Early Blight

Early Blight, Alternia solani, of the common tomato, Lycopersicon esculentum, is a fungal disease which affects Solanaceous crops world wide. The first reported incidence of early blight was reported on potato in 1882. The fungus overwinters on either infected crop debris or Solanaceous weeds causing infections to occur under favorable conditions in the field. Early blight development is frequent in areas of high humidities, heavy dew, frequent rainfall or overhead irrigation and warm temperatures.

Early blight may effect foliage, stems, and, in more severe cases, fruit. The pathogen has two distinct types of disease, the collar rot and leaf blight phases. The collar rot phase is considered a seed-bed disease. Infected plants are carried out to the field as transplants and dark sunken lesions begin to appear on the stem near the soil line. These lesions eventually girdle the stem forming "collars" which inhibit vascular processes. The leaf blight phase, refered to as early blight, is charcterized by dark brown lesions which are necrotic in the centers. As the lesions expand they produce a "target-like" concentric ring appearance. Lesions eventually coalesce resulting in yellowing of the leaves and giving a blighted appearance.

The pathogen progresses upward throughout the plant as it developes. It is likely that inoculum splashes up from the ground to infect the lower foliage first and then moves up into the plant. This may be due to rain or irrigation practices. Early blight is associated with the physiological maturity of the plant. Older, sensecing leaves are more susceptible than younger immature leaves. Heavy fruit set can enhance the prevelance of the disease as well. As plant sugars decrease with age, susceptibilty increases.

Present control methods incorporate crop rotation, field debris sanitation, utilization of disease free seeds and transplants, resistance cultivars and a preventitive spray schedule. Soil fumigation and sanitation can reduce primary infections by the pathogen but secondary infections are possible from sources such as weeds. In western North Carolina a five day spray schedule is recommended beginning soon after transplanting with a tighter schedule during weather conditions which favor disease development. Without protective fungicide applications unsprayed plots may exhibit reductions in yield of 30-50% compared to treated plots.

Due to the advent of chemical fungicides, breeding for resistant varieties had not been a priority in most breeding programs. However reducing spray applications would be both a economic and ecological benefit for growers. Current control stratigies look at incorporating resistant lines to reduce fungicide applications for both the ecological and economic advantages. Resistance was first reported in 1945 and work by the USDA in the late 60's and 70's produced several sources of resistance. To the best of my knowledge these lines were never utilized in developing varieties for commercial/public release.

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C. Diseases characterized by leaf spots and spotting or decay of fruits, but no wilting of plant.

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