"Powdery mildew is a common disease on many types of plants. Several powdery mildew fungi cause similar diseases on different plants (such as Podosphaera species on apple and stone fruits; Sphaerotheca species on berries and stone fruits; Erysiphe necator on grapevines). Powdery mildew fungi generally require moist conditions to release overwintering spores and for those spores to germinate and infect a plant. However, no moisture is needed for the fungus to establish itself and grow after infecting the plant. Powdery mildews normally do well in warm, Mediterranean-type climates." (W. D. Gubler, S. T. Koike, 2011)
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. (R. M. Davis, Plant Pathology, UC Davis)
Recommended controls for powdery mildew
Each control will have its own set of parameters that will be best suited for individual environments. Certain controls may only be available for commercial application.
Commercial farmers are required to reference their own state laws to ascertain if the recommended controls fall within compliance of their states regulatory guidelines.
Cultural, MECHANICAL AND PHYSICAL CONTROLS
- Plant in a sunny location wherever possible
- Make sure there’s good air circulation (through plant spacing and pruning)
- Promote steady growth with moderate application of nitrogen fertilizer (or use a time release fertilizer)
OVERUSE OF NITROGEN WILL CAUSE THE LEAVES TO "OFFGASS" THE NITROGEN, LEAVING VULNERABLE OPENINGS ON THE LEAF SURFACE SUSCEPTIBLE TO INFECTION.
- In a sunny dry location, an occasional spray of water can be used to wash the PM from the plant.
(DO NOT SPRAY WITH WATER DURING ONSET OF FLOWERING)
- Remove and discard infected material, take special care in the fall to reduce the amount material available for over wintering (if applicable)