How to Manage Pests
UC Pest Management Guidelines
SYMPTOMS AND SIGNS
Powdery mildew primarily affects leaves on pepper plants. Although the disease commonly occurs on older leaves just before or at fruit set, it can develop at any stage of crop development. Symptoms include patchy, white, powdery growth that enlarges and coalesces to cover the entire lower leaf surface. At times the powdery growth is present on the upper leaf surface as well. Leaves with mildew growing on the undersurface may show a patchy yellowish or brownish discoloration on the upper surface. The edges of infected leaves may roll upwards exposing the white, powdery fungal growth. Diseased leaves drop from the plants and leave the fruit exposed to the sun, which may result in sunburning.
COMMENTS ON THE DISEASE
Powdery mildew can be severe during the warmest part of summer and can cause heavy yield losses. The pathogen has a very wide host range and inoculum from one host plant species can cross-infect other host plants. In California, powdery mildew inoculum can come from crops such as onion, cotton, tomato, all varieties of peppers, and weeds such as annual sowthistle and groundcherry.
This powdery mildew pathogen differs from powdery mildew pathogens in other genera in that it primarily occurs inside the leaf rather than on the leaf surface. Cleistothecia (sexual spores) of the Leveillula perfect stage rarely occur in California, but asexual spores (conidia) are produced and disseminated by wind. In general, high humidity favors germination of conidia. Infection of plants can occur over a wide temperature range (64° to 91°F or 18° to 33°C) under both high and low humidity. Under favorable conditions, secondary infections occur every 7 to 10 days, and disease can spread rapidly. Temperatures over 95°F that commonly occur in the interior valleys of the state can temporarily suppress development.
Regular monitoring to detect powdery mildew, especially during warm weather, is important to time fungicide applications early enough to prevent damage. Powdery mildew is managed primarily with fungicides.
The fungi that cause powdery mildew can survive between crop seasons on other crops and on weed species. The degree of survival depends on environmental conditions. Because of the wide host range of the fungus, it is difficult to control the amount of inoculum that overwinters in California. Thus, simple sanitation methods in and around pepper fields may not provide a sufficient reduction in the primary inoculum to provide disease control.
Most pepper cultivars used in California do not possess acceptable levels of resistance to powdery mildew. Currently, there are no breeding programs aimed at developing resistant cultivars to pepper powdery mildew.
Organically Acceptable Methods
Sprays of sulfur and potassium bicarbonate are acceptable for use on organically grown peppers.
Fungicides can provide satisfactory control and prevent economic loss if applied during the early stages of the infection. Effective control requires spraying with high pressure and high volume of water for optimum penetration of the crop canopy by the fungicide. Good coverage is necessary for satisfactory control; ground applications give better coverage than air.
UC IPM Pest Management Guidelines:
S. T. Koike, UC Cooperative Extension, Monterey County
Acknowledgment for contributions to Diseases:B. W. Falk, Plant Pathology, UC Davis