Funky Leaf Spot, Viruses, and Xylella Update – Winter 2011

Phillip M. Brannen

University of Georgia Plant Pathology Department

Background: Systemic Blueberry Diseases

� At least nine species of plant viruses infect blueberries in North America

� Some (such as scorch and shock viruses) are very damaging, whereas most others are highly variable in level of damage

� Historically, southern blueberries have been essentially virus-free– 2000: a few plants positive for Blueberry shoestring virus or

Strawberry latent ringspot virus in south GA– 2001: no positives in sample of 93 suspects in large NC & GA

virus survey– Since 2005: increased prevalence and severity of viral and

virus-like diseases on southern highbush

Blueberry red ringspot virus (BRRV)Fall Spring

Blueberry red ringspot virus (BRRV)

BRRV on Ozarkblue fruit in NC (courtesy Bill Cline, NCSU)

� Mosaic, mottling, ringspotting� Foliar symptoms most

pronounced in fall (absent in spring)

� Stem symptoms most pronounced in spring

� Fruit symptoms not seen in GA� No known in-field vector, most

likely transmitted via cuttings� Effects on plant growth and yield

poorly documented

Prevalence of Blueberry red ringspot virus (BRRV) and necrotic ringspot disease in different counties in Georgia, based on a survey conducted in fall of 2008.

Scherm et al., 2008

Scherm et al., 2008

Prevalence of Blueberry red ringspot virus (BRRV) and necrotic ringspot disease on different southern highbush cultivars in Georgia, based on a survey conducted in fall of 2008.

Phil Harmon (Univ. of Florida) did observe transmission of the disease in a greenhouse. Infected plants on one table, collected for experimentation, were placed opposite non-infected plants. Within two months, most of the non-infected plants were showing the disease symptoms. Mites were found in association with the diseased plants, but data is preliminary at best.

Blueberry Shock

�Spread through propagation or pollen (bees). � In theory, the bees coming from Michigan to

Georgia should not be a problem, since the pollen would not be active when the bees come to Georgia (pollen too old).

� 1-2 year latent period before symptoms.�Plants eventually recover, but losses are severe

for several years.

Blueberry Scorch

�Spread through propagation or aphids. � 1-2 year latent period before symptoms.�Plants eventually die, and losses are significant

until death occurs.

Temecula isolate (WT))

Xylella fastidiosa

A small, fastidious, Gram negative, xylem-inhabiting bacterium related to Xanthomonas spp.

Tissue specific, confined to tracheary elements, tracheids or vessels

Photo by H.C. Hoch, Cornell University; Journal of Bacteriology.

189:7507–7510.

XF1968F3/XF1968R3 Primer Results

1 kb BB21 BB1 BB9 G11 Teme OL2 T1B T5C Dixon M12 FL GA NegLadder Oak Plum Ctrl

Several strains of X. fastidiosa courtesy of Dr. Jianchi Chen, USDA-ARS, Parlier, CA

Blueberry Grape Oleander Almond

Results from multiple primers indicate the blueberry X. fastidiosa is not piercei

L. Nissen and T. Denny

Blueberry survey locations with confirmed Xylella fastidiosa infections. Plants symptomatic for bacterial leaf scorch were confirmed through ELISA (map developed using Google Earth).

Incidence (percentage of symptomatic plants) of bacterial leaf scorch by cultivar at one 7th leaf producer site in Colquitt County. The number of rows surveyed (n) is shown in parentheses next to the cultivar name.

FL 86-19 [V1]

Emerald

Millennium

Windsor

Southern Belle

FL 86-19 (V1) y = 7.7299x - 1.7131R2 = 0.4022F = 0.002

0

20

40

60

80

100

0 2 4 6 8 10 12

Years

Dis

ease i

ncid

en

ce (

%)

Star y = 3.2974x - 4.2256R2 = 0.2758F = 0.00004

0

20

40

60

80

100

0 2 4 6 8 10 12

Years

Dis

ea

se

in

cid

en

ce

(%

)

Disease incidence (percentage of symptomatic plants) of bacterial leaf scorch observed on cultivars FL 86-19 (V1) and Star over time (years in the field). Both cultivars are susceptible to the disease, but FL 86-19 is clearly more susceptible to disease development and spread than Star. Based on the trend lines projected through use of survey data, ~75% of FL 86-19 plants would be symptomatic by the tenth year in the field, versus ~30% in Star.

Can we screen for resistance in the greenhouse to obtain rapid evaluations of blueberry lines?

Blueberry leaf scorch symptom Development with 10-fold dilution of 0.25 OD BB08-2 Inoculum (2009)

0

1

2

3

4

6wk

8wk

10wk

12wk

14wk

16wk

18wk

V1StarWindsorEmerald

Blueberry leaf scorch symptom development with 0.25 OD BB08-3 inoculum (2010 test)

0

1

2

3

4

2 mo 3 mo 4 mo 5mo

V1StarWindsorEmerald

Blueberry leaf scorch symptom development with 0.25 OD BB08-3 inoculum (2010 test)

0

1

2

3

4

2 mo 3 mo 4 mo 5 mo

V1ClimaxSouthern BelleTif BlueRebel

Cultivar Field GreenhouseV1 Susceptible SusceptibleStar Susceptible Moderate Res.Windsor Resistant ResistantEmerald Resistant SusceptibleRebel Susceptible SusceptibleSouthern Belle Resistant ResistantClimax ? ResistantTif Blue ? Resistant

Comparison of Field Resistance to Blueberry Leaf Scorch and Greenhouse Resistance

0

0.5

1

1.5

2

2.5

3

Leaf

sco

rch

ratin

g

Four months after Inoculation

BB08-3 EB06-1SCBB08-1 B1-10 GAB2-64 GA B2-66 GABuffer

Pathogenicity of Xylella fastidiosa strains to Emerald, 2010 test

Greenhouse Screening for Resistance to Leaf Scorch

� Resistance to blueberry leaf scorch in greenhouse screening agreed with observed field resistance for most cultivars.

� Emerald was more susceptible to Xylella fastidiosa in the greenhouse than in the field.

� Star appeared to be more resistant in greenhouse tests than has been observed in the field.

� Differences between greenhouse and field observations could be due to host specificity of strains, aggressiveness of strains, or loss of virulence in some strains during culture.

Evaluation of Benign Strains of X. fastidiosa for the Biological Control of Blueberry Leaf Scorch by Cross-Protection

� Various strains of X. fastidiosa are being evaluated in the greenhouse for biological control of blueberry leaf scorch in ‘Star’ and ‘V1’, including strains from elderberry, blackberry, sycamore, and mulberry.

� ‘Star’ and ‘V1’ blueberry plants inoculated with these strains were challenged 3-4 weeks later with the blueberry leaf scorch strain BB08-2 in 2009 or BB08-3 in 2010 to evaluate biological control of the disease.

� Disease incidence and severity are rated once a month.

Don Hopkins, UF

0

1

2

3Le

af s

corc

h ra

ting

Five months after Inoculation with BB08-3

Buffer EB92-1

Syc86-1 BKB95-1

Dixon PD00-2

Biological Control of Blueberry Leaf Scorch in V1 (2010 test)

Biocontrol of Blueberry Leaf Scorch

� In greenhouse tests, X. fastidiosa strains EB92-1 and Syc86-1 reduced leaf scorch in both Star and V1. BKB95-1 provided biocontrol in Star and PD00-2 was effective in V1.

� Strain PD00-2 is not a candidate for biocontrol of blueberry leaf scorch, because it is pathogenic to grapevine.

� Further greenhouse and field tests are warranted for EB92-1, Syc86-1, and possibly BKB95-1.

Summary � In addition to X. fastidiosa strains from blueberry, strains

from lupine, almond, and elderberry were pathogenic to blueberry.

� Resistance to blueberry leaf scorch in greenhouse screening agreed with observed field resistance for most cultivars. The development of a quick screening test for resistance to leaf scorch is possible.

� Differences in virulence or host specificity were observed in X. fastidiosa strains used in greenhouse tests.

� Greenhouse tests for the biological control of leaf scorch were positive. Field tests are needed.

Vectors of Xylella species diseases are limited to sharpshooter and spittlebug insects, of which the glassy-winged sharpshooter (Homalodisca vitripennis) is primary in the Southeast.

University of California

Homalodisca insolita Paraulacizes irrorata

Credits: Susan Ellis

Draeculacephala sppOncometopia nigricans

0.25% 0.77% 0.25% 1%

Graphocephala versuta 0.51%

Emphasis on glassy-winged sharpshooter (Homalodisca vitipennis), which constituted 97% of leafhoppers sampled within bushes season-long at two field sites 97%

Credit: University of California

Dr. Moukaram Tertuliano, 2009

Feeding preference of glassy-winged sharpshooter in choice test

� Total of 4 plants (reps) per cultivar tested sequentially

� For each rep, 20 leafhoppers released in cage containing one plant per cultivar

� Five days observation, plant position rotated every day

� Number of leafhoppers settled/feeding per plant recorded daily

Emerald

Star

V155%

27%

17%

GWSS feeding on blueberry cultivar

Early plant growth (3 months after transplanting) was reduced for BRRV and Xylella cuttings, although Xylella cuttings were still asymptomatic at that time. In fall 2010, 15 months after transplanting, 100, 8.9 and 0% of plants derived from the summer 2009 BRRV, Xylella, and funky spot cuttings, respectively, showed visual disease symptoms.

Scherm et al., 2010