Early blight is a fungal disease that causes yield loss each year on all varieties of tomatoes. Eggplant and pepper also are susceptible.
Foliar symptoms generally occur on the oldest leaves and start as small, pencil-point-size brownish to black lesions. These leaf spots enlarge up to ½-inch and usually have readily visible, concentric rings that look somewhat like a bull’s-eye. These concentric leaf spots are distinctive enough to make early blight one of the easier tomato diseases to diagnose.
The area around the spot may become yellow as could entire severely affected leaves. Early blight symptoms are most pronounced in the lower canopy. Under favorable conditions, significant defoliation of lower leaves may occur, leading to sun scald of fruit.
Green or red fruit may be infected by the fungus, which invades at the point of attachment between the stem and fruit, and through growth cracks and wounds made by insects. Dark lesions enlarge in a concentric fashion and may affect large areas of the fruit. Mature lesions in fruit are typically covered by a black velvety mass of fungal spores.
Stem lesions are dark, slightly sunken, and enlarge concentrically. Basal girdling and death of seedlings may occur. This expression of the disease is sometimes called collar rot.
Survival And Spread
Alternaria solani survives between crops in plant debris and on seed. It also can survive on volunteer tomato plants (warm climates) and on other cultivated and wild solanaceous plants (potato, eggplant, horse nettle, and black nightshade)
Spores are formed on debris, or infected tomatoes or other hosts, when temperatures of 60°F to 90°F occur — provided wet weather is present. Spores dislodged by wind or rain land on susceptible host tissue and germinate when the tissue is wet and penetrate leaf, stem, petiole, or fruit tissue. Infection by the fungus is most rapid under warm (82°F to 86°F), wet conditions.
Disease severity and prevalence are highest when plants are loaded with fruit. Despite it name, early blight is normally more prevalent later in the season in South Florida.
Where possible, growers should strive to use long rotations away from tomato and other solanaceous crops and avoid planting tomato near tomatoes or potatoes that are more mature. Look for and destroy volunteer tomato and potato plants and eliminate solanaceous weed hosts in and around the field and burn down or plow down adjacent fields planted to potatoes or tomatoes immediately after harvest. Control insects to avoid damage, which can allow entry of the pathogen and spread of the fungus.
Begin a fungicide spray program at first sign of disease or before. Contact fungicides, such as chlorothalonil and mancozeb, provide moderate levels of control when applied preventively.
Maintain spray applications on a five- to 14-day interval throughout the growing season. Use the shorter intervals if rainfall is frequent or where history of early blight has been severe or when temperatures from 75°F to 85°F prevail.
By Gene McAvoy, Regional Vegetable Specialist IV, University of Florida/IFAS