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Symptoms and Signs

Potato stems and leaves
Late blight of potato is identified by black/brown lesions (Figures 2,3) on leaves and stems that may be small at first and appear water-soaked or have chlorotic borders, but soon expand rapidly and become necrotic. In humid conditions, Phytophthora infestans produces sporangia and sporangiophores on the surface of infected tissue. This sporulation results in a visible white growth at the leading edge of lesions on abaxial (lower) surfaces of leaves (Figures 4,5,6). As many lesions accumulate, the entire plant can be destroyed in only a few days after the first lesions are observed (Figure 7).

Figure 2	Figure 3
Figure 4	Figure 5
Figure 6	Figure 7

Potato tubers
Potato tubers become infected in the field when sporangia are washed from the foliage into the soil. Infections generally begin in tuber cracks, eyes or lenticels. Infected tuber tissues (Figures 8,9) are copper brown, reddish or purplish in color. Sporulation (Figure 10) may occur on the surface of infected tubers in storage or on cull piles. Infected tubers are often invaded by soft rot bacteria which rapidly convert adjoining healthy potatoes into a smelly, rotten mass that must be discarded (Figure 11).

Figure 8	Figure 9
Figure 10	Figure 11

Tomato stems and leaves
Tomato plants are also susceptible to late blight, and the foliar symptoms are similar. Like potato, infected tomato plants (Figure 12) may be rapidly infected and destroyed by P. infestans. White sporulation (sporangia and sporangiophores) (Figure 13) may be visible in humid weather.

Figure 12	Figure 13
Figure 14

Tomato fruit
Late blight infections produce dark brown, firm lesions (Figure 14) which may enlarge and destroy the entire tomato fruit. Late blight lesions on tomato fruit are often followed by soft rot and disintegration as described for potato tubers.

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