Causal Agent:
Phytophthora infestans 
At least two races (0 and 1) are known.

Distribution:
Worldwide.

Symptoms:
The first symptom of the disease is a bending down of the petiole of infected leaves. Leaf and stem lesions are large, irregular, greenish, water-soaked patches. These patches enlarge and turn brown and paper-like. During wet weather the underside of the leaf may have a white sporulating fungal growth. A rapid blighting of the entire foliage may occur during moist, warm periods. Entire fields can have extensive foliar and fruit damage. Fruit lesions are firm, large, irregular, brownish-green blotches. The surface of the fruit lesion has a greasy, rough appearance.

Conditions for Disease Development:
This fungus can survive on volunteer and home garden potatoes and tomatoes, in potato cull piles and on solanaceous weeds. Spores of the fungus can be carried long distances by storms. Cool, wet weather favors the development of this disease. Under these conditions the disease progresses rapidly and can completely destroy a mature tomato field in a few days.

Control:
A fungicide spray program combined with a blight forecasting system is the most effective means of controlling late blight. Avoid planting on land previously cropped to potatoes or close to a potato field because this disease is frequently associated with potatoes.

Burnt foliar appearance in the field.                   Fluffy white sporulation

                                                                                  on the leaf.

Large necrotic stem lesions.     Necrotic expanding leaf lesion.

Rough brown fruit lesions.