Early blight or target spot is caused by Alternaria solani, an opportunistic fungus that attacks older leaves as the plant ages or when the plant is under stress. Severe infestations also attack stems. Tubers that get exposed to early blight spores during harvest can develop a rot called tuber blight after a few months in storage.
I. Vine infection can only come from spores floating in the air. The onset of early blight is dependent on the age of the plant as advanced by air temperature. To estimate this and determine when plants are susceptible, a predictive model is used based on Physiological Days. See Panel for more information.
a) Early symptoms appear on lower, older leaves. Lesions appear as round dark spots with concentric rings inside, hence the name target spot. These lesions occur between the veins and as they grow remain bordered by them thereby getting an angular shape. As the plant ages, lesions move up the plant infecting younger leaves and, in severe cases, infect the stem.
b) Later as the disease progresses, the leaf spots join forming larger lesions. Sometimes a yellow edge to the lesions can be seen. Leaf clorosis (yellowing) occurs then cells die (necrosis). The leaves desiccate and fall (defoliation). In severe cases, early blight will also infect the stem, usually in the upper half of the plant. Stem lesions appear as rectangular or oblong blotches that enlarge and turn necrotic thereby hastening vine death. Spores are spread leaf to leaf and plant to plant by breezes.
II. During harvest, spores of A. solani can move from the ground or infected vines onto tubers. The disease that forms in storage due to this spore transfer is referred to as tuber blight. Symptoms start as dark, sunken skin surfaces areas with raised borders. Lesions are dry, leathery and brown underneath. As storage progresses, the skin lesions enlarge, turn watery with a yellowish decay, and in time the tubers shrivel. See Tuber Blight under the Tuber Blemishes section.
III. Early blight spreads faster when plants are stressed. Also weather conditions that result in alternating wet and dry periods promotes it. Vines should be well desiccated before harvest. At harvest, avoid tuber bruises and allow the skin to set. Cultivars differ in their sensitivity to early blight and independently to tuber blight. There are many chemistries effective against foliar early blight such as the EBDCs (mancozebs), chlorothalonils, triphenyltin, strobilurins, and coppers.
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Early Blight - stem |
Early Blight - target spot |
Early Blight - Vine |
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Early Blight - Vine death and spores |
Early Blight on nightshade |
References:
- "Compendium of Potato Diseases" 2nd Edition. 2001. Eds. Stevenson, W.R., Loria, R., Franc, G.D., and Weingartner, D.P., Publ. Amer. Phypathological Soc. Press, St. Paul, MN.
- Reifschneider, F.J., Furumoto, O. and Filgueira, F.A.R. 1984. Illustrated key for the evaluation of early blight of potatoes. FAO Plant Protection Bulletin 32:91-94.