HPIPM:Alternaria Leaf Spot

Author: Howard F. Schwartz and David H. Gent

Taxonomy

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

Scientific Name

Alternaria brassicicola

Scientific Name Synonyms

Alternaria circinans

Alternaria oleracea

Common Name

black spot

Identification and Life Cycle

Alternaria leaf spot is caused by the fungus Alternaria dauci. Infection can be initiated by seedborne or windborne spores (conidia), which germinate in the presence of free moisture and penetrate leaves through wounds or natural openings. The fungus sporulates profusely on dead and dying plant tissue, especially during moderate to warm weather (60 to 77ºF) with prolonged periods (8 to 12 hours) of leaf wetness. Spores (conidia) are readily disseminated by wind and splashing water in and among adjacent fields; large numbers of conidia become airborne during harvesting. The pathogen can be moved among fields on contaminated equipment. A. dauci survives between carrot crops as a pathogen of wild and volunteer carrot, in infested crop debris, in the soil for up to one year, and in and on contaminated seed.

Plant Response and Damage

Alternaria leaf spot symptoms first appear as greenish brown, water-soaked lesions. Lesions quickly become dark brown to black with or without yellow halos. Under disease-favorable conditions, lesions coalesce and cause entire leaves to become yellow, collapse, and die. Older leaves are most susceptible to infection and often the first to develop symptoms, but all leaves can be infected. Petiole lesions appear similar to leaf lesions, but are more elongated; petiole lesions quickly kill entire leaves. Disease symptoms are often confused with Cercospora leaf spot and bacterial blight, and microscopic examination is often necessary for accurate diagnosis.

Carrot seedlings can also be attacked by A. dauci within 2 to 3 weeks after emergence. Infection occurs at or just below the soil line. Infected hypocotyls and upper roots become girdled and shrivel, turning a gray or black color. Seedling infection appears similar to Pythium damping-off, but A. dauci infection and decay is drier than that of Pythium.

Biological Control

No biological control practices have been developed for Alternaria leaf spot.

Cultural Control

Plant high quality seed free of the Alternaria leaf spot pathogen. Hot water treatment can reduce seed contamination, but may reduce seed storability and germination. Select varieties resistant or tolerant to Alternaria leaf spot, such as ‘Orlando Gold,’ if available. Practice a three-year or longer crop rotation to nonhosts such as small grains. Eliminate potential sources of the pathogen by deeply burying crop debris after harvest and controlling wild and volunteer carrot. Avoid prolonged periods of leaf wetness by avoiding dense planting, orientating rows parallel to the prevailing wind direction, and timing irrigations to end before dusk. Avoid overhead irrigation if possible.

Chemical Control

Fungicides are rarely necessary for Alternaria leaf spot control in many High Plains carrot production regions, but are essential in warm, humid environments. Disease forecast models have been developed that can improve the timing and efficiency of sprays. Chemical controls are most effective when combined with as many cultural control strategies as possible.

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.