Ring Spot Virus
· Gurke · Zucchini · Papaya · Melone
Symptoms may vary slightly depending on plant age at time of infection, the plant vigor and the strength of the virus. Dark-green blister-like structures first appear on leaves. Later, these develop into a mottled pattern in different shades of green. At later stages of the disease, leaves have a shoestring appearance and show a mosaic pattern with yellow and brown necrotic spots. Leaf size is considerably reduced, resulting in stunted growth and smaller canopy. Water-soaked chlorotic spots and oily streaks also appear on stems and petioles. Infected fruits show innumerable, dark green, often sunken, oily ring spots, are of reduced size and deformed shape. If infection takes place at an early age, fruits are not marketable.
The virus is transmitted by numerous species of aphids in a non-persistent manner. Since it does not multiply in the aphids, the transmission from plant to plant must happen within short periods of time (no longer than one minute). The virus has a variety of alternative hosts such as watermelon and other cucurbits, but its preferred target is papaya. Infection can spread rapidly in a plantation if it coincides with large populations of winged aphids. Cool weather can also worsen the symptoms on the leaves (mosaic pattern and distortion).
Spray white oil emulsions at concentrations of 1% to hinder the uptake and transmission of the virus by aphids. Mixtures of beneficial microorganisms including some types of bacteria, yeasts, actinomycetes and photosynthetic bacteria can reduce the incidence of the disease.
Always consider an integrated approach with preventive measures together with biological treatments if available. There is no direct chemical treatment for viral infections. However, a foliar spray of di-methoate or azadirachtin can reduce the population of aphids. Apply the spray every fortnight after the first symptoms appearance.