Alternaria porri

Author: Howard F. Schwartz and Michael E. Bartolo

Identification and Life Cycle

Purple blotch is caused by the fungus Alternaria porri. It can infect all above-ground parts of the plant in addition to the bulb. Initial symptoms appear on older leaves, usually late in the summer as spores are blown from infested debris. Older leaves and plants are more susceptible to infection. The disease is most severe when frequent rain or persistent dews occur during moderate (77 to 81°F) temperatures. Almost no infection occurs below 55°F. The fungus is disseminated within and among fields by splashing water and wind, and overwinters in and on infested crop debris. The pathogen may also be seed-borne.

Plant Response and Damage

Lesions are elongate, small, sunken and whitish with a purple center. Concentric light and dark zones later appear over part or all of the purple area. These blotches may enlarge (up to four inches long) and become covered with black fruiting bodies (spores). Leaves wilt and die. Bulbs can be infected at harvest if the pathogen enters neck wounds. Storage symptoms appear as a dark yellow to wine-red spongy rot of outer or inner scales of bulbs. The disease can reduce bulb yield and grade 20% or more.

Management Approaches

Biological Control

Biological control strategies have not been developed for purple blotch.

Cultural Control

Plant high quality seed and transplants free from the purple blotch pathogen. Practice a three-year or longer rotations to non-hosts such as small grains and corn. Reduce over- wintering and survival of the pathogens by eliminating culls, onion debris, and volunteers. Avoid dense plantings of late maturing varieties. Apply adequate but not excessive nitrogen fertilizer before bulb initiation. Undercut onion roots and harvest at full maturity when intact necks top over naturally, ideally during dry weather. Thoroughly cure bulbs in the field and before storage with ambient to warm (90 to 95°F) air. Maintain storage temperatures near 32°F with a 70% or less relative humidity.

Chemical Control

Chemical controls are most effective when combined with sound cultural practices. Copper fungicides are registered for control of purple blotch, but are not highly effective. Contact your local extension service for the recommendations that are appropriate to your state.

Taxonomy

Domain	Eukarya
Kingdom	Fungi
Phylum	Ascomycota
Subphylum	Pezizomycotina
Class	Dothideomycetes
Subclass	Pleosporomycetidae
Order	Pleosporales
Family	Pleosporaceae
Genus	Alternaria

Scientific Name

Alternaria porri

Common Name

purple blotch