The pathogen completes reproduction cycles rapidly, with spore production occurring frequently. Through cuts and scrapes in the surfaces of tomato fruit.In potatoes, wounds in tuber surfaces allow for the pathogen to migrate directly from the soil to the plant.Migrating to low-hanging leaves when they contact the soil, or when infected soil is splashed onto leaf surfaces.solani has multiple means of infecting plants: The disease is capable of suspending growth and reproduction during dry, cool periods, and resuming when conditions improve.Ī. However, consistent warmth and moisture isn’t necessary. Hence the disease is more frequently seen in humid climates, or in fields where poor drainage allows for standing water and waterlogged soils. solani also need a great deal of moisture. These same temperatures are also enough to reactive active infections which have gone into dormancy. When conditions warm in the spring, to where temperatures reach highs in the mid-80s Fahrenheit for at least half an hour or so, dormant spores germinate. In more mild climes, the pathogen can survive in crops and weeds. solani spores overwinter in infected plant debris. The success of this pathogen is thanks in large part to its ability to happily live in and feast upon the thousands of wild and cultivated plants in the family Solanaceae, the nightshades, which in addition to tomatoes and potatoes include peppers, eggplants, paprika, tobacco, eggplants, horse nettles, and a variety of ornamentals and weeds. tomatophilia-is a soil-dwelling fungus found just about anywhere. tomatophilia, which prosper in warm, damp conditions.Īlternaria solani-and its tomato-favoring cousin A. Early blight is caused by Alternaria solani and A. In addition, infection spreads into seeds and eyes, meaning that the next generation of plants start life already infected. This damage makes it even more difficult to sell visibly affected crops. The tissues beneath surface lesions darken and harden. The damage to tomatoes and potatoes is not merely aesthetic. With potatoes, tubers which may look only lightly infected when harvested may become significantly more disfigured while in storage. In tomatoes, the disfigured fruit may survive, or drop prematurely. Infected tissue becomes leathery and produces larger versions of the ridged bullseye spots found on leaves. Potato tubers and tomato fruit may be infected at any stage of development, producing spots which are dark and sunken. (Image source: Jerzy Opioła, “ Alternaria solani,” licensed via CC BY-SA 4.0) As the disease advances, spots will proliferate on affected leaves, as with this tomato plant. On older, more robust plants, these points of infection are usually oval or irregular in shape, eventually forming dark concentric rings like those found on the leaves. On seedlings, these infections can circle the stem (“collar rot”), causing the seedling to wither and die. Eventually, the disease spreads to the stems, where it causes brown, sunken spots. In potatoes, early blight also causes extensive premature defoliation.Īs the disease progresses, spots appear on younger leaves increasingly higher up the plant. Dead leaves may remain attached to the plant, or drop to the ground where they can serve as a vector for the disease. Eventually, infected leaves turn yellow, develop fissures, and die. (Image source: Clemson University – USDA Cooperative Extension Slide Series,, licensed via CC BY 3.0 US)Īs spots grow, more spots form nearby. This ridging produces the well-known ‘bullseye’ spots associated with early blight. Note the ridging of the spot near the center of the image. A tomato leaf showing the initial symptoms of early blight. These spots may be surrounded by yellow leaf tissue. Over time, increasingly large rings of tissue around the lesions turn brown or black, forming a ridged spot often described as resembling a bullseye, but which can also be characterized as looking like tree rings. Spots don’t form all at once, instead starting as small pinpoint lesions near leaf centers. The first symptoms of early blight usually appear on the oldest, lowest leaves. Early blight is easily identified by its bullseye-patterned spots, as well as the general pervasiveness of symptoms throughout infected plants. The disease is usually caused by the fungus Alternaria solani, though the closely related Alternaria tomatophila is a common cause of early blight in tomatoes.Įarly blight can be a devastating disease, as it affects every part of the plant, including the leaves, stem, and fruit, causing spots and wilting, and often resulting in leaves and fruit dropping off prematurely. Early blight is a disease which infects tomatoes and potatoes, as well as other crops including okra and eggplant.
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