EARLY BLIGHT OF POTATO EPIDEMIOLOGY

Epidemiology

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.

Symptoms of early blight
occur on fruit, stem and foliage of tomatoes and stem, foliage and tubers of potatoes. Initial symptoms on leaves appear as small 1-2 mm black or brown lesions. Under conducive environmental conditions, the lesions enlarge and are often surrounded by a yellow halo (Figures 2 and 3). Lesions greater than 10 mm in diameter often have dark pigmented concentric rings. This so-called "bullseye" type lesion is highly characteristic of early blight (Figure 4). As lesions expand and new lesions develop, entire leaves may turn chlorotic and dehisce, leading to significant defoliation. Lesions on stems are often sunken and lens-shaped with a light center and have the typical concentric rings (Figure 5). On young tomato seedlings, lesions may completely girdle the stem, a phase of the disease known as "collar rot," which may lead to reduced plant vigor or death.

Infection of both green and ripe tomato fruit normally occurs through the calyx with lesions sometimes reaching a considerable size (Figure 6). The lesions appear leathery and may have the characteristic concentric rings. Infected fruit will frequently drop prematurely. Symptoms on potato tubers are characterized by sunken, irregular lesions (Figure 7), which are often surrounded by a raised purple border. Beneath the lesion surface, the tuber tissue is leathery or corky with a brown discoloration. Early blight lesions on tubers tend to be dry and are less prone to invasion by secondary organisms compared to lesions of other tuber rots. After prolonged storage, severely diseased tubers may become shriveled.