![]() ![]() In contrast, spores of downy mildew grow on branched stalks that look like tiny trees. 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. Most powdery mildew fungi grow as thin layers of mycelium on the surface of the affected plant part. On strawberry the fungus can survive on leaves that remain on the plants through winter. 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. LIFEĪll powdery mildew fungi require living plant tissue to grow. When foliage infections are severe, flowers and fruit may also be infected. ![]() Infected leaves later develop dry, brownish patches along with nondescript patches of white powdery fungus on the lower surface and reddish discoloration on the upper surface. On strawberry, affected leaf edges curl upward. Grapes with a severe infection may also crack or split and fail to grow and expand. On tree fruits a rough corky spot on the skin will develop where infection occurred. On apple and grape and to a lesser extent apricot, nectarine, and peach, infected young fruits develop weblike, russetted scars. New growth is dwarfed, distorted, and covered with a white, powdery growth. The disease can be serious on woody plants such as grapevines, caneberries, and fruit trees where it attacks new growth including buds, shoots, and flowers as well as leaves. 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, sometimes on flowers and fruit, and on shoots. Thus powdery mildews are more prevalent than many other diseases in California’s dry summer and fall seasons. Powdery mildews normally do well in warm, Mediterranean-type climates. However, no moisture is needed for the fungus to establish itself and grow after infecting the plant. Powdery mildew fungi generally require moist conditions to release overwintering spores and for those spores to germinate and infect a plant. 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, see Table 1). ![]() Powdery mildew is a common disease on many types of plants. White, flattened, pointed apple buds are overwintering sources of powdery mildew, Podosphaera leucotricha. ![]()
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