Gloeocercospora sorghi (copper spot of creeping bent grass)

Taxonomy

Domain	Eukarya
Kingdom	Fungi
Phylum	Ascomycota
Subphylum	Pezizomycotina
Class	Sordariomycetes
Subclass	Xylariomycetidae
Order	Xylariales
Family	Microdochiaceae
Genus	Microdochium

Scientific Name

Microdochium sorghi

Scientific Name Synonyms

Gloeocercospora sorghi

Common Name

Lee Miller, University of Missouri

Reviewed by:Name, Organization

Pathogen

Gloeocercospora sorghi is an ascomycete with no known teleomorph. The pathogen produces abundant sporodochia that emerge through stomates on the leaf surface. Sporodochium bear numerous conidia in a salmon-colored slime. Conidia are hyaline, needle-shaped, have 1-7 septations, and are 20 – 195 μm long and 1.4-3.2 μm wide. Small, black sclerotia (0.1-0.2 mm diameter) may be observed in necrotic leaf lesions.

Symptoms and Signs

In the early stages of disease, lesions are small and reddish but enlarge to cover the entire blade. Since the disease occurs most often on low mown putting greens, the stand symptom is often first observed in the field. Copper to reddish-pink colored patches approximately 1-3 inches in diameter. In warm wet weather, or dew events leaves may be covered with mycelium and/or a gelatinous matrix of salmon-colored spores.

Ecology and Spread

G. sorghi overwinters as sclerotia and thick mycelium in thatch. Initial infection occurs at air temperatures of 68°F. Hyphae germinating from sclerotia penetrate leaf tissue through direct penetration or through stomates. Masses of conidia formed by sporodochia are distributed to other leaf surfaces by splashing water. The pathogen can produce secondary inoculum rapidly in conducive temperatures of 79-86°F and extended periods of wetness, resulting in large epidemics.

Geographic Distribution

Copper spot occurs mostly on Agrostis spp., and is particularly prevalent in the northeastern U.S. on Agrostis canina. Recently, more outbreaks have been observed on A. stolonifera in the central U.S. The pathogen also causes zonate leaf spot in numerous other Poaceae, which is also an important foliar disease on sorghum.

Cultural Control

As with other foliar diseases, minimizing leaf wetness duration through early morning mowing, irrigation, dragging a hose, or dew-whipping can aid in preventing copper spot development. In active infections, however, do not mow or move morning dew across the green as this will aid in conidia spread. Also limit the use of water-soluble nitrogen sources when copper spot is active. Maintaining a soil pH between 6 and 7 is ideal to suppress copper spot.

Host Resistance

No information on resistant cultivars is available.

Diagnostic procedures

Copper spot and dollar spot may occur simultaneously on Agrostis. Copper spot, however, is a darker brown to rust colored, while dollar spot symptoms are typically a bleached color. Copper spot stand symptoms may also resemble red leaf spot and the initial stages of brown patch. The most characteristic diagnostic feature of copper spot is the numerous filiform shaped spores, which are formed en masse on leaves and easily found when examined under a compound microscope. If the sample is dry, an overnight incubation in a humidity chamber at room temperature will cause pathogen sporulation.

Acknowledgments

• National Institute of Food and Agriculture, U.S. Department of Agriculture, under Agreement No. 2011-41530-30818 as part of "Diagnostic Image Series Development for Supporting IPM in the North Central Region" (USDA-NIFA-RIPM-003349)