Powdery mildew of Cucumbers

Host. Plants and Control Measures for Powdery Mildew Species:
Erysiphe cichoracearum - cucumbers, endive, lettuce, melons, potato, pumpkin, squash.
Erysiphe polygoni - beets.
Symptoms. Powdery mildew first appears as white, powdery spots that may form on both surfaces of leaves, on shoots, and sometimes on flowers and fruit. These spots gradually spread over a large area of the leaves and stems. An exception is one of the powdery mildews that affects artichokes, onions, peppers, and tomatoes: it produces yellow patches on leaves but little powdery growth.
Leaves infected with powdery mildew may gradually turn completely yellow, die, and fall off, which may expose fruit to sunburn. On some plants, powdery mildew may cause the leaves to twist, buckle, or otherwise distort. Powdery mildew fungal growth does not usually grow on vegetable fruits, although pea pods may get brownish spots. Severely infected plants may have reduced yields, shortened production times, and fruit that has little flavor.
Life Cycle. All powdery mildew fungi  require living plant tissue to grow. Year-round availability of crop or weed hosts is important for the survival of some powdery mildew fungi. Special resting spore are produced, allowing overwinter survival of the species that causes the disease in cucurbits, lettuce, peas, and certain other crops.
Most powdery mildew fungi grow as thin layers of mycelium (fungal tissue) on the surface of the affected plant part. Spores, which are the primary means of dispersal, make up the bulk of the white, powdery growth visible on the plant’s surface and are produced in chains that can be seen with a hand lens; in contrast, spores of downy mildew grow on branched stalks that look like tiny trees.
Powdery mildew spores are carried by wind to new hosts. Although humidity requirements for germination vary, all powdery mildew species can germinate and infect in the absence of free water. In fact, spores of some powdery mildew fungi are killed and germination is inhibited by water on plant surfaces for extended periods. Moderate temperatures (60° to 80°F) and shady conditions generally are the most favorable for powdery mildew development. Spores and fungal growth are sensitive to extreme heat (above 90°F) and direct sunlight.