Cucurbit Genetics Cooperative Report 19:83-84 (article .30) 1996
A Taiwan Strain of Papaya Ringspot Virus from Carica papaya Causing Prominent Symptoms on Cultivated Cucurbits
Department of Plant Pathology. Cornell University, New York State Agricultural Experiment Station, Geneva, N. Y. 11446
Papaya ringspot virus (PRV), an economically important viral agent, usually occurs in tropical and semitropical areas of the world. Its particles are flexuous rods about 780 x 12 mm containing a single strand of RNA, which in nature are spread by a number of aphid species in the non[persistent manner. This potyvirus is not seed-borne, but can be easily transferred mechanically. Most of the strains and variants of PRV are serologically related and belong to one of two groups: Type W (PRV-W)(= watermelon mosaic virus 1) and Type P (PRV-P)(Papaya-infecting) (6).
PRV-W occurs in warm regions of the world, but occasionally it has been found in temperate zones. It is one of the major viruses affecting Cucurbitaceae, but it cannot infect any member of the Caricaceae. Symptoms incited by PRV-W include severe plant stunting, foliar mosaic, and extreme reduction of leaf lamina. Fruits are prominently affected by knobby overgrowths and color break. Resistance has been located in Cucumis melo (9), Cucumis sativus (8), Cucurbita species (6), Lagenaria siceraria (2), Luffa acutangula and L. aegyptica (1), and in a few other cucurbit species, including Cucumis metuliferus (5).
PRV-P is mainly confined to tropical areas and he major hosts are Caricacae, and particularly affected is the cultivated papaya (Carica papaya). Symptoms include plant stunting, leaf mosaic, distortion, necrotic streaks on stem and petioles, and concentric rings on fruits. In nature, however, PRV-P is not of common occurrence in Cucurbitaceae, in which most of the strains cause mild to moderate symptoms. Summer squashes and other cultivated cucurbits infected with PRV-P wee found to be inadequate for the production of large amounts of antiserum to this virus. However, a susceptible line (Acc. 2549) of Cucumis metuliferus (4) is presently used extensively for this purpose. Conversely, other accessions of this cucurbit from South Africa were reported to be resistant to both PRV-P and PRV-W.
Recently an isolate of PRV-P from Taiwan (PRV-PIT) was found to cause on cucurbits (particularly on zucchini) more prominent foliage symptoms than those incited by PRV-W. Extensive greenhouse studies have also demonstrated that PRV-P/T is controlled by the same genes conferring resistance to PRV-W in cucurbits (1,2,5,6,8,9). PRY-P/T has been maintained in the Hawaiian papaya 'Solo', which responds with the typical symptoms incited by other strains of PRV-P. However, young plants of this cultivar initially develop some foliage necrosis and prominent stunting. With special care, infected plants slowly recover from this acute stage, but during the chronic stage foliage symptoms are still prominent. Other cultivars should be tested to find one that is less sensitive than 'Solo'. PRSV/P/T can be kept in papaya plants for many months.
One of the major inconveniences in working simultaneously with several cucurbit viruses is possible accidental mixtures. By losing the purity of a given virus, breeding for resistance can encounter severe problems. Viral contamination can be eliminated if each virus is kept in a host that is highly resistant to, or is not infected by other cucurbit viruses. In breeding cucurbits for resistance to PRV, this PRV-P/T from Taiwan is very valuable and a good substitute for PRSV-W. It can be kept free of contaminants if maintained routinely in papaya. Extracts from infected papaya leaves usually have a low virus titer and are not suitable for screening a large number of cucurbit plants for resistance. Thus, the best results can be obtained by transferring PRSV-P/T from papaya to zucchini (or any other summer squash) and using them as sources of virus inoculum. Infected zucchini can also provide large amounts of the virus to produce an effective antiserum. However, at 28-30 C the incubation period for zucchini inoculated with PVR-W is 5-6 days, whereas for PRV-P/T it is longer,, 8-10 days.
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