineffectual in

controlling the spread of potato viruses (de Bokx & Huttinga, 1981; Radcliffe &

Ragsdale, 2002). This is due to the non-persistent nature of PVY transmission, i.e. the

virus may be transmitted within the first few seconds of probing an uninfected plant

(Bradley, 1954). PVY may therefore be transmitted before the insecticide has time to kill

the vector (Radcliffe & Ragsdale, 2002).

However, pesticide usage may be required to curb aphid outbreaks resulting from

insecticide applications targeting other insect pests (Radcliffe & Ragsdale, 2002). The

11


following chemicals have been used to control aphid populations either as in-furrow

applications on seed potato plantings (imidacloprid; provides approximately 60-90 days of

aphid protection, but has no effect on reducing PVY spread, Boiteau & Singh (1999)) or

foliage applications: imidacloprid, methamidophos and thiamethoxam. Most insecticide

classes registered for usage on potatoes, such as organophosphates, carbamates and

pyrethroids, may cause outbreaks of potato-colonising aphids (ffrench-Constant et al.,

1988; Harrington et al., 1989). Myzus persicae is resistant to most insecticide classes,

making chemical control of this species particularly problematic (Radcliffe & Ragsdale,

2002). A new insecticide, pymetrozine, affects aphid stylet penetration and is likely to be

used substantially by the industry in the future (Radcliffe & Ragsdale, 2002). Pyrethroids

may cause outbreaks in aphid populations, increasing virus spread (Ragsdale et al., 2001).

In addition, some insecticides are irritating to aphids and increase PVY spread. Extreme

care must be taken when applying insecticides in PVY infected areas (Ragsdale et al.,

2001).

Where action thresholds of aphid abundances for the prevention of PVY spread in seed

potato crops have been established, it has become clear that these vary across regions

(PVY spread is not reduced when aphid densities are maintained at the lowest levels

possible) (Ragsdale et al., 1994). It is therefore essential to monitor aphid populations

within the crop and to establish an action threshold relevant to a particular locality. An

important point to remember in aphid control is that of cumulative vector intensity

(Basky, 2002; Halbert et al., 2003). This is a function of both the abundance of the aphid

species within a field and their efficiency at transmitting the virus (Basky, 2002).

Therefore, a rare (low abundance) aphid species may be a serious threat to crops if its

transmission efficiency is high, or a highly abundant, less efficient vector may pose a

problem regarding PVY spread (Halbert et al., 2003). Also, non-colonising alate (winged)

aphids are important vectors of PVY (Radcliffe & Ragsdale, 2002).

The disadvantages of chemical control include the build-up of aphid genotypes that are

resistant to chemicals (as mentioned above, Myzus persicae is resistant to many classes of

insecticides) and environmental degradation (e.g. pollution of water sources (Radcliffe &

Ragsdale, 2002)).

12


By reducing insecticide usage on leaf litter under the plants and on surrounding natural

vegetation, predator numbers may be enhanced, thereby reducing aphid numbers.

Certain mineral oils are known to reduce aphid colonization on plants, and thus the

transmission of virus diseases (Simons & Zitter, 1980; Martín et al., 2004). Mechanical

control with mineral oils reduces the incidence of PVY in tubers (Marco, 1986). Use of

mineral oils has resulted in reducing virus spread by more than 50 % in Minnesota, USA,

without severely affecting natural enemies or the environment (Ragsdale et al., 2001).

However, phytotoxicity of mineral oils applied in-field needs to be established (e.g. oils

may be phytotoxic when temperatures are high) (see Ragsdale et al., 2001; Martín et al.,

2004).

Although whitewash sprays also aid in PVY control, these may be less efficient than net

covers and may attract, rather than repel, some aphid species (Marco, 1986).

2.5.3. Host plant resistance to the virus

Numerous potato cultivars are available, some of which are symptom-free carriers

(infected with PVY and/or PLRV but few symptoms are visible), while a few cultivars

appear to be immune or almost immune to PLRV net necrosis (Mowry, 1994). Symptom-

free carriers are useful for table and processing potatoes. However, these cultivars are

often the bane of seed potato producers as infected plants and tubers are not visibly

discernable from uninfected plants, making roguing difficult (Radcliffe & Ragsdale,

2002). The planting of virus resistant potato cultivars is an important avenue of potential

control of the viruses but these are costly to develop and need to be durable sources of

resistance (Mowry, 1994; Lecoq et al., 2004; Shubert et al., 2004). In addition, the

benefits gained (e.g. yield, firmness) from established cultivars also need to be

incorporated into the new cultivars (Mowry, 1994).

Enzyme-linked immunosorbent assay (ELISA) and polymerase chain reaction (PCR) tests

are the only certain means of ascertaining the infection status of potato tubers, especially

symptomless carriers (Robert et al., 2000). ELISA is used in seed certification

programmes to process large amounts of samples rapidly and ensures that infection levels

in seed potatoes are low (Robert et al., 2000).

13


2.5.4. Host plant resistance to the vector

Although the planting of aphid resistant cultivars was suggested as a potential virus

control strategy (Adams, 1975), only wild potato species are truly resistant to aphids

(Radcliffe & Ragsdale, 2002). A study by Raman (1988) indicated that when resistant and

susceptible cultivars were exposed to insecticides commonly used in the potato industry,

no difference in M. persicae mortality was observable.

2.6. Recommendations for seed producing growers: PVY

Potato virus management problems are regional and effective control may only be

maintained through co-operation on a regional level between growers (Radcliffe &

Ragsdale, 2002). As stated by Ragsdale et al. (2001), a reduction of virus inoculum

(preventative) and reduction in virus vectors (therapeutic) is required to manage potato

viruses. When seed certification programmes (including multiple inspections, roguing and

testing of tubers for viruses) are successful, virus inoculum is all but eliminated (Radcliffe

& Ragsdale, 2002). This is the case in the US where, in general, PLRV is no longer a

major threat to potato growers (pers. comm. Dr W. Pett, Michigan State University).

However, on occasions, disease incidence may increase and other measures need to be

implemented to reduce disease incidence (Radcliffe & Ragsdale, 2002). Different

management strategies are required depending on which virus (PVY or PLRV) is causing

the major problem.

2.6.1. Reduction of virus inoculum within the crop

Immediate roguing of plants expressing secondary infection symptoms reduce PVY

sources within the crop. Infected plants should be removed before winged aphids arrive.

Arrival dates may differ for each region in South Africa. For example, peak M. persicae

flight activity occurs in October - November and January - February. An additional peak

occurs in June/July in Pretoria (see Daiber, 1965). In addition, different species appear to

colonise the crop at different times (Daiber, 1962).

14


2.6.2. Reduction of virus inoculum outside the crop

Potential host plants should be identified and volunteer potato plants eliminated. If virus

incidence in the crop exceeds 5 %, then the crop is likely to be the greatest source of virus

inoculum. Commerical fields located close to seed potato fields are likely to serve as PVY

inoculum sources for seed fields.

2.6.3. Monitoring aphid flight to minimize transmission

Alate aphids should be monitored in traps or apterae should be counted on potato foliage

to provide an early warning system to growers to initiate mineral oil spraying or early

harvest.

2.6.4. Isolation

An effective strategy to curb the spread of PVY is to grow seed potatoes in isolation from

commercial potatoes (Radcliffe & Ragsdale, 2002). The separation distance between seed

and commercial potato fields will depend on the acceptable level of risk, weather

conditions, time when aphids first colonise the crop, crop age at first aphid flight and

cultivar, i.e. it varies between localities (Ragsdale et al., 2001; Radcliffe & Ragsdale,

2002). Thresholds to determine when haulm destruction must occur are region-specific

and no recommendations can be made in this regard without knowledge of these regional

factors.

2.6.5. Barriers

Mechanical barriers (e.g. polypropylene sheets) have been shown to reduce PVY spread

(Ragsdale et al., 2001). Barrier crops, planted with a fallow border to the outside and

without a gap between it and the crop, will likely reduce PVY spread.

15


2.6.6. Insecticides

Insecticides do not act quickly enough to reduce PVY spread as aphids usually transmit

the viruses before they die. Mineral oils may be effective.

3. Potato leaf roll virus (PLRV)

Potato leaf roll virus (PLRV) is one of the most damaging viruses of seed, processing and

fresh market potatoes globally. The virus is transmitted naturally through aphid vectors.

Infection of potato with PLRV may cause yield losses through stunting of plants and a

reduction in tuber number and size. In addition, infection can lead to internal net necrosis,

resulting in tubers being unsuitable for processing. Furthermore, PLRV infection in seed

potato crops leads to rejection from certification schemes if the rate of infection exceeds

tolerance levels (Ragsdale et al., 2001; Radcliffe & Ragsdale, 2002).

3.1. The disease and virus

Potato leafroll virus (genus Polerovirus, family Luteoviridae) is a single-stranded RNA

virus (Harrison, 1984). Strains from potato have been determined based on the severity of

symptoms induced in Solanum tuberosum (potato), Physalis floridana and Montia

perfoliata, as well as by ease of transmission through M. persicae (Harrison, 1984).

However, these strains did not differ antigenetically (Tamada et al., 1984).

Symptoms of PLRV through primary infection (current season inoculation) in potato

plants include pallor or reddening of leaf tips, which may roll and become erect.

Secondary symptoms in plants grown from infected potato tubers include stunting of

shoots and leaflets rolling upwards, starting with the oldest leaves. In plants with primary

infection the virus is transmitted through a variable portion of tubers, whereas all tubers

on plants with secondary infection are viruliferous (Harrison, 1984).

3.2. Geographic distribution

The distribution of PLRV is global (Harrison, 1984).

16


3.3. Host range

The majority of known hosts of PLRV belong to the Solanaceae. Non-solanaceous hosts

have been reported from a few species belonging to nine plant families. These include the

Amarathaceae (Chenopodiaceae), Brassicaceae, Malvaceae, Asteraceae, Cucurbitaceae,

Lamiaceae, Nolanaceae, and Portulacraceae (1). (Harrison, 1984; Thomas, 1984; Natti et

al., 1953; Tamada et al., 1984) (Table 3). Examples of non-host plants are given in Table

4.

Table 3. Examples of host plants of potato leafroll virus (PVLR)

Family Species Plant recorded from South Africa

Amaranthaceae (Chenopodiaceae)

Amaranthus caudatus L. X


Celosia argentea L. X


Gomphrena globosa L. X

Brassicaceae Capsella bursa-pastoris (L.) Medik. X

Brassicaceae Sisymbrium altissimum L.

Nolanaceae Nolana lanceolata

Portulacaceae Montia perfoliata Donn ex Willd.

Solanaceae Atropa spp.

Solanaceae Capsicum spp.

Solanaceae Datura stramonium L. X

Solanaceae Hyoscyamus spp.

Solanaceae Lycopersicon esculentum Mill. (tomato) X

Solanaceae Nicandra physalodes (L.) Gaertn. X

Solanaceae Nicotiana clevelandii A. Gray

Solanaceae Nicotiana spp.

Solanaceae Physalis floridana L. (husk-tomato)

Solanaceae Solanum tuberosum L. subsp. andigenum (Juz. & Bukasov) Hawkes

Solanaceae Solanum tuberosum L. subsp. tuberosum (potato) X

17


Table 4. Insusceptible hosts of potato leafroll virus (PVLR)

Family Species

Brassicaceae Brassica rapa L. subsp. campestris (L.) A. R. Clapham

Brassicaceae Brassica oleracea L. var. capitata L. (cabbage)

Brassicaceae Brassica rapa L. subsp. pekinensis (Lour.) Hanelt (Chinese cabbage)

Fabaceae Pisum sativum L. (pea)

Brassicaceae Raphanus sativus L. (radish)

Asteraceae Senecio vulgaris L.

Fabaceae Vicia faba L. (broad bean)

3.4. Virus transmission

The virus is not transmitted by manual inoculation with sap nor through true seed but is

transmissible by vector aphids and by grafting. Because PLRV is not seed-transmitted,

annual weeds are not thought to be an important virus source in spring in regions where

winters are cold (Thomas, 1983). However in warmer regions the situation may differ and

perennial and tuber-forming weeds can harbour PLRV through winter (Natti et al., 1953;

Fox et al., 1993).

PLRV is transmitted by aphids in a persistent circulative non-propagative manner

(Thomas, 1987; Harrison, 1984). As is the case generally with persistently transmitted

viruses, the longer the acquisition and inoculation times, the higher is the rate of

transmission of PLRV. Persistently transmitted viruses usually have a narrow range of

vectors, are passed on when the vectors moult and need a latent period. PLRV does not

replicate in the vector (circulative-non propagative) virus.

Vectors must feed on the phloem tissue of potato plants. PLRV is therefore only

transmitted by aphids colonizing potatoes and, once acquired, the aphid remains infective

for life (Radcliffe & Ragsdale, 2002; Harrison, 1984). Aphids colonizing the potato crop

require 1.6 – 15 minutes to acquire PLRV from an infected plant and become infective

within 8 – 72 hours (latent period) (Radcliffe, 1982; Radcliffe & Ragsdale, 2002).

18


The efficiency of virus transmission by aphids varies and depends on the species, and

within a species on the biotype, morph or instar, e.g. nymphs or adults of winged or

apterous viviparous (wingless aphids bearing living young instead of eggs) species, on

viviparous morphs producing sexual forms (gynoparae) and on sexual morphs themselves

(oviparae and males) (Table 5).

Myzus persicae is the most efficient vector of PLRV and is able to acquire more of the

virus and transmit it faster when feeding at comparatively higher temperatures (Syller,

1994; Bradley & Rideout, 1953). The role of Macrosiphum euphorbiae in PLRV

transmission is variable; many populations are poor vectors while the species has been

reported as an important vector in the early-season spread of the virus at some localities

(see Radcliffe & Ragsdale, 2002). Generally, monitoring of aphid populations in the field

for PLRV control focuses on M. persicae (Ragsdale et al., 1994). In South Africa, nine

aphid species known to transmit PLRV have been recorded in the vicinity of potato fields

(Table 5). Unlike PVY, within-field spread of PLRV is usually by apterae (wingless

aphids) (Boiteau, 1997; Mowry, 2001). However, Radcliffe & Ragsdale (2002) suggest

that within-field spread of PLRV may yet be associated with alatae and not apterae

because maiden flights of alate M. persicae seldom exceed 100 m.

3.5. Disease management

3.5.1. Crop management and tuber storage

a) Adjustment of planting date and avoidance of vector (spatial and temporal

segregation)

As for PVY, isolation of seed and commercial potatoes is an effective strategy to curb the

spread of PLRV (Radcliffe & Ragsdale, 2002). The separation distance varies between

localities but is likely to be > 30 km (Ragsdale et al., 2001; Radcliffe & Ragsdale, 2002).

Changes in the cropping practices in a region must be noted as these may reduce the

effective isolation distance.

19


Isolation may also be via modified planting and harvesting dates (Ragsdale et al., 2001).

Delayed planting (planting after the colonizing flight of M. persicae) has been suggested

as a potential PLRV control strategy (Hanafi et al., 1995; Boiteau, 1984). Yield and tuber

size are reduced with delayed plantings.

b) Roguing

In the Pacific Northwest, USA, twelve plants surrounding the infective PLRV-infected

plant need to be removed as neighbouring plants may be infective but not symptomatic

(Mowry, 1994). This is unlikely to occur in the field where symptoms are mild and/or

fields are large (Mowry, 1994).

c) Crop mulching, covers and barriers

Whitewashes substantially reduce PLRV incidence in tubers, but are less efficient than

white net covers (Marco, 1986). However, crop covers reduce yield (Marco, 1986) and

where infective aphids penetrate the covers, virus spread is often dramatic because of

delayed control measures. This risk of increased virus spread under covers and the cost of

crop covers make this management strategy unlikely to be adopted at large scales.

Mechanical barriers (sticky polyethylene sheets) may reduce PLRV spread by 38 %

(Marco, 1981).

d) Elimination of virus sources

Weeds and volunteer potatoes may be important sources of viruliferous aphids and PLRV

inoculum (e.g. Hanafi et al., 1995; Thomas & Richards, 2004). Many South African plant

species that occur naturally may be sources of virus inoculum. However, when

commercial potato growers plant seed with acceptable levels of virus (1-4 % according to

US certification standards) in neighbouring areas, the potato crop itself becomes a greater

source of inoculum in the area than weeds (Ragsdale et al., 2001). In South Africa, these

values are zero for PLRV levels for the first two generations and both inspections used for

the production of seed potatoes (South African Seed Certification Programme). In line

with international standards, higher levels are virus are acceptable for commercial

20


production in South Africa (South African Seed Certification Programme). Therefore,

potato fields are the likely to be the greatest source of PLRV inoculum in South Africa.

Crop rotation is an important mechanism used to reduce the quantity of volunteer potato

plants in the Pacific Northwest but farms need to be separated by large distances and three

to four year rotation cycles need to be maintained (Mowry, 1994).

e) Tuber storage management

The probability of net necrosis in tubers increases to a maximum at 90 days of storage

(Marsh et al., 1998).

3.5.2. Chemical control of vectors

Systemic insecticides and/or accurately timed foliar insecticide applications are useful to

reduce within-field spread of PLRV, especially if colonizing aphids are virus-free on

arrival. Some insecticides (e.g. the carbamate aldicarb) control within-field spread of

PLRV by apterae but do not prevent transmission by alatae (see Ragsdale et al., 2001).

The chloronicotinyl class of insecticides (imidacloprid) and the new insecticide

pymetrozine appear effective at controlling within-field spread of PLRV (Ragsdale et al.,

2001). Imidacloprid applied systemically reduces PLRV spread (Boiteau & Singh, 1999).

Recommendations for the timing of insecticide applications for PLRV control in seed

potatoes are affected by aphid biology and arrival time relative to crop age (Ragsdale et

al., 2001). Vector control is usually focused primarily on M. persicae, as it is the most

efficient vector species colonizing potatoes (Ragsdale et al., 1994). For PLRV, action

thresholds of aphid abundances for seed potato crops are 10 M. persicae apterae per 100

leaves, but such thresholds are region-specific (Mowry, 1994; Ragsdale et al., 1994).

Weekly monitoring may occur at too coarse a scale to provide rapid results to apply

control measures (aphid populations expand rapidly and exponentially (Radcliffe &

Ragsdale, 2002). A 3-4 day aphid monitoring programme, instead of 7 days, has been

recommended by some authors (Mowry, 2001). This provides sufficient time to apply

foliage sprays before aphids have exponentially spread the virus to plants throughout the

fields (Mowry, 2001). A zero-tolerance policy has been advocated to prevent the spread

of PLRV in seed potatoes in Idaho (Mowry, 2001). However, such intensive insecticide

usage is likely to contribute to insecticide resistance in the future (Mowry, 1994). The

21


negative consequences of the uncontrolled use of chemicals have led to the development

of economic risk assessments where the end-use of the potatoes is taken into

consideration in an attempt to minimize insecticide applications (Marsh et al., 1998).

Antifeedants and repellants such as neem (azadirachtin) have also been used with varying

results to slow the spread of PLRV (van den Heuvel, 1998; Nisbet et al., 1996). These

may be applied as oil formulations (Cranshaw & Baxendale, 2005).

3.5.3. Host plant resistance to the virus

Commercial potato cultivars do not show significant resistance (immunity) to PLRV but

some (e.g. ‘Norgold Russet’) are resistant to tuber net necrosis (Mowry, 1994; Thomas et

al., 2000). This review is not concerned with establishing to which degree the potato

cultivars currently grown in South Africa are susceptible to PLRV.

3.5.4 Host plant resistance to the vector

Genetically modified potato cultivars may decrease PLRV spread between plants by

Myzus persicae (Thomas et al., 2000).

A summary of distribution, host range, symptomatology, vectors, mode of transmission

and control of PVY and PLRV is provided in Table 6.

3.6 Recommendations for seed producing growers: PLRV

3.6.1. Reduction of virus inoculum within the crop

Plants expressing secondary infection symptoms should be removed to reduce within crop

virus inoculum. Symptom-free carriers should not be planted in seed potato regions,

thereby reducing the ease of roguing.

3.6.2. Reduction of virus inoculum outside the crop

Virus inoculum must be reduced by eliminating weeds and volunteer potatoes (Hanafi et

al., 1995).

22


3.6.3. Monitoring aphid flight to minimize transmission

Alate aphids should be monitored in traps or apterae counted on potato foliage to provide

an early warning system for growers to initiate mineral oil spraying or early harvest.

3.6.4. Isolation

Isolate seed and commercial potato production. Effective isolation distances need to be

established after determining cropping practices in the region.

3.6.5. Barriers

Mechanical barriers (sticky polyethylene sheets) may reduce PLRV spread by 38%

(Marco, 1981). Crop barriers may also slow PLRV spread.

3.6.6. Insecticides

Action thresholds based on counts of wingless aphids need to be developed for each

region in South Africa where seed potatoes are grown (Ragsdale et al., 1994; Ragsdale et

al., 2001). Clear guidelines need to be formulated to prevent the over-use of chemicals

that will likely lead to the development of increased resistance in aphid populations and to

further outbreaks. Minimizing damage to the environment is also a concern.

3.6.7. Cultivars

Cultivars should be planted that are not susceptible or are less susceptible to net necrosis

but that are still symptomatic (or else roguing is affected).

The incidence of PLRV and PVY in tubers is presently high in certain growing areas (Ben

Pieterse, pers comm.). It will potentially take a number of years before the levels of virus

inoculum will decline and stabilize to lower levels than are currently experienced and for

the epidemic to end once additional control measures are implemented (Radcliffe &

Ragsdale, 2002). Over this time, it will be essential for growers to follow the guidelines

provided by their managing organization (Potatoes South Africa) and the Seed

Certification Programme.

23


Table 5. Aphid vectors of potato virus Y (PVY) and potato leaf roll virus (PLRV) recorded in the vicinity of seed potato fields in South Africa and aphid species known to colonize potato Species PVY vector Percent

transmission efficiency of PVY

a

c

PLRV vectora Percenttransmission efficiency of PLRVc

Known to colonize potato b

Acyrthosiphon pisum (Harris) X 3.8 - 14

Aphis fabae (Scopoli) X 7.6 - 24 X not reported X

Aphis gossypii (Glover) X 12 - 31 X 4-74 X

Aphis nasturtii Kaltenbach X 19.0 - 50 X 20 X

Aphis spiraecola (Patch) X

Aulacorthum circumflexum (Buckton) X 75 X

Aulacorthum solani (Kaltenbach) X 5.0 X not reported X

Brachycaudus helichrysi (Kaltenbach) X 4.8 - 12.5

Brachycaudus rumexicolens (Patch) X not reported

Brevicoryne brassicae (L.) X

Capitophorus hippophaes (Walker) X 3.0 - 3.1

Diuraphis noxia (Mordvilko) X 4.0 - 7.0

Dysaphis (Pomaphis) spp. X

Hyalopterus pruni (Geoffroy) X 13.9

Hyperomyzus lactucae (L.) X 0.4 - 17.4

24


Table 5 continued

Species PVY vector Percenttransmission efficiency of PVY

a

c

PLRV vectora Percenttransmission efficiency of PLRVc

Known to colonize potato plantsb

Lipaphis erysimi (Kaltenbach) X 10

Macrosiphum euphorbiae (Thomas) X 4.0 - 29.0 X 0 - 25 X

Metopolophium dirhodum (Walker) X 0.5 – 10

Myzus ornatus Laing X not reported X

Myzus persicae (Sulzer) X 4.7 - 71.1 X 2.4 - 83.8 X

Rhopalosiphoninus latysiphon (Davidson) X X not reported X

Rhopalosiphum maidis (Fitch)d X not reported

Rhapalosiphum rufiabdominalis (Sasaki) X

Rhopalosiphum padi (L.) X 0.5 - 11.5

Schizaphis graminum (Rondani) X not reported

Sitobion sp. X

Sitobion avenae (F.) X 0.06 - 1.8

Sitobion fragariae (Walker) X 0.09 – 10

Smynthurodes betae Westwood X

Uroleucon sonchi (L.) X a Thompson, 1997; Ragsdale et al., 2001 b I. Millar (pers. comm., ARC-PPRI. Although all of the species marked are known to colonize potato, some have not been collected on potato in South Africa, which could be a reflection of collecting effort). c Ragsdale et al., 2001; d This species has not been recorded from potato

25


Table 6. Tabulated summary of distribution, host range, symptomatology, vectors, mode of transmission and control of i) Potato virus Y

and ii) potato leafroll virus

Distribution Host range Symptoms Identified vectors Transmission Control6

i) Potato Y potyvirus (PVY)

Worldwide, in potato and outdoor crops of pepper, tobacco and tomato in warmer countries (de Bokx and Huttinga 1981)

Mainly Solanaceae (Solanum tuberosum (potato), Capsicum spp. (peppers), Nicotiana spp. (e.g. tobacco), Lycopersicon esculentum (tomato)), but also some members of Chenopodiaceae (Amaranthaceae), Compositae, Leguminosae are susceptible

a) leaf mottling, b) ‘leaf drop streak’ with vein necrosis, c)‘stipple-streak’1 primary symptoms mild to very mild mottle, may appear late in growing season; secondary symptoms more obvious, may appear early in growing season depending on climatic conditions; differences often indistinct because of diversity of potato cultivars and virus strains (de Bokx and Huttinga 1981), necrosis may occur in tubers

Myzus persicae (most efficient), Aphis fabae, Macrosiphum euphorbiae, Myzus (Nectarosiphon) certus, Myzus (Phorodon) humuli, Rhopalosiphum insertum2

-Non-persistent-Mechanical inoculation (25 spp of aphids) -grafting -no helper virus required

Insecticides ineffective i) avoid infection ii) avoid growing crops near established crop of same species iii) roguing iv) spray with mineral oils to reduce transmission3

v) breed for resistance vi) reflective surfaces and sticky yellow sheets4

26


Table 6 continued

Distribution Host range Symptoms Identified vectors Transmission Control6

ii) Potato leafroll virus (PLRV)

Worldwide in most potato crops; tomato yellow top diseases reported in some countries but not determined that all caused by PLRV

Potato (Solanum tuberosum) Tomato 20 spp (most Solanaceae); confined mainly to potato

Potato: Primary: pallor/reddening of leaf tips, may become rolled; Secondary (plants grown from tubers): shoot stunting, upward rolling of leaflets esp. lower leaves, may develop marginal necrosis; internal net necrosis may develop in tubers with primary/secondary infection Tomato: stunting, marginal yellowing, curling of leaflets, death of flowerbuds7

Several spp reported to transmit virus2; Myzus persicae (most efficient), Macrosiphum euphorbiae (good vector of Australian yellow top isolates)

-not transmitted by manual inoculation -persistent -transmissible by aphid vectors and grafting

i) select tubers from symptom-free clones of mother plants ii) heat treatment of tubers5 iii) grow seed-potato crops where vectors are few or arrive late in growing season or unfavourable weather conditions for aphid activity iv) roguing v) apply insecticides to minimize aphid activity vi) harvest/destroy haulms (early ‘vine kill’) before virus pass from shoots to tubers vii) isolate healthy seed-potato crops from infected viii) Plant field-resistant cultivars ix) Assess tuber health after harvest (serological tests esp. ELISA)

1Beemster and Rozendaal 1972; 2Kennedy et al. 1962, Van Hoof 1980; 3Bradeley et al. 1966, Vanderveken 1977; 4Loebenstein and Raccah 1980; 5Kassanis 1949; 6de Bokx and Huttinga 1981; Harrison 1984; 7 leaf drop streak = necrosis, starts as spots or rings on leaflets, may cause leaves to collapse and drop from plant or remain hanging on stem stipple streak = necrotic lesions and streaks on leaves and variable streaking on petiole and stems

27


4. References

Adams, J. B. 1975. Comments on Aphid Resistance in Potatoes. American Potato Journal 52:

313-315.

Annecke, D.P. and V.C. Moran. 1982. Insects and Mites of Cultivated Plants in South Africa.

Butterworths, Durban.

Basky, Z. 2002. The relationship between aphid dynamics and two prominent potato viruses

(PVY and PLRV) in seed potatoes in Hungary. Crop Protection 21: 823-827.

Boiteau, G. 1984. Effect of planting date, plant spacing, and weed cover on populations of

insects, arachnids, and entomophthoran fungi in potato fields. Environmental Entomology

13: 751-756.

Boiteau, G. 1997. Comparative propensity for dispersal of apterous and alate morphs of three

potato-colonising aphid species. Canadian Journal of Zoology 75: 1396-1403.

Boiteau, G. and R.P. Singh. 1999. Field assessment of imidacloprid to reduce the spread of PVYo

and PLRV in potato. American Journal of Potato Research 76: 31-36.

Bradley, R. H. E. 1954. Studies of the mechanism of transmission of potato virus Y by the green

peach aphid, Myzus persicae (Sulz.) (Homoptera: Aphididae). Canadian Journal of

Zoology 32: 64-73.

Bradley, R. H. E. and D. W. Rideout. 1953. Comparative transmission of potato virus Y by four

aphid species that infect potatoes. Canadian Journal of Zoology 31: 333-341.

Büchen-Osmond, C. 1987. Plant Viruses Online- Descriptions and lists from the VIDE database.

Potato Y potyvirus. http://image.fs.uidaho.edu/vide/descr652.html.

Caldiz, D. O., A. J. Haverkort and P. C. Struik. 2002. Analysis of a complex crop production

system in interdependent agro-ecological zones: a methodological approach for potatoes

in Argentina. Agricultural Systems 73: 297-311.

Cranshaw, W.S. and B. Baxendale. 2005. Insect control: horticultural oils.

http://www.ext.colostate.edu/pubs/insect/05569.html.

Daiber, C.C. 1962. Notes on the population dynamics of aphids on potatoes and the spread of the

leaf roll virus in South Africa. Journal of the Entomological Society of Southern Africa

25: 157-169.

Daiber, C.C. 1965. Notes on potato aphids and leaf roll spread at different South African

localities. Journal of the Entomological Society of Southern Africa 27: 191-215.

28


Daiber, C.C. and Schöll, S.E. 1959. Further notes on the overwintering of the green peach aphid,

Myzus persicae (Sulzer), in South Africa. Journal of the Entomological Society of

Southern Africa 22: 495-520.

de Bokx, J. A. and H. Huttinga. 1981. CMI/AAB Descriptions of Plant Viruses. Potato Virus Y

242 (No. 37 revised). www.dpvweb.net/dprv/showdpv.php?dpvno=242.

DiFonzo, C. D., D. W. Ragsdale, E. B. Radcliffe, N. C. Gudmestad and G. A. Secor. 1996. Crop

borders reduce potato virus Y incidence in seed potato. Annals of Applied Biology 129:

289-302.

DiFonzo, C. D., D. W. Ragsdale, E. B. Radcliffe, N. C. Gudmestad and G. A. Secor. 1997.

Seasonal abundance of aphid vectors of potato virus Y in the Red River Valley of

Minnesota and North Dakota. Journal of Economic Entomology 90: 824-831.

ffrench-Constant, R. H., R. Harrington and A. L. Devonshire. 1988. Effect of repeated

applications of insecticides to potatoes on numbers of Myzus persicae (Sulzer)

(Hemiptera: Aphididae) and on the frequencies of insecticide-resistant variants. Crop

Protection 7: 55-61.

Fletcher, J.D. 2001. New hosts of Alfalfa mosaic virus, Cucumber mosaic virus, Potato virus Y,

Soybean dwarf virus, and Tomato spotted wilt virus in New Zealand. New Zealand

Journal of Crop and Horticultural Science 29: 213 – 217.

Fox, L., K.D. Biever, H.H. Toba, J.E. Duffus and P.E. Thomas. 1993. Overwintering and

monitoring of potato leafroll virus in some wild crucifers. American Potato Journal 70:

505-515.

Halbert, S. E., D. L. Corsini and M. A. Wiebe. 2003. Potato virus Y transmission efficiency for

some common aphids in Idaho. American Journal of Potato Research 80: 87-91.

Hanafi, A., E. B. Radcliffe and D. W. Ragsdale. 1995. Spread and control of potato leafroll virus

in the Souss Valley of Morocco. Crop Protection 14: 145-153.

Harrington, R., E. Bartlet, D. K. Riley, R. H. ffrench-Constant and S. J. Clark. 1989. Resurgence

of insecticide-resistant Myzus persicae on potatoes treated repeatedly with cypermethrin

and mineral oil. Crop Protection 8: 340-348.

Harrison, B. D. 1984. CMI/AAB Descriptions of Plant Viruses. Potato leafroll virus 291 (no. 36

revised). www.dpvweb.net/dpv/showdpv.php?dpvno=291.

Jones, R. A. C. 2004. Using epidemiological information to develop effective integrated virus

disease management strategies. Virus Research 100: 5-30.

Latorre, B. A. 1983. Disease Incidence Gradient and the Effect of Seedbed Infection on Potato

Virus Y Outbreaks in Tobacco. Plant Disease 67: 302-304.

29


Lecoq, H., B. Moury, C. Desbiez, A. Palloix and M. Pitrat. 2004. Durable virus resistance in

plants through conventional approaches: a challenge. Virus Research 100: 31-39.

Marco, S. 1981. Reducing potato leafroll virus (PLRV) in potato by means of baiting aphids to

yellow surfaces and protecting crops by coarse nets. Potato Research 24: 21-31.

Marco, S. 1986. Incidence of Aphid-Transmitted Virus Infections Reduced by Whitewash Sprays

on Plants. Phytopathology 76: 1344-1348.

Marsh, T.L., R.G. Huffaker, R.C. Mittelhammer, R.J. Folwell, G.E. Long, D.R. Horton and H.H.

Toba. 1998. Potato leafroll virus net necrosis: identifying pest management tradeoffs

among inoculation interval, storage length, and tuber weight. Journal of Economic

Entomology 91: 923-932.

Martín, B., I. Varela and C. Cabaleiro. 2004. Effects of various oils on survival of Myzus persicae

Sulzer and its transmission of cucumber mosaic virus on pepper. The Journal of

Horticultural Science and Biotechnology 79: 855-858.

Mowry, T. M. 1994. Potato leafroll virus management in the Pacific Northwest (USA), pp. 111-

123. In: G. W. Zehnder, Powelson, M.L., Jansson, R.K. and Raman, K.V. [ed.], Advances

in potato pest biology and management. American Phytopathological Society, Minnesota,

USA.

Mowry, T. M. 2001. Green peach aphid (Homoptera: Aphididae) action thresholds for controlling

the spread of potato leafroll virus in Idaho. Journal of Economic Entomology 94: 1332-

1339.

Natti, J.J., H.C. Kirkpatrick and Ross, A.F. 1953. Host range of potato leafroll virus. American

Potato Journal 30: 55-64.

Nisbet, A. J., J. A. T. Woodford and R. H. C. Strang. 1996. The effects of azadirachtin on the

acquisition and inoculation of potato leafroll virus by Myzus persicae. Crop Protection 15:

9-14.

Radcliffe, E. B. 1982. Insect pests of potato. Annual Review of Entomology 27: 173-204.

Radcliffe, E. B. and D. W. Ragsdale. 2002. Aphid-transmitted potato viruses: The importance of

understanding vector biology. American Journal of Potato Research 79: 353-386.

Radcliffe, E. and D. Ragsdale. 1998. Aphids cause big problems for industry. Valley Potato

Grower April ’98. pp. 4-6, 31.

Ragsdale, D. W., E. B. Radcliffe and C. D. DiFonzo. 2001. Epidemiology and field control of

PVY and PLRV, pp. 237-270. In: G. Loebenstein, Berger, P.H., Brunt, A.A. and Lawson,

R.H. [ed.], Virus and virus-like diseases of potatoes and production of seed-potatoes.

Kluwer Academic publishers, Dordrecht.

30


Ragsdale, D. W., E. B. Radcliffe, C. D. DiFonzo and M. S. Connelly. 1994. Action thresholds for

an aphid vector of potato leaf roll virus, pp. pp99-110. In: G. W. Zehnder, Powelson,

M.L., Jansson, R.K. and Raman, K.V. [ed.], Advances in potato pest biology and

management. American Phytopathological Society, Minnesota, USA.

Raman, K. V. 1988. Insecticide toxicity to three strains of green peach aphid (Myzus persicae

Sulzer) reared on resistant and susceptible potato cultivars. Crop Protection 7: 62-65.

Robert, Y., J. A. T. Woodford and D.G. Ducray-Bourdin. 2000. Some epidemiological

approaches to the control of aphid-borne virus diseases in seed potato crops in northern

Europe. Virus Research 71: 33-47.

Roosen, J., R. G. Huffaker, R.J. Folwell, T.L. Marsh and R.C. Mittelhammer. 1997. The relation

of potato leaf roll virus net necrosis in potato tubers to the interval between planting and

inoculation. Crop Protection 16: 533-539.

Schubert, J., J. Matousek and D. Mattern. 2004. Pathogen-derived resistance in potato to Potato

virus Y-aspects of stability and biosafety under field conditions. Virus Research 100: 41-

50.

Simons, J.N. and T.A. Zitter. 1980. Use of oils to control aphid borne viruses. Plant Disease 64:

542-546

Syller, J. 1994. The effect of temperature on the availability and acquisition of potato leafroll

luteovirus by Myzus persicae. Annals of Applied Biology 125: 141-145.

Tamada, T., B.D. Harrison and I.M. Roberts. 1984. Portulacraceae as PLRV host ?Variation

among British isolates of potato leafroll virus. Annals of Applied Biology 104:107-116

Thomas J.E. 1984. Characterisation of an Australian isolate of tomato yellow top virus. Annals of

Applied Biology104: 79-86.

Thomas, J. E. 1987. Potato leafroll luteovirus. Plant Viruses Online. Descriptions and lists from

the VIDE Database. http://micronet.im.ac.cn/vide/descr644.html.

Thomas, J.E. 1993. Alternative hosts and the epidemiology of poato leafroll virus in Queensland.

Australian Journal of Agricultural Research 44: 1905-1916.

Thomas, P.E. 1983. Sources of dissemination of potato viruses in the Columbia basin of the

northwest United States. Plant Disease 67: 744-747.

Thomas, P. E. and K. Richards. 2004. New weed hosts of potato viruses and their impact on

potato virus epidemiology. Phytopathology 94: S154.

Thomas, P. E, K.S. Pike and G.L. Reed. 1997. Role of green peach aphid flights in the

epidemiology of potato leaf roll disease in the Columbia Basin. Plant Disease 81: 1311-

1316.

31


Thomas, P. E., E. C. Lawson, J.C. Zalewski, G.L. Reed and W.K. Kaniewski. 2000. Extreme

resistance to Potato leafroll virus in potato cv. Russet Burbank mediated by the viral

replicase gene. Virus Research 71: 49-62.

Thompson, G. J., D. C. A. Hoffman and P. J. Prins. 1987. A deviant strain of potato virus Y

infecting potatoes in South Africa. Potato Research 30: 219-228.

Thompson, G.J. 1997. Study and control of virus diseases of potatoes. In: Landbounavorsingraad

Roodeplaat: Aartappelnavorsing 1996/1997. Agricultural Research Council, Pretoria.

van den Heuvel, J. F. J. M., S. A. Hogenhout, M. Verbeek and F. van der Wilk. 1998.

Azadirachta indica metabolites interfere with the host-endosymbiont relationship and

inhibit the transmission of potato leafroll virus by Myzus persicae. Entomologia

Experimentalis et Applicata 86: 253-260.

Vorster, L. L., L. H. Nel and J. M. Kotzé. 1990. Differentiation of strains of potato virus Y

affecting tobacco in South Africa. Phytophylactica 22: 129-131.

32

Date post:	04-Feb-2022
Category:	Documents
Upload:	others
View:	5 times
Download:	0 times

REVIEW: Potato virus Y (PVY) and Potato leaf roll virus

Documents