At first, irregular greenish black water-soaked areas appear on the oldest leaves. The spots enlarge rapidly, and in humid weather a bluish gray growth of fungus sometimes develops on the lower leaf surfaces. Brownish cankers are often found on the stems and leafpetioles. In moist weather the infection develops so rapidly that large patches in affected fields have a frost-damaged appearance.
where heavy dews and showery weather prevail during August and September, late blight occurs regularly on tomatoes and potatoes.under favorable conditions, the disease then breaks out in epidemic proportions on fruiting plants. Late blight does not over-winter on dead plant debris in the soil, but it does persist in infected potato tubers in cull piles. It may appear in potato plants from diseased tubers used as seed or on volunteer plants from tubers in potato refuse piles, thereby gaining entry to potato and tomato fields.
