Powdery Mildew Risk Model for Pepper

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.

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.

Risk Model:

  • if temperature is in between 22°C and 32°C for 6 hours or more => Risk increases by 20 Points
  • If temperature is higher than 32°C for 6 hours or more => Risk decreases by 10 Points
  • if temperature is lower than 22°C for the whole day => Risk decreases by 10 Points
  • if there is more than 6 hours of leaf wetness => Risk decreases by 10 Points

If the risk is below 20 it can be assumed that powdery mildew can not propagate fast and the spry program can by very extensive. If the risk is higher than this the spray program should start. if the risk exceedes 60 points a strict and effective spray program has to be used. In organic growing this will include the reducton of the spray intercelt for sulfor.

In FieldClimate the risk is displayed in daily values. Because of the temperatures of 20 to 30°C at the beginning of August we have a risk of 100%.