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Disease Cycle and Epidemiology

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Alternaria solani overwinters primarily on infected crop debris. The dark pigmentation of the mycelium increases resistance to lysis which extends the survival time in the soil to several years. Thick-walled chlamydospores have been reported, but they are found infrequently. In mild climates the pathogen can survive from season to season on volunteer tomato and potato plants as well as other weedy Solanaceous hosts such as horsenettle and nightshade.

Warm, humid (24-29°C/ 75-84°F) environmental conditions are conducive to infection. In the presence of free moisture and at an optimum of 28-30°C (82-86°F), conidia will germinate in approximately 40 min. Desiccated germ tubes are able to renew growth when re-wetted, and, hence, infection can occur under conditions of alternating wet and dry periods. Germ tubes penetrate the leaf epidermis directly or enter through stomata. Infection of potato tubers usually occurs through wounds in the tuber skin. Wounds caused at harvest, that coincide with wet conditions favorable for spore germination, can lead to significant tuber infection. High soil moisture levels can cause swollen lenticels on tubers which are also easily invaded.

Time from initial infection to appearance of foliar symptoms is dependent on environmental conditions, leaf age, and cultivar susceptibility. Early blight is principally a disease of aging plant tissue. Lesions generally appear quickly under warm, moist conditions on older foliage and are usually visible within 5-7 days after infection.

A long wet period is required for sporulation, but it can also occur under conditions of alternating wet and dry periods. Conidiophores are produced during wet nights and the following day. Light and dryness induce them to produce spores, which emerge on the second wet night. Early blight is considered polycyclic with repeating cycles of new infection. Secondary spread of conidia is mainly by wind and occasionally by splashing water.

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