Tomato mosaic

Damage. This disease may cause serious losses in tomatoes. Infected seedlings have a stunted, spiky appearance. On more mature plants leaves have a pale green mottled appearance, or sometimes a bright yellow (aucuba) symptom. The stem may show brown streaks in summer when growing conditions are poor, a condition often resulting in death of the plant. Fruit yield and quality may be lowered, the green fruit appearing bronze, and the ripe fruit hard, making the crop unsaleable (see Figure 15.19).

Life cycle and spread. The virus is a rod shaped virus. The period from plant infection to symptom expression is about 15 days. The virus may survive within the seed coat (testa) or endosperm of the tomato seed. It is very easily spread by human contact as it is present in large numbers in the leaf hairs of infected plants.

Control. Heat treatment of dry seed at 70°C for 4 days by seed merchants helps remove initial infection. Infected debris, particularly roots, in the soil enables the virus to survive from crop to crop, and soil temperatures of 90°C for 10 min are normally required to kill the organism. Peat-growing bag and nutrient-film methods enable the grower to avoid this source of infection. Hands and tools should be washed in soapy water after working with infected plants. Clothing may harbour the virus.

Cultivars and rootstocks containing several factors for resistance are commonly grown, but newly arriving virus strains may overcome this resistance. A mild strain spray inoculation method has been used at the seedling stage to protect non-resistant cultivars from infection with severe strains. Great care is required to avoid mosaic-contaminated equipment when using this method.

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