Leaves of plants with powdery mildew have irregular, yellow blotches. Severely affected leaves die, but seldom drop. As blotches enlarge, leaf tissue in center dies.
Identification
On plants infected with powdery mildew spots of dead tissue, sometimes with rings like those in early blight lesions, may appear in the blotches. However, there are no lesions on stems or fruit, and there is no vascular discoloration as with Verticillium or Fusarium wilt. Rarely, a gray mycelium develops on the leaf surface.
Life cycle
All powdery mildew fungi require living plant tissue to grow. Most powdery mildew fungi grow as thin layers of mycelium on the surface of the affected plant part. Spores, which are the primary means of dispersal, make up the bulk of the powdery growth and are produced in chains that can be seen with a hand lens. Spores are carried by the wind to new hosts. Spores are killed and germination is inhibited by free moisture. Moderate temperatures and shady conditions are generally the most favorable for powdery mildew development. Spores are sensitive to extreme heat and direct sunlight.
Solutions
This disease occurs late in summer or fall and does not cause significant losses unless very severe, so no control is normally needed. If young plants are attacked, applications of sulfur will control the disease. Avoid water stress.
For more information, see the Powdery Mildew Pest Note.
