By Mary Hausbeck, Michigan State University Extension, Department of Plant Pathology
Late blight is a disease that most commonly affects potatoes, but can affect tomatoes when the weather is cool, rainy and humid. The pathogen is called Phytophthora infestans and is well known to potato growers. Late blight is troublesome this season for tomato and potato growers in the northeast United States that have had an especially wet spring and summer. Michigan's dry weather does not favor this disease.
Late blight symptoms include blighting on all above ground parts of the tomato plant. Lesions on leaves often appear dark and oily with production of sporangia, or seeds of the pathogen, occurring on the undersides of the leaves resulting in a whitish-purplish appearance, especially when conditions are wet and humid. These sporangia can be carried long distances from diseased plants to nearby healthy plants via wind currents and storm fronts.
Blackened lesions on the stems also occur and are typical of late blight disease. Late blight affects green and ripe tomato fruit. The blighting on the fruit appears as dark, greasy areas that enlarge rapidly, encompassing the entire fruit. During wet and humid conditions, white masses (sporangia and threads) of the late blight pathogen can be seen on the diseased leaves and fruit.
Late blight lesions on tomato leaves.
Between cropping seasons, the fungus survives on volunteer and abandoned potatoes in cull piles. Control measures include eliminating all potato and tomato cull piles and destroying volunteer potato plants that grow from overwintered tubers. Infected potato plants established from diseased seed potatoes are another source of late blight. Most tomato varieties are susceptible to late blight.
'Mountain Magic' is a late blight-resistant variety recently developed at North Carolina State University.