Plant Virology Section, Punjah Agricultural Research Institute,Faisalahady Pakistan
Studies with a Whitefly-transmitted Yellow Vein Mosaicof Digera alternijolius
By
MusHTAQ A H M A D
With one figure
Received March J, 1978
Introduction
Digera alternifolius Linn., locally called *Tandala', is an endemic plantbelonging to the family Amarantaceae. It is an annual weed ubiquitous in thesummer crops in the plains of the Punjab, Pakistan. In the field, aroundLyallpur and some other places, this weed, however, often shows symptoms ofa yellow vein mosaic (YVM) disease, presumed to be caused by a virus.
Endemic plants like D. alternifolius may serve both as a source and analternate host of a virus, so that the viruses harboured by weeds could infectcultivated plants and conversely the viruses originating in commercial plantscould persist in the endemics. So, as a basic approach to work out the possiblerelationships of virus diseases of weeds and those of economic plants, thestudies on the nature of the causal agent and the modes of transmission ofYVM of D. alternifolius were undertaken.
Materials and Methods
The present research was carried out during the summer months i.e.. May to Septem-ber. To provide insect-free conditions, the plants were grown in crodts placed in isolationcages. Plants about one month old were preferred for transmission purposes. For acquisitionor inoculation feeding of the suspect insect vectors, sleeve-type cages (20 cm X 7.5 cm X7.5 cm) made of pla.'itic and muslin cloth, that could cover 4 to 5 leaves, were used (designedafter KAISER 1972).
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22 AHMAD
Symptomatology
The vein clearing first starts followed by veina! chlorosis of leaves. Tlien the networkof veins becomes bright yellow whidi is very conspicuous and exhibits the characteristicsymptomatology of the disease (Fig.). As the plants grow old, the yellow colour of the veinsfades away a little. The size of the leaves is reduced and the veins and veinlets are thickened.In severe cases, almost complete chlorosis of the leaves is evident. The disease is most pre-valent in mid-summer.
Fig. Yellow vein mosaic symptoms on leaves of Digera alternifolius
Results
In order to find out whether the causal agent of the disease is transmittedthrough seed, soil or nematodes, seeds from diseased plants were planted infield soil collected from around Digera plants with YVM. Ten erodes, with15 plants each, were maintained in insect-proof cages. No disease developmentwas observed in these plants up to maturity, proving negative transmissionthrough seed, soil or nematodes.
To determine medianical transmission, YVM-affected foliage was macer-ated with a mortar and pestle, after the addition of 0.01 phosphate bufferpH 7.0. The sap was applied with the thumb and forefinger to carborundum-dusted (325 mesh) leaves of young indicator plants. No transmission occurredto the healthy plants.
A Whitefly-transmitted Yellow Vein Mosaic of Digera alternifolius 23
Dodder (Cuscuta reflexa) vines were placed on affected plants for para-sitization. After the dodder had fully established on the diseased plants in25 days, it was placed in contact with 20 healthy Digera plants in the insect-exclusion cages. No test plants became infected before maturity.
Twenty scions from healthy weed plants were grafted by cleft to YVM-affected stalks. Graft transmission succeeded in 15 plants which showedtypical symptoms of the disease within two months. This reveals that YVM ofD. alternifolius is a graft-transmissible agent, either a virus or a mycoplasma.
The suspect vectors tested in transmission experiments were the insectsoften found associated with Digera plants in the field such as thrips, leaf-hoppers, whiteflies, bugs and beetles. These potential vectors were then testedseparately. Non-viruliferous insects were first fed on diseased plants for 24 hbefore being transferred to healthy test plants in groups of 15—20 insects fora 3-day inoculation feeding. Before each test, insects were starved for 1 to 2 hand then transferred to host plants for inoculations or acquisitions. These testswere repeated thrice during the summer and 20 to 25 plants were used for eaditreatment at any one time. Transmission succeeded only in the case of white-flies, Bemisia tahaci Genn., 20 days after inoculation. In another test 35 moreplants were infected by whiteflies in the above manner for confirmation. Inparallel tests, whiteflies first fed on cotton, supposed to be virus-free, wereplaced on 30 test plants whidi yielded no symptoms. This proves that thewhitefly, B. tahaci Genn. is the only known vector of yellow vein mosaicdisease of D. alternifolius.
Whiteflies from diseased Digera plants were allowed to feed on 25 healthyplants each of mungbean {Vigna radiata) and urdbean (V. mungo); but noinfection occurred. Conversely, bade transmission tests to this weed fromthese beans infected with another whitefly-transmitted yellow mosaic disease(AHMAD and HARWOOD 1973) also yielded negative results, indicating thatYVM of D. alternifolius has no correlation with the yellow mosaic of urd andmung beans.
Four hundred whitefly adults were caged with the diseased plants for a3-day acquisition feeding. Afterwards, they were transferred to 25 cottonplants, which were supposed to be immune to this pathogen, and allowed tooviposit. After seven days, a few insects still living were killed with mala-thlon spray. After 20 days, the progeny of the whiteflies developed tomaturity. Two hundred whitefly adults were then collected from this newgeneration and caged with 20 healthy plants of D. alternifolius for a 72-hourinoculation feeding. No infection occurred in this way, which proved that thispathogen does not have transovarial transmission in the whitefly.
Discussion
This yellow vein mosaic disease exhibits all the transmission diaracter-istics whidi are typical of other whitefly-transmitted viruses (GOSTA 1969).Consultation of literature by the author and the reviews made by GOSTA(1969) and VARMA (1963) indicate that no whitefly-transmitted virus disease
24 AHMAD, A WKitefly-transmitted Yellow Vein Mosaic of Digera alternifoUus
has so far been described on D. alternifoUus. It, therefore, seems logical toassume that the pathogen under study is either a new one or infects this newhost from an indigenous host already infected.
A review of diseases caused by mycoplasma indicates that so far no virusof the whitefly-vectored group has been proved to be a mycoplasma (HULL1972). Conversely, no mycoplasma has thus far been shown to be whitefly-transmitted (COSTA 1969). It may, therefore, be hypothesized that the patho-gen under investigation may be a virus, similar to that described recently byGALVEZ and CASTANO (1976) and GOODMAN et al. (1977).
Summary
A yellow vein mosaic disease, presumed to be caused by a virus, showingthe symptoms of bright yellow colouration in the leaf veins was recorded onDigera alternifoUus Linn. It was only transmitted by grafting and the white-fly, Bemisia tabaci Genn.; no transmission occurred by medianical means,dodder, seed and soil or nematodes. There was no transovarian passage of thepathogen in the whitefly.
Zusammenfassung
Untersudiungen eines durdi die Weifie Fliege ubertragenen Gelbadernmosaiksvon Digera alternifolias
Auf Digera alternifoUus Linn, wurde von einer Gelbadernmosaikerkran-kung berichtet, vermutlich durch ein Virus verursacht, die die Symptome hell-gelber Farbung der Blattadern aufwies. Diese Erkrankung konnte nur durdiPfropfung und die WeifSe Fliege, Bemisia tabaci Genn., ubertragen werden;durch medianisdie Mittel, Seide, Saatgut und Boden oder Nematoden erfolgtekeine Obertragung. Es gab keine transovariale Passage des Erregers in derWeifien Fliege.
Literature
AHMAD, M., and R. F. HARVX'OOD, 1973: Studies on a whitefly-transmitted yellow mosaic ofurdbean {Phaseolus niungo). Plant Dis. Reptr. 57, 800—802.
COSTA, A. S., 1969: Whiteflies as virus vectors. In: K. MARAMOROSCH (Ed.), Viruses, Vectors,and Vegetation, Chapter 4, 95—119. Intersci. Publ., New York.
GALVEZ, G. E., and M. J. CASTANO, 1976: Purification of the whitefly-transmitted beangolden mosaic virus. Turriaiha 26, 205—207.
GOODMAN, R. M., J. BIRD, and P. THONGMEEARKOM, 1977: An unusual viruslike particleassociated with golden yellow mosaic of heans, Phytopathology 67, 37—42.
HULL, R., 1972: Mycoplasma and plant diseases. PANS 18, 154—164.KAISER, W. J., 1972: Diseases of food legumes caused by pea leaf roll virus in Iran. FAO
Plant Prot. Bull. 20, 127—133.VARMA, P . M., 1963: Transmission of plant viruses by whiteflies. Bull. Nat. Inst. Sci. India
24,11—33.
Author's address: Plant Virology Section, Punjab Agricultural Research Institute,Faisalabad (Pakistan),
