Biotic Diseases

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Biotic Diseases

• Fungal diseases (cont.)

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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.
• 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.

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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.

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Bremia lactuca on lettuce.
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Pathogen Bremia lactuca
Saucer shaped
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Plasmopara viticola on grape.
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Pathogen Plasmopara viticola
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Perenospora parasitica on crucifers.
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Pathogen Perenospora parasitica
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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.

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Cleistothecia
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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.

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Erysiphe sp. on.
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Pathogen Erysiphe sp.
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Pathogen Erysiphe sp.
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Leveillula taurica on artichoke.
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Pathogen Leveillula taurica
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Uncinula necator on grapes
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Pathogen Uncinula necator
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Sphaerotheca pennosa-rosa on roses
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Pathogen Sphaerotheca pennosa-rosa