The source of infection is caused by the fungus Phytophthora infestans.
In moist weather, spores produced on the white and downy affected areas, are spread by wind. Cool wet weather is essential for infection and disease development.
Late Blight can attack all parts of the tomato plant. Leaf lesions first appear as irregularly shaped, water-soaked spots which may enlarge rapidly into pale green to brown lesions that cover large areas of the leaf.
During humid conditions the undersides of these lesions may develop a white downy growth. As the lesions develop, this downy growth may be seen only the perimeter of the lesion.
Badly infected foliage will eventually turn brown, shrivel and die. Petioles and stems become infected in a similar way so that eventually the whole plant may die.
Fruit lesions appear as dark, olive colored greasy spots which may enlarge until the entire fruit is infected. In moist weather a thin downy film may develop on the infected lesion.
Soft Rot follows Late Blight infection which leads to decay and disintegration of fruit.
Late Blight Management
In addition to fungicide applications, preventive measures can reduce the infection by Late Blight:
- Planting certified seed
- Planting cultivars with resistance to Late Blight
- Choosing well drained sites and those that allow adequate air movement
- Avoiding planting downwind of another potato or tomato crop
- Destroying volunteer tomatoes
- Reducing periods of leaf wetness with appropriate irrigation strategies ensuring that foliage dries-off before nightfall.