Late blight is caused by the fungus Phytophthora infestans, which is a pathogen of potato and tomato. This disease can spread quickly and devastate a tomato or potato field within a few weeks if not properly controlled.
The disease thrives under cool and wet conditions. Temperatures between 50º and 80º F combined with moist conditions such as rain, fog, heavy dews, or relative humidity above 90% are conducive for disease development. Night temperatures in the mid-fifties with daytime temperatures from the mid-fifties to mid-seventies are ideal for this disease.
Since the disease can spread so rapidly, growers should scout their fields thoroughly each day, especially when cool and wet conditions conducive to disease development prevails.
Late blight symptoms on leaves appear as irregularly shaped brown to purplish lesions with indefinite border lesions that can span veins. The lesions may be seen any time of day, on any stage of plant growth and on leaves of any age. Velvety, white fungal growth may appear on the lower surface of affected leaflets early in the morning before leaves dry and/or in the lower canopy.
On stems, purplish lesions may be found any where on the stem. Cottony, white growth of fungus on stems with lesions can often be seen early in the morning and/or in the lower canopy. Stems with lesions are brittle and break easily. Lesions are confined to epidermis and cortex. Leaf rolling and wilting is often associated with stem lesions and purpling of leaflets may occur in some varieties.
Here are pictures of late blight on tomatoes: https://www.google.com/search?q=late+blight+on+tomatoes&tbm=isch&tbo=u&source=univ&sa=X&ved=0ahUKEwiVuJnh_qbLAhXhtoMKHfgcBNEQsAQIHw&biw=1200&bih=579
Begin a spray program with fungicides if late blight is in your area or weather conditions are suitable for late blight development. After harvest, kill infected foliage to minimize tuber infection.
Fungicides slow the rate at which the disease develops in by creating a protective barrier on the foliage.
Just applying a chemical, however, does not necessarily equate with effective disease control. Relative effectiveness of a product, coverage, and timing must be factored into the equation for maximum benefit.
As home gardeners, our only controls are copper and Garden Friendly Fungicide.
