Cladosporium iridis (heterosporium leaf spot on iris)

Taxonomy

Domain	Eukarya
Kingdom	Fungi
Phylum	Ascomycota
Subphylum	Pezizomycotina
Class	Dothideomycetes
Subclass	Dothideomycetidae
Order	Capnodiales
Family	Davidiellaceae
Genus	Cladosporium

Scientific Name

Cladosporium iridis

Scientific Name Synonyms

Davidiella macrospora

Didymellina macrospora

Heterosporium gracile

Heterosporium iridis

Mycosphaerella macrospora

Common Name

Heterosporium leaf spot on iris

Author: Mary Ann Hansen, Virginia Tech

Reviewed by: Anton Baudoin, Virginia Tech and Karen Rane, University of Maryland

Pathogen

The fungus Davidiella macrospora (syn. Mycosphaerella macrospora) is a member of the Ascomycetes. Its imperfect stage, Cladosporium iridis, (syn., Heterosporium iridis, H. gracile) is a member of the Deuteromycetes (Hyphomycetes). On plant tissue the pathogen is most commonly found in its imperfect stage. Conidia are borne on erect conidiophores, which arise in fascicles through stomates. Conidiophores are flexuous, brown, smooth, and up to 150µm long and 8-20µm wide. Conidia vary widely in size, from 35 to 70µm long by 13-25µm wide. The description by Ellis, M.B. (1971) states that conidia are "usually solitary, straight or slightly curved, oblong or cylindrical, rounded at the ends", pale to olivaceous brown, densely echinulate, and 1-4 septate, often with a constriction at the septum.

Symptoms and Signs

The first evidence of this disease is the appearance of small brown spots with watersoaked margins near the leaf tips. As the spots enlarge, they become oval in shape and develop a yellow halo. Later, spots develop gray centers with a darker, reddish brown border. Tufts of grayish brown conidia may be visible in the leaf spots. During the early part of the growing season, leaf spots are inconspicuous, but after bloom, the spots enlarge rapidly, coalesce, and may cause death of the leaf from the tip back. Although the fungus does not attack the bulbs or rhizomes, premature killing of the leaves may weaken the underground plant parts and cause reduced bloom and a gradual death of the entire plant.

Ecology and Spread

This disease occurs on both bulbous and rhizomatous iris, although it tends to be more severe on rhizomatous iris. The fungus overwinters on dead iris leaves. Conidia or ascospores that form on old plant debris in the spring initiate new infections. Spore germination can occur at a wide temperature range (50-77C or 10-25C) with an optimum temperature near 68F (20C). Conidia that form on leaf spots are splashed to other plants by rain or irrigation water, or they may be moved by air currents. Multiple cycles of infection may occur in one growing season.

Geographic Distribution

The pathogen is widely distributed in the United States. It has been reported in most of New England, the Mid-Atlantic, the Southeast, the Midwest, and in Montana and Oregon. The disease is also found in Europe, South Africa, Australia and the Middle East. The disease has also been reported on Gladiolus sp., Freesia sp., and Narcissus sp.

Management

Cultural Control

The most important step in control is sanitation. Remove all old leaves in the fall to reduce overwintering of the fungus. Leaves should be buried, bagged and removed from the site, or burned, if local regulations allow burning. This will greatly reduce the amount of fungal inoculum available for infection the following spring. Plant in locations with full sun and well drained soil. Avoid overhead irrigation, which spreads spores.

Chemical Control

A fungicide spray program can be used to prevent new infections. Usually 4 to 6 sprays of a registered fungicide, starting when the leaves are 4 to 6 inches high and repeated at 7 to 10 day intervals, will control the disease. Because of the waxy nature of iris leaves, it is important to add a commercial spreader-sticker to the fungicide spray to aid in wetting the foliage.

Resistance

Iris germanica, the commonly grown German bearded iris, is more susceptible to the disease than non-bearded cultivars. Siberian iris (Iris siberica) has good resistance to the disease.

Diagnostic procedures

The fungus sporulates readily on diseased plant tissue under moist chamber incubation and can be identified by preparing a wet mount of the conidia and observing with the compound microscope.

Resources and References

• Ellis, M.B. 1971. Dematiaceous Hyphomycetes. Commonwealth Mycological Institute, Kew, Surrey, England.
• Gleason, M.L., Daughtrey, M.L., Chase, A.R., Moorman, G.W., and D.S. Mueller. 2009. Diseases of Herbaceous Perennials. American Phytopathological Society, St. Paul, MN.
• Mycobank. www.mycobank.org. International Mycological Association. Cladosporium iridis
• Pataky, N. 1982. Iris Leaf Spot. RPD No. 628. University of Illinois Extension.

Acknowledgement

• National Institute of Food and Agriculture, U.S. Department of Agriculture, under Agreement No. 2011-41530-30708 as part of "Diagnostic Image Series Development for Supporting IPM in the Southern Region" (USDA-NIFA-RIPM-003351)