Pest and disease management

Pathogen Profile: Alternaria

Troy Buechel Horticulture Specialist
Troy Buechel
 Horticulture Specialist at Premier Tech Growers and Consumers
Watermelon in greenhouse

Alternaria is a genus of ascomycete fungi often associated with the decay and decomposition of plant residues. It is found everywhere in the natural environment. However, many species are also plant pathogens that typically infect foliage, flowers or fruits in many types of annuals, perennials and vegetable plants. In some cases, Alternaria has been found to parasitize plants at or below the soil line. Alternaria is not commonly associated with a root disease pathogen, but it can cause root damage that may result in plant death.

Root and Stem Infection

Alternaria survives and can overwinter as spores and mycelium in infected plant residues (leaves, stems, tubers, etc.) and on or in seeds. If Alternaria spores are on or in seed, they can germinate with the seed causing damping off in seedlings. More frequently, Alternaria diseases come from plant residues that can infect young and more mature plants causing stem lesions or collar rot. During periods of rainfall, heavy dew or high moisture conditions in the soil, Alternaria will grow and sporulate outdoors or inside a greenhouse. Alternaria infections often appear as dark sunken lesions that can appear on roots, tubers, stems and fruits. Mycelium growth appears on lesions as fine, black fuzzy growth that quickly produces gray to black spores. 

Alternaria collar rot of tomato
Young tomato plants showing collar rot from Alternaria. Notice the constriction and browning of the stems just above the root ball. Source: http://www.omafra.gov.on.ca/IPM/english/tomatoes/diseases-and-disorders/damping-off.html

Foliar Infection

On leaves, it appears as round ‘targets’ with concentric circles around the infection site. This is a unique expression and can be used as a diagnostic tip to help identify this disease. Commonly there is a yellow halo around these spots due to the toxins emitted by Alternaria. As with other plant tissue, these spots will have fine, black fuzzy growth with gray to black spores. 

Alternaria LeafBlight melon
Cantaloupe leaves infected with Alternaria. Notice the concentric rings in both pictures giving the spot a target look. Source: https://www.bighaat.com/pages/fungal-diseases

Disease Development

Alternaria spores can be spread by wind or splashing water onto plant surfaces where infection can begin. Like Botrytis, the spores need free moisture to germinate and they will grow into stomatal openings or wounds. Plants that are under-fertilized, have weak growth, damaged tissue or are under stress, are more susceptible to infection from Alternaria. Alternaria prefers warmer temperatures around 80oF (27oC) and infection rates significantly drop when the temperature is below 53oF (12oC).

Disease Management

Like most pathogens, if plant roots are infected with Alternaria, do not reuse the growing medium. Discard containers or properly sanitize them before reusing them. Remove infected plant residues from the greenhouse and eliminate weeds as they can become infected by Alternaria which can spread to greenhouse crops. Use disease-free seed as Alternaria spores can, in a few cases, be found inside the seed under the seed coat. Some species of Alternaria require ultraviolet light to produce spores, so the use of ultraviolet light-absorbing film can greatly reduce the incidence of this disease. Many chemical products can be drenched or applied as a foliar spray to control Alternaria so consult with your distributor or university extension service for more information.

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