LECTURE 9: The Oomycetes continued

Content of Lecture

5. The Downy Mildews

Class: Oomycetes

Order: Peronosporales

Family: Peronosporaceae


Pages 427 - 433

Family: Peronosporaceae (Different family from the Pythiaceae)

Differ from Pythiaceae in being biotrophic and completely specialized to an aerial existence. They are homothallic.

These are foliage blights attacking tender green tissue such as leaves, buds, twigs and fruit. They are highly specialized pathogens a logical step from the Phytophthora infestans type of foliar blight.

Historically very serious diseases - eg. The grape Downy Mildew story

Downy mildews continue to be serious pathogens in humid parts of the world both tropical and temperate. They are very serious greenhouse pathogens and can cause total losses of young seedlings and seed plants.

There are many genera and species of downy mildew but here are a few of the more common and serious: See page 428.

Bremia lactucae - Downy Mildew of Lettuce

Peronospora destructor - Onion

Peronospora tabacina - Blue mold of Tabacco

Peronospora trifoliorum - Downy mildew of alfalfa and clover

Plasmopara viticola - Grape

Peronosclerospora - Sorghum, Maize and Sugarcane

Pseudoperonospora - cucurbits and hops

Sclerospora graminicola - Downy mildew of grasses and millets

Downy mildews over season as oospores and sporangia can germinate directly (at higher temperatures) or indirectly at lower temperatures. Some downy mildews (eg Peronospora ONLY germinate directly.


Common symptoms include white downy mycelium on plant tissues, necrotic spots and lesions are common. Distinguish from powdery mildews? Very characteristic sporangiophores (cf P. infestans) make these about the easiest of all fungal diseases to identify. Branching distinguishes the different genera.

Require Lots of moisture...and cool to warm conditions (depending if temperate or tropical.

The classic example is downy mildew of grape caused by Plasmopara viticola.

Dry areas are usually free of the disease, which is why California and Australia even New Mexico are good grape/wine areas.

Downy mildew affects the leaves, fruit and herbaceous stems of grape causing at worst defoliation and stunting of the vine.

DISEASE CYCLE 11-32 page 431

First observed as small chlorotic spots on the top of leaves with a downy growth of sporangiophores directly under these spots on the bottom of leaves. (Difference with Powdery Mildews). These spots turn necrotic on older leaves and may result in complete defoliation, the sporophores turn dark grey. The fruit and shoots may become infected and covered with sporangiophores.

The pathogen is Plasmopara viticola. It is biotrophic and grows intercellularly and sends haustoria into cells. Sporangiophores come out of stomata on under surface of leaf in humid weather. These sporangiophores are the basis of identification.

Overwinters as Oospores in dead leaves and as mycelium in infected (not dead) shoots and twigs.



Covered on Pages 434 - 438.


Last of the lower fungi only two pathogenic species - postharvest rots

Rhizopus - soft rot of fruit and vegetables - postharvest...very, very common fungus a good saprophyte and sporangiospores arrive at a wound and enter to rot the underlying tissue.

LIFE CYCLE (Figure 11 - 37 page 437)

Choanephora - attack sensing floral parts of squash and pumpkin.

Also Mucor - Bread mold

Glomus and Gigaspora are zygomycetes which form VAM. Only five plant families are known which DO NOT have mycorrhizal associations.

Sum up the lower fungi

Show - fruiting bodies and overview.

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