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Late blight is the most important fungal disease in potato cultivation. The pathogen Phytophthora infestans is found in almost all regions where potatoes are grown. Most damage is done in moist and cool areas.
Typical symptoms are brown, blighted zones on the leaves showing a white, downy fungus growth on the underside. Infected tubers show purplish or brownish blotches. The affected areas become firm and dry and kind of sunken.
Infections start out from infected tubers. The mycellium grows into the new shoots and emerges through sporangia via the stomata. New sporangia may form within 4 days, leading to a large number of asexual generations and new infections in one growing season. The fungus requires two mating types for sexual reproduction. In the last few years both mating types have become present in most countries which resulted in a greater genetic variability. Recent isolates show a wide range of virulences. Resistance gene mediated resistance of Phytophthora infestans is rapidly overcome by the pathogen. Therefore potato breeding now focuses on a horizontal resistance which may last longer.
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