Potyviruses

Each virus produces similar symptoms and mixed infections are common. The first symptom is a clearing of veins, followed by development of mosaic patterns or mottling consisting of irregularly shaped, dark green areas alternating with light green or yellow areas. Leaves on some species and cultivars are drastically reduced in size and growth is often retarded. Watermelon mosaic virus tends to cause raised, blisterlike areas on leaves and to reduce leaf size severely. Zucchini yellow mosaic virus typically causes the leave lobes to become long and narrow. Malformations of the fruit can occur with all three of the viruses.

All three viruses are found in wild and volunteer cucurbits. Watermelon mosaic virus also occurs in weeds such as goosefoot, lambsquarters, Russian thistle, various legumes, cheeseweed and other related plants. All three viruses are transmitted by many species of aphids. After feeding on an infected plant, aphids only retain the ability to transmit these viruses for very short periods of time (minutes to a few hours). In general, spread of potyviruses in the field occurs when aphid activity is high and is often very rapid and localized.

Start looking for potyviruses during the vegetative growth stage. Note infections to make management decisions before the next crop because during the season, mosaic viruses are difficult to control. Spraying with insecticides is rarely effective for control because the insect transmits the virus before being killed by the insecticide. Because their occurrence is erratic and unpredictable, control of these viruses is not attempted. In studies, silver reflective plastic mulches applied at planting have been shown to be effective in repelling aphids from plants, thereby reducing or delaying virus infection. These mulches help plants off to a healthy start and are effective until expanding foliage covers the reflective surface. Reflective mulches may need to be removed in the desert areas when summer temperatures are excessive for optimal growth of plants. However, in the Central Valley and cooler areas, these mulches have not caused plant damage in the summer; in fact, they improve soil moisture and nutrient retention, which may further aid plant productivity.