Early blight of tomatoes and potatoes
Early blight on potato leaves
Photo: U of MN Plant Disease Clinic
Early blight (target spot) of tomato and potato is caused by the fungus Alternaria solani. This common disease may also occur on other solanaceous plants, such as pepper and eggplant, as well as certain Brassica spp. Although this disease usually affects older, senescing plants, it can cause complete defoliation when environmental conditions are optimal for disease development.
Early blight on potato tubers
Photo: Chad Behrendt
Symptoms of early blight usually appear near the end of the season, though symptoms may appear earlier. Brown lesions first appear on older, lower leaves, and spread up toward new growth. Lesions are small (1-2 mm), dry, and papery and may develop characteristic dark concentric rings of raised and necrotic tissue. Leaf tissue often turns chlorotic (yellow) at the edge of the lesion. As the disease progresses, the entire leaf can become chlorotic and then necrotic (brown). Severely infected tomato plants may become completely defoliated exposing fruit to sunscald. Infected potato leaves, however, usually do not fall off.
Lesions may also develop on the stems. These lesions begin as small, dark, slightly sunken areas that enlarge and may develop the target-appearance as on the leaves.
Early blight on fruit rot
Symptoms may also appear on the tomato fruit or potato tubers. Symptoms on tomato fruit are usually seen during wet periods at harvest but can also develop on green fruit. Lesions develop near the stem attachment and can occasionally spread over the entire fruit. Lesions appear leathery and may also have concentric rings like foliar lesions. In addition, lesions may become covered with a mass of black fungal spores.
Infected potato tubers develop dark, sunken lesions that are often surrounded by a purplish raised border. Under these lesions, the tissue is dry, leathery and brown. Lesions will increase in size in storage, though they will remain superficial.
Early blight on tomato leaves
Disease cycleThe fungus overwinters in infected residue in the soil and on weedy solanaceous hosts. Spores infect leaves directly through the epidermis when the leaves contact the soil, or when spores are blown or splashed to susceptible leaves. Leaves that are the most susceptible are those that are older and under stress from other diseases or nitrogen deficiency. Under cool, moist conditions, the fungus can produce numerous spores that are wind-blown to adjacent leaves and plants, leading to new infections. Potato tubers usually become infected with the fungus when immature or wounded tubers are lifted from the ground through soil infested with fungal spores.
ManagementAs with many diseases, no single management strategy will cure the disease. Rather, a combination of methods should be used to decrease its effects.
- Plant disease-free seeds and plants; avoid bruising seed tubers before planting.
- Maintain plant health through proper water and fertilization regimes; nitrogen and phosphorus deficiencies increase susceptibility to early blight.
- Garden areas should be rotated out of potatoes, tomatoes, pepper and eggplant for 3-4 years so that hosts of the disease are not present.
- Minimize leaf wetness by watering plants in the morning or at the base of the plant.
- Harvest mature tubers when soil is not wet, and minimize injury to the tubers.
- Remove infested plant material after harvest, or incorporate material into the soil.
- Several foliar fungicides are registered for early blight. Mancozeb, chlorothalonil and copper are effective against early blight when applied at approximately 7-10 days intervals. Spraying should commence at the first sign of disease or immediately after bloom.
- Plant varieties that are resistant or have a lower susceptibility to early blight.