"Copper fungicides are estimated to extend the growing period (before the potato foliage has to be destroyed to prevent the spread of blight to the tubers and neighbouring fields) by between 2-4 weeks. As required by the certification process, farmers must use all available alternative practices to manage late blight, and describe these in the Organic System Plan, before deciding to apply a copper product. In regions of the world where late blight epidemics occur frequently, prophylactic copper applications are applied. Once late blight is initiated in a field, disease progress can occur very rapidly, killing all aboveground plant tissues within one or two weeks. However, in some regions, even when all best organic management strategies are adopted, if inoculum is blowing in from off-farm sources, potato cultivars are not resistant, and weather conditions are conducive to disease development, a late blight epidemic can occur. Organic farmers should practice best cultural management to manage this disease. Late blight (causal agent Phytophthora infestans) is a very difficult disease to control organically in regions where potatoes are grown on large acreage and rain occurs during the production period. New York State Agricultural Experiment Station. Cornell University College of Agriculture and Life Sciences. Resource guide for organic insect and disease management.
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