Return to:

Hosts

In Hawaii, globe amaranth (locally called "bozu") and Celosia are commonly diseased by Alternaria gomphrenae. In Florida, bullace or muscadine grape (Vitis rotundifolia) is also reported to be afflicted with a leaf spot caused by Alternaria gomphrenae.

Diseases Names:

Leaf spot of globe amaranth (Gomphrena globosa).

Leaf spot and blight of Celosia plumosa, commonly called cockscomb.

Distribution

Common in the tropical areas of countries such as Burma, Cuba, India, Jamaica, Malaya, Sabah, Sri Lanka, Trinidad, and the U.S. This pathogen is found on most islands in Hawaii.

Symptoms

Leaf spots begin as tiny reddish purple spots which expand into tan to gray spots with a reddish or purple border. Occasionally, spots are initially circular to irregular in shape, white to light tan, and have no purple borders. Entire leaves can be killed and almost all of the leaves can be heavily diseased during cool, moist weather. 

Floral bracts, petioles, and small stems are also infected. Stem lesions are usually reddish or purplish.

Biology

Taxonomy

Kingdom: Fungi

Phylum: Ascomycota (teleomorph or sexual stage)

Order: Dothideales

Family: Pleosporaceae

Genus: Lewia

Traditional: Fungi Imperfecti: Deuteromycotina (anamorph or asexual stage)

Hyphomycetes

Genus: Alternaria

The teleomorph or sexual stage of Alternaria gomphrenae is not known but other Alternaria species are reported to have Lewia as the sexual stage.

Biology

Alternaria gomphrenae produces microscopic spores that are golden brown, 70 to 140 um long, and multicellular (9 to 12 cells). Spores have a pointed tip called a beak. 

This fungus grows well between 18° and 24° C. Sporulation is favored at lower temperatures(16° to 20° C).

Epidemiology

Spores of Alternaria gomphrenae are produced in large numbers on the surface of diseased leaves and other infected plant parts. Cool, moist weather favors spore production by this pathogen. Spores are spread by wind, splashing water and contact with gloves, hands, clothing, etc. Infected floral bracts contaminate seeds, allowing the fungus to move long distances when seeds are sold. Transport of infected plants will also move the pathogen great distances.

Spores germinate when moisture is available and produce a thread like growth called a germ tube. This germ tube will penetrate the leaf and initiate leaf spot formation.

The pathogen grows within the leaf causing leaf spots to develop. Specialized fungal threads called conidiophores emerge from the leaf and produce spores .

Infected debris harbors the pathogen in soil until the next crop of plants.

Management

Severely diseased plants should be discarded. If only a few plants are involved, remove all blighted leaves. Keep the foliage as dry as possible to reduce initiation of new spots. Water during the day to avoid leaf wetness at night.

Applications of Dithane M45 or iprodione will also reduce new infections.

To begin new plantings, purchase clean seeds. Dip seeds in a solution of 10% household bleach, e.g. 1 part Clorox and 9 parts water) with a few drops of detergent for 1 minute. Immediately plant seeds in clean potting mix such as Sunshine Blend 4. As seeds germinate check continuously for any signs of rot. If rots develop, discard all seeds in that pot.

If young plants are purchased at garden shops, check all plants for disease. If any plant has leaf spots, avoid all plants in that group.

Ideally, all old, diseased plants should be destroyed before new plants are started. Fields with diseased plants will harbor the pathogen for months and surviving plants produce spores that will blow over to clean plants. Alternaria survives in diseased plant tissue in the soil. Avoid reinfestation by growing the new crop in areas which have not been used for gomphrena for at least a year. Homeowners should consider planting gomphrena in large pots to allow time for Alternaria to die in contaminated gardens. Without such measures, this pathogen will persist season after season.

References

Ellis, M. B. 1976. More Dematiaceous Hyphomycetes. Commonwealth Mycological Institute, Commonwealth Agricultural Bureaux. Farnham Royal, Slough, England.

Alfieri, S. A. Jr., K. R. Langdon, J. W. Kimbrough, N. E. El-Gholl, C. Wehlburg. 1994. Diseases and Disorders of Plants in Florida. Bulletin No. 14. Florida Department of Agriculture and Consumer Services. Gainesville, Florida.

BA-GOMP

Photos