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.

• 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.

Bremia lactuca on lettuce.

Pathogen: Bremia lactuca

Saucer shaped

Plasmopara viticola on grape.

Pathogen: Plasmopara viticola

Perenospora parasitica on crucifers.

Pathogen: Perenospora parasitica

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.

Cleistothecia

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.

Erysiphe sp. on.……

Pathogen: Erysiphe sp.

Pathogen: Erysiphe sp.

Leveillula taurica on artichoke.

Pathogen: Leveillula taurica

Uncinula necator on grapes

Pathogen: Uncinula necator

Sphaerotheca pennosa-rosa on roses

Pathogen: Sphaerotheca pennosa-rosa