In many cases employing sound cultural practices that maintain potato and tomato plants in good health will keep early blight losses below economic levels. This is the period when the disease has the potential to spread rapidly and build up to damaging levels in the crop. Early blight is considered polycyclic with repeating cycles of new infection. Secondary spread of the disease results from conidia being dispersed mainly be wind and occasionally by splashing rain or overhead irrigation. 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. Lesions generally appear quickly under warm, moist conditions on older foliage and are usually visible within 5-7 days after infection.Ī long wet period is required for sporulation but it can also occur under conditions of alternating wet and dry periods. Early blight is principally a disease of aging plant tissue. Time from initial infection to appearance of foliar symptoms is dependent on environmental conditions, leaf age, and cultivar susceptibility. Wet conditions at harvest provide a favorable environment for spore germination as well as causing swollen lenticels on the tubers which are easily invaded. Germ tubes penetrate the leaf epidermis directly or enter through stomata. Infection of potato tubers usually occurs through wounds in the tuber skin inflicted during harvest. Desiccated germ tubes are able to renew growth when re-wetted, and, hence, infection can occur under conditions of alternating wet and dry periods. In the presence of free moisture and at an optimum of 28-30☌ (82-86☏), conidia will germinate in approximately 40 min. Warm, humid (24-29☌/ 75-84☏) environmental conditions are conducive to infection. 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. Thick-walled chlamydospores have been reported, but they are found infrequently. The dark pigmentation of the mycelium increases resistance to lysis which extends the survival time in the soil to several years. solani has given rise to claims of the existence of races, although this remains unproven.Īlternaria solani overwinters primarily on infected crop debris. Morphological and pathogenic variability among isolates of A. The beaked conidia normally possess 9–11 transverse septae ( Figure 8). The asexual conidia are borne singly or in a chain of two on distinct conidiophores. Sporulation in culture can be stimulated by exposure to fluorescent light. The mycelium is haploid and septate, becoming darkly pigmented with age. The fungus is readily cultured on artificial media such as V8 juice where it produces a deeply pigmented gray/black hairy colony. The genus Alternaria is a large and important group of pathogenic fungi, which cause a significant number of important diseases. There is no known sexual stage and hence it is classified as a Deuteromycete. The causal pathogen of early blight is the fungus Alternaria solani. After prolonged storage severely diseased tubers may become shriveled. Early blight lesions on tubers tend to be dry and are less prone to invasion by secondary organisms than lesions of other tuber rots. Symptoms on potato tubers are characterized by sunken, irregular lesions ( Figure 7), which are often surrounded by a raised purple border. Beneath the surface of the lesion the tuber tissue is leathery or corky with a brown discoloration. ![]() ![]() Infected fruit will frequently drop prematurely. ![]() The lesions appear leathery and may have the characteristic concentric rings. Infection of both green and ripe tomato fruit normally occurs through the calyx with lesions sometimes reaching a considerable size ( Figure 6). Lesions occurring 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. As lesions expand and new lesions develop entire leaves may turn chlorotic and dehisce, leading to significant defoliation. This so-called “bullseye” type lesion is highly characteristic of early blight ( Figure 4). Lesions greater than 10 mm in diameter often have dark pigmented concentric rings. Initial symptoms on leaves appear as small 1-2 mm black or brown lesions and under conducive environmental conditions the lesions will enlarge and are often surrounded by a yellow halo ( Figures 2 and 3). ![]() Symptoms of early blight occur on fruit, stem and foliage of tomatoes and stem, foliage and tubers of potatoes. HOSTS: Tomato ( Lycopersicon esculentum), Potato ( Solanum tuberosum)ĭow AgroSciences, 9330 Zionsville Road, Indianapolis, IN, USA
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