Powdery mildew (Sphaerotheca pannosa)
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 plant tissue. However, no moisture is needed for the fungus to establish itself and grow after infecting the plant. Powdery mildews normally favour warm, Mediterranean-type climates.
Powdery mildew can be recognized easily on most plants by the white to gray powdery mycelium and spore growth that forms on both sides of leaves, flowers, fruits and on shoots. On tree fruits a rough corky spot on the skin will develop where infection occurred.
All powdery mildew fungi require living plant tissue to grow. On deciduous perennial hosts such as grapevine, raspberry, and fruit trees, powdery mildew survives from one season to the next in infected buds or as fruiting bodies called chasmothecia, which reside on the bark of cordons, branches, and stems.
Most powdery mildew fungi grow as thin mycelium layer 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. In contrast, spores of downy mildew grow on branched stalks that look like tiny trees. Also downy mildew colonies are gray instead of white and occur mostly on the lower leaf surface.
Powdery mildew spores are carried by wind to host plants. Although humidity requirements for germination vary, many powdery mildew species can germinate and infect in the absence of water. In fact, spores of some powdery mildew fungi are killed and germination and mycelial growth are inhibited by water on plant surfaces. Moderate temperatures and shade are generally the most favorable conditions for powdery mildew development, since spores and mycelium are sensitive to extreme heat and direct sunlight.
This fungus overwinters as mycelia inside the budscales, primary infection occurs as leaves emerge from these infected buds. Secondary infections occur when conidia produced by primary and subsequent secondary infections are blown or splashed by rain onto susceptible tissues. Fruit (before pit hardening) and succulent terminal growth are susceptible to infection.
The average minimum, optimum, and maximum temperatures for S. pannosa are about 5°, 24° and 24°C. Many more conidia are formed in dry air than in humid air at all temperatures (C.E. Yarwood, Soliman Sidky, Morris Cohen, Vincent Santilli; 1954)
Powdery mildew is common under similar relative humidity and temperatures as cherry powdery mildew.
Fieldclimate Model: Fungal disease is modeled by the factors temperature and duration of leaf wetness. For example on May 11th the leaf wetness period under moderate temperatures supported the development of the disease and a risk of 100% could be determined.
C.E. Yarwood, Soliman Sidky, Morris Cohen, Vincent Santilli (1954): Temperature relations of Powdery Mildews. HILGARDIA. A Journal of Agricultural Science Published by the California Agricultural Experiment Station. University of California. Volume 22/Number 17.