Powdery Mildew
caused by the fungus Erysiphe cichoracearum, affects cucumber, muskmelon, pumpkin, and squash. It is caused by a fungus that appears as a white powdery growth on leaves. Crown leaves are affected first and may wither and die. The fungus can be introduced on greenhouse-grown plants or by wind from areas with relatively warm winter climate where the fungus can over-winter. Disease development is favored by high temperatures.
Downy Mildew
on cucumber and muskmelon is caused by the fungus Pseudoperonospora cubensis. Irregularly shaped yellowish to brown spots appear on upper sides of leaves, usually at the center of plants. Under moist conditions, a purplish mildew develops on the underside of leaf spots. Leaves die as spots increase in size. Spread is rapid from the crown toward new growth. The causal fungus overwinters in areas with a relatively warm climate and can be introduced to other areas by wind. Moist conditions favor disease development.
Target Spot
caused by Corynespora cassiicola, is similar to downy mildew in appearance. Leaf spots begin as small yellow flecks which become angular or as circular spots with light brown centers and dark brown spots. Spots vary from 1/8 to 3/8 inch in diameter. Lesion coalescence may result in large areas of dead tissue, which may shred and fall out.
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