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.

SYMPTOMS
Foliar

Early blight on potato leaves

Early blight on potato tubers

Early blight Fruit Rot

Early blight on Tomato leaves

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.

Fruit
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.

DISEASE CYCLE
The 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.

MANAGEMENT
As with many diseases, no single management strategy will cure the disease. Rather, a combination of methods should be used to decrease its effects.

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