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Biotic Diseases. Fungal diseases (cont.). Downy Mildew. A disease caused by a group of fungi belonging to the family Peronosporaceae class oomycetes. Infect a wide range of plants including grain crops, vegetables, field crops, ornamentals, shrubs and vines.
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Biotic Diseases Fungal diseases (cont.)
Downy Mildew • A disease caused by a group of fungi belonging to the familyPeronosporaceae class oomycetes. • Infect a wide range of plants including grain crops, vegetables, field crops, ornamentals, shrubs and vines. • Infection occurs through stomata on lower surface by sporangia itself or its zoospores, then mycelium spreads into the intercellular spaces. Finally, sporangia are produced on sporangiophores that emerge from stomata. • Sporangiophores form a visible mat of oomycete growth on lower side or both sides of leaves and other infected organs.
Symptoms • At first, small pale yellow irregular spots appear on upper surface of leaves and a white downy growth of sporangiophores or oomycete appears on lower surface of spots. Spots of enlarge, coalesce forming large dead areas. • Later, infected leaf areas are killed and turn to brown while spots turn gray.
Pathogen:Bremia lactuca Saucer shaped
Powdery Mildew • A disease caused many species of fungi belonging to the family Erysiphaceae class Ascomycetes. The genera are distinguished by number of asci per cleistothecium and morphology of appendages. • Infect all plants except gymnosperms. • Infection starts when conidia fall on plant surface and germinate producing mycelium that spreads on plant surface not inside. Mycelium feeds by haustoria and produce conidia on short conidiophores on plant surface. In unfavorable conditions, the fungus produces cleistothecia containing one or more ascus.
Symptoms White to grayish powdery mildewy growth as patches or spots turns to yellow brown and finally black cleistothecia. Symptoms appear first on upper side of leaf then lower side. It also affects all plant organs.
