Late blight is a disease that most commonly affects potatoes, but can affect tomatoes in some years. When the weather is favorable as it has been this year, late blight can be especially troublesome. This disease is being reported in Michigan this year from homeowners and organic growers. Growers practicing IPM are not reporting this disease since the fungicides they are likely using for Alternaria, Septoria, and Anthracnose are also providing adequate protection against late blight. LATE BLIGHT SYMPTOMS include blighting on all aboveground parts of the tomato plant. Lesions on leaves often appear dark and oily with production of spores occurring on the undersides of the leaves resulting in a purplish appearance especially when conditions are wet and humid. Blackened lesions on the stems also occur and are unique to late blight disease. Late blight affects green and ripened tomato fruit. The blighting on fruit appears as dark, greasy areas that enlarge rapidly encompassing the entire fruit. During wet and humid conditions, white threads (mycelium) can be seen on the fruit.
Between cropping seasons, the fungus survives on volunteer and abandoned potato and tomato plants in fields, cull piles, and homeowner gardens. Cool nights, moderately warm days, and weather that keep foliage/fruit wet for several hours are ideal for late blight development. The spores can be carried from diseased plants to nearby healthy plants via wind.
Control measures include eliminating all potato/tomato cull piles in the vicinity of tomato plantings and destroying volunteer potato plants that grow from overwintered tubers. All tomato varieties are susceptible to late blight. There are several highly effective fungicide sprays that can be used to protect tomato plants from late blight.
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