Drechslera (Melting out/Leaf Spot of turfgrasses)

Taxonomy

Domain	Eukarya
Kingdom	Fungi
Phylum	Ascomycota
Subphylum	Pezizomycotina
Class	Dothideomycetes
Subclass	Pleosporomycetidae
Order	Pleosporales
Family	Pleosporaceae

Scientific Name

Drechslera

Common Name

Drechslera leaf spot

Author: Lee Miller, University of Missouri

Reviewed by:Name, Organization

Pathogen

Drechslera spp. are dothideomycetes in the order Pleosporales. Diseases caused by Drechslera spp. were once considered Helminthosporium leaf, crown, and root diseases, which also included Bipolaris, Exserohilum, and Marielliottia. With one exception, most Drechslera diseases occur on cool season grasses with pathogen species being specific to one particular host. The most prevalent Drechslera diseases include leaf spot/melting out of Poa by D. poae, red leaf spot of Agrostis caused by D. erythrospila, and net blotch of Festuca and Lolium by D. dictyoides. Drechlsera fungi produce dark brown mycelia, and copious dark brown, 3-10 septate conidia born on conidiophores that emerge from leaf tissue or mycelium along the leaf surface. Drechslera spp. can be speciated by conidial characters including the size, shape and number of septations.

Symptoms and Signs

Symptoms of Drechslera diseases on turfgrasses vary depending on the host species and mowing height. Initial symptoms of leaf spot on Poa are small, water-soaked lesions on the leaf blade. As the disease progresses to the melting out phase, lesions turn uniformly reddish brown to black (possibly surrounded by a chlorotic halo) and coalesce to blight entire leaves and leaf sheaths. Drechslera poae also invades crown tissue, roots and rhizomes of Poa pratensis, resulting in a dark brown “foot rot”. Net blotch symptoms on Festuca and Lolium may also occur on roots and crowns, but foliar symptoms appear as a number of small leaf lesions that form a longitudinal network of brown blotches or streaks across the leaf blade. Due to the lower mowing height, red leaf spot on Agrostis is normally first observed as small, reddish-brown spots (~ 5 cm diameter) irregularly distributed throughout a bentgrass stand. In heavy infections, these spots may coalesce to form larger areas of blighted turf. Individual leaf lesions are small, brown to reddish brown, circular or oval spots, which may have a tan-colored interior. This section contains images of symptoms grouped by host.

Ecology and Spread

Drechslera spp. overwinter as conidia or mycelium in the thatch layer and infected leaf tissue. The pathogens are active and sporulate at a wide variety of temperatures, with peak activity occurring when thatch temperatures range from 55-65°F. Except for red leaf spot, which occurs during warm, wet periods, Drechslera diseases are more frequent during cool, wet periods that correlate with peak sporulation. Airborne or water-splashed conidia are the main form of longer distance pathogen dissemination, while mycelium spread via traffic, machinery, or growth from plant to plant facilitate localized infections.

Geographic Distribution

Diseases caused by Drechslera spp. have been found worldwide and distribution is correlated with host presence. Leaf spot/melting out of Poa pratensis has been found in all regions where the host is utilized. Red leaf spot has been reported in the United States, Australia, and Great Britain. Net blotch is widely distributed in North America, Europe, Australia, and Asia.

Management

Management of Drechslera diseases on cool season turfgrasses centers on properly timed nitrogen applications, minimizing leaf wetness duration, mowing at appropriate heights, and proper cultivar selection.

Cultural Control

Leaf spot/melting out diseases are most severe on turf that is growing slowly due to environmental stresses. Nitrogen applications that cause excessive turf growth during the hot summer months can predispose turfgrass to these diseases and result in extreme infections. Areas that are extensively shaded, lack drainage and air movement, and are prone to long periods of leaf wetness are more prone to these diseases. Irrigation, mowing, rolling, or dew whipping early in the morning or using fans can minimize leaf wetness duration and reduce incidence of this disease. Leaf spot/melting out is particularly more severe on lower mown Poa pratensis, so raising mowing heights during infection periods is necessary to limit this disease.

Host Resistance

Several cultivars of Poa pratensis, including “Merion”, “Aldephi”, “Avid”, and “Midnight”, are available which are resistant to leaf spot/melting out (www.ntep.org). Resistant cultivars should be used alone or as part of a cultivar blend or species mixture to reduce leaf spot severity.

Chemical Control

Several contact and systemic fungicides in the benzimidazole, dicarboximide, demethylation inhibitor (DMI), dithiocarbamate, oxidative phosphorylation uncoupler, quinone oxidase inhibitor (QoI), nitrile, and succinate dehydrogenase inhibitor (SDHI) classes provide control of Drechslera diseases. These fungicides must be applied at regular control intervals during environmental conditions conducive to this disease.

Diagnostic procedures

Based on stand symptoms, several other diseases may be mistaken for those caused by Drechslera spp. For example, red leaf spot on bentgrasses may be confused with anthracnose, copper spot, or Pythium root rot. In most instances of severe infection, profuse sporulation of the pathogen also occurs. Observation of the dark, elongated, multi-colored spores in infected leaf tissue can be made with a hand lens in the field or with a dissecting microscope. Incubation for 12-24 h in a humidity chamber may also facilitate conidia production and diagnosis.

Acknowledgements

• 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)