Sclerotinia homoeocarpa (dollar spot of turfgrasses)

Taxonomy

Domain	Eukarya
Kingdom	Fungi
Phylum	Ascomycota
Subphylum	Pezizomycotina
Class	Leotiomycetes
Subclass	Leotiomycetidae
Order	Helotiales
Family	Sclerotiniaceae
Genus	Sclerotinia

Scientific Name

Sclerotinia homoeocarpa

Common Name

dollar spot

Author: Lee Miller, University of Missouri

Reviewed by:Name, Organization

Pathogen

Since it’s first description in Britain in 1937, Sclerotinia homoeocarpa F.T. Bennett is the currently recognized name for the pathogen causing dollar spot. The genus Sclerotinia is known for producing tuber-like sclerotia, which is not a demonstrated character of the dollar spot pathogen in its current state. Hence, taxonomists have separated it from the genus, and since the teleomorph is rare in nature, are utilizing phylogenetic evidence to rename the organism. In nature, the pathogen exists as sterile mycelia, which in culture are characterized as fast growing and floccose, and turns shades of olive, grey or brown with age. A flat, black or dark brown stroma is produced in aged cultures, which may run vertically through media and form sectors.

Symptoms and Signs

Dollar spot symptoms can appear slightly different depending on the host and corresponding mowing height. Symptoms on lower cut, high amenity turfgrasses such as creeping bentgrass are characterized by small, (~ 5 cm diameter) round spots which are bleached or straw-colored. These small spots eventually coalesce and destroy larger areas of turf if not treated. Greyish-white mycelium is often observed in the morning while the grass is still wet from dew. On higher mown turfgrass, individual patches are somewhat larger (~ 15 – 30 cm diameter), and straw colored. Individual lesions on grass blades may be white to light brown with reddish brown margins. On some cool-season species, dollar spot lesions will span the entire width of the leaf blade, resulting in an hourglass shape. Leaf lesions on warm-season species are oblong- or oval-shaped and separated from healthy tissue by the reddish brown border.

Ecology and Spread

S. homoeocarpa survives unfavorable conditions as mycelium in infected plants and plant debris, or as stroma on the leaf surface. Dollar spot can occur through much of the growing season. Highly favorable conditions occur during periods of high humidity, day temperatures between 60 and 90°F and cool nights that produce heavy dews. Inoculum is commonly spread as mycelia in infected leaf debris transported by people, animals, equipment, water, and wind.

Geographic Distribution

Dollar spot is widespread globally and common in most areas, except for very arid regions.

Management

Nitrogen fertilization, moisture management, and minimizing leaf wetness duration can play significant roles in managing this disease. In high amenity turfgrasses, chemical control may also be necessary to supplement these practices.

Cultural Control

Dollar spot severity and incidence is greater in under-fertilized turfgrasses, so nitrogen applications may aid in reducing this disease. This is particularly true of warm-season species, as nitrogen fertilization may be all that is required to recover from an outbreak. Care must be taken, however, when fertilizing cool-season turfgrasses, as other diseases such as Pythium or brown patch may be favored. Drought-stressed turf is more susceptible to dollar spot, so care must be taken to provide enough water to the plant. In contrast, leaf wetness is a key component in the infection process, therefore irrigating early in the morning is recommended to minimize duration of the leaf wetness event. Methods of removing morning dew and guttation fluid, including mowing, rolling, pole- whipping, increasing air movement, or using fans, are suggested cultural controls. Biological controls, including strains of Pseudomonas auerofacians, Trichoderma harzianum, Bacillus spp., and others, may provide adequate dollar spot suppression during periods of light or moderate disease pressure, but do not provide satisfactory control in conditions promoting high disease severity.

Chemical Control

Many contact and systemic fungicides in the benzimidazole, dicarboximide, demethylation inhibitor (DMI), oxidative phosphorylation uncoupler, nitrile, and succinate dehydrogenase inhibitor (SDHI) classes can provide dollar spot control. Pathogen populations with demonstrated field resistance to the benzimidazole, dicarboximide, and DMI classes are known to occur. Strategies aimed at delaying or preventing fungicide resistance such as alternating and tank-mixing of effective chemistries should be employed.

Host Resistance

While no completely resistant variety developed, the degree of disease incidence among cultivars may vary greatly. Several new bentgrass cultivars are much less susceptible than older varieties. Several cultivars are also considerably more susceptible to dollar spot. The National Turfgrass Evaluation Program website (ww.ntep.org) hosts information regarding disease severity conducted in variety trials.

Diagnostic procedures

Dollar spot leaf and stand symptoms may be confused with other diseases such as copper spot, brown patch, red thread, or Pythium blight. Disease detection enzyme-linked immunosorbent assay (ELISA) kits have been used to identify the pathogen in plant tissue. Since dollar spot does not produce spores, mycelial characteristics are most often used for diagnosis. After incubation in a humidity chamber for 24 h, floccose mycelium will often emerge from leaf lesions. Hypha of S. homoeocarpa are differentiated from other pathogens that commonly do this (Pythium and Rhizoctonia) in that it is non-pigmented, septate, forms a Y-type instead of right angle branch pattern, and generally has a larger diameter.

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)