Powdery mildew of Cucumbers and Mints

Cucumbers. 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.
Mints. Powdery mildew is a fungal disease that commonly occurs on mint in the Columbia Basin and Yakima Valley of Washington. Scotch spearmint can be severely damaged while peppermint and native spearmint are usually not seriously affected. The semiarid environment in south central Washington is an ideal environment for powdery mildews. Free moisture on plant foliage is not required for infection by powdery mildews as is the situation for many other foliar diseases of plants caused by fungi. However, humid conditions such as those that occur in mint canopies when days are warm and nights are cool favor powdery mildew.  The disease is most severe on young, succulent plants such as those resulting from heavy nitrogen fertilization and irrigation. Powdery mildew is often severe on mint grown in the greenhouse due to humid, shady conditions. Mint infected with powdery mildew is sensitive to winter injury.
Powdery mildew appears on mint leaves, stems, and petioles as a powdery, white to gray coating of fungal mycelium and spores. Infections consist of discrete circular colonies of up to ½ inch in diameter and can become numerous, coalesce and spread over the entire plant. Colonies turn gray with age. Leaves may turn yellow and drop with severe infections.