Papaya ringspot (virus-W)
Papaya ringspot virus-W. Previously, it was known as Watermelon mosaic virus-1. However, tests using antisera showed that it was a strain of Papaya ringspot virus. The other strain is Papaya ringspot virus-P, which infects papaya and cucurbits (see Fact Sheet no. 194). Papaya ringspot virus-W does not infect papaya.
Note, a third virus, Watermelon mosaic virus, also infects watermelon and other cucurbits; this virus causes light and dark mottling on the leaves and a slight roughening of the skin of the fruit. It was previously known as Watermelon mosaic virus-2.
Worldwide. Asia, North, South and Central America, the Caribbean, Europe, Oceania. It is recorded from Australia, and most Pacific island countries: Cook Islands, Federated States of Micronesia, Fiji, French Polynesia, Guam, Kiribati, Nauru, New Caledonia, Northern Mariana Islands, Palau, Papua New Guinea, Republic of the Marshall Islands, Samoa, Solomon Islands, Tokelau, Tonga, and Vanuatu.
Chayote (Sechium edule), cucumber, melon, pumpkin, squash, and watermelon. Weeds in the cucurbit family are also hosts, for example, wild bitter melon (Momordica charantia), and ivy gourd (Coccinia grandis).
The virus causes distortions, blisters and light and dark mosaic patterns on leaves. Characteristic ring spots develop on the fruit (Photos 1&2), merging into large dark green areas, more conspicuous on light coloured varieties (Photo 3).
Spread of the disease occurs in two ways. First, aphids spread the virus from infected crops and wild cucurbits to healthy watermelons' and secondly, infected nursery plants spread the virus over short and long distances.
There are many species of aphids that spread Papaya ringspot virus-W naturally, although some species are more efficient than others. In Pacific islands, the aphid Aphis gossypii is common and transmits the virus in a non-persistent way; this means that it acquires the virus on its mouth parts after a short feed on an infected plant (30 seconds or less), infects a healthy plant after another short feed, and then loses the ability to infect. Spread is very rapid by aphids.
This is a serious disease of cucurbits, and in Florida it is said to be a limiting factor on production. The virus can severely reduce fruit set in watermelon. It also affects the quality of the fruit produced as growers are unable to market fruit with ringspots.
Look for the light and dark green mosaic patterns and distortions on the leaves, and the characteristic ringspots on the fruits. However, these are not conclusive, and identification needs confirmation using serological or molecular methods. Other viruses, for example, Cucumber mosaic virus, Watermelon mosaic virus and the papaya strain of Papaya ringspot virus-P can produce similar symptoms.
Although some zucchini varieties are tolerant to Papaya ringspot virus, Watermelon mosaic virus, and Zucchini yellow mosaic virus, commercial seed catalogues are yet to announce watermelon with resistance to Papaya ringspot mosaic virus-W.
Chemical control is not effective against this disease because aphids are able to transmit the virus so quickly. Usually, by the time the insecticide has killed the aphid, the aphid has fed, and the virus has been transmitted. An exception to this is horticultural oils, either vegetable or petroleum based. Sprays of soaps, white oil (vegetable) or commercial horticultural oils (petroleum) may give some protection if applied frequently (see Fact Sheet no. 56). Use the following recipe for home-made preparations:
Use commercial horticultural oils as indicated on the label. White oil, soap and horticultural oil sprays work by blocking the breathing holes of insects causing suffocation and death. They also stop some insects from feeding, without killing them.
AUTHOR Grahame Jackson & Mani Mua
Information from Davis RI, Ruabete TK (2010) Records of plant pathogenic viruses and virus-like agents from 22 Pacific island countries and territories: a review and an update. Australasian Plant Pathology 39, 265-291; and Papaya ringspot virus. Wikipedia. (https://en.wikipedia.org/wiki/Papaya_ringspot_virus); and from Diseases of vegetable crops in Australia (2010). Editors, Denis Persley, Tony Cooke, Susan House. CSIRO Publishing.
Produced with support from the Australian Centre for International Agricultural Research under project HORT/2016/18: Responding to emerging pest and disease threats to horticulture in the Pacific islands, implemented by the University of Queensland and the Pacific Community.
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