Late blight (Phytopthora infestans) is perhaps the world’s most important potato disease because it not only attacks plant foliage but also attacks tubers in the field as spores drop from the foliage and travel through soil cracks and pores in irrigation and rainwater to attack tubers. Early tuber infections are difficult to detect and tubers may be stored with devastating results unless the grower takes special precautions. Late blight tuber decay is almost invariably followed by soft rot bacteria which can rapidly reduce tubers and an entire storage to mush. Late blight survives primarily on living plant debris, so volunteer plants, cull piles and infected seed are major sources. Infection is greatly favored by cool, wet weather. Control is usually achieved by spraying fungicides (Mancozeb, Bravo, Coppers, Duter, Polyram, Super Tin, etc.) both before and during infection. Prevention is much more effective than curing with regard to late blight. In situations with bad weather and heavy blight pressure, crop loss can be very difficult to prevent. Growers must sometimes salvage a crop and market it immediately without attempting to store. Severe late blight and long-term storage are mutually exclusive.
