HPIPM:Onion Downy Mildew

Taxonomy

Domain	Eukarya
Kingdom	Chromista
Phylum	Oomycota
Class	Oomycetes
Order	Peronosporales
Family	Peronosporaceae
Genus	Peronospora

Scientific Name

Peronospora destructor

Scientific Name Synonyms

Peronospora schleideni

Common Name

downy mildew

Author: Howard F. Schwartz and Michael E. Bartolo

Identification and Life Cycle

This foliar disease is caused by the fungus-like organism Peronospora destructor. Infection is favored by temperatures less than 72°F, and free moisture from rain, dew, or overhead irrigation in early spring or late summer. The fungus overwinters in volunteer onion, culls, and wild Allium species.

Plant Response and Damage

Downy mildew symptoms appear on older leaves as elongated patches which vary in size and are slightly paler than the rest of the foliage. With moisture, these areas become covered with a violet gray mycelium, which contain spores that may be spread to surrounding healthy tissue. Leaves generally fold over at affected areas, and leaf tips die. Another fungus (Stemphylium botryosum) may develop on primary lesions and produce a brown to black fungal growth. Affected plants may be dwarfed and leaves become pale green, deformed and often are covered by fungal mycelium. Bulb size and quality are reduced by infection occurring before maturity.

Biological Control

Biological control strategies have not been developed for downy mildew.

Cultural Control

Practice a three-year or longer to non-hosts such as small grains and corn. Reduce over wintering and survival of the pathogen by eliminating culls and volunteers. Avoid dense plantings of late maturing varieties and overhead irrigation. Apply adequate but not excessive nitrogen fertilizer before bulb initiation. Orientate rows parallel to the prevailing wind direction and avoid wind breaks to minimize periods of leaf wetness. Onion varieties with high levels of downy mildew resistance are being developed.

Chemical Control

Chemical controls are most effective when combined with sound cultural practices. Copper fungicides and chlorothalonil are registered for control of downy mildew, but are not highly effective.

Due to constantly changing labels, laws and regulations, the sponsors of this web site assume no liability for the suggested use of any chemicals noted in the management section. Contact your local pest management personnel for current information on labeled pesticides that can be used for the intended plant disease, insect pest or weed.