Last week at the UMass Plant Diagnostic Lab, fungal disease caused by Drechslera species was diagnosed from fairways at two golf courses in Berkshire County. Diseases caused by Drechslera and Bipolaris are sometimes referred to collectively as “melting-out” or “the Helminthosporium diseases.” These diseaes may manifest as leaf spots, melting out, and/or crown and root rot, depending on weather conditions and the species of pathogen and host.
When leaf spot is severe, whole leaves and tillers can die, producing the effect known as melting out. On bluegrass, leaf spot is generally considered a cool weather phenomenon, while melting out may be seen in cool or warm temperatures and crown and root rots tend to appear in warmer weather. On bentgrass, red leaf spot is generally a warm weather disease, but crown and root rots may occur in cool or warm conditions. With the wild temperature swings we’ve seen in the Northeast in the past 2-3 weeks, it would not be surprising to see either category.
Several different species of Drechslera cause disease on cool season turfgrasses. Drechslera erythrospila is associated with red leaf spot on Agrostis. Symptoms of red leaf spot often appear in random, irregular patches. Spots frequently coalesce and give the turf a reddish tint. Lesions may girdle leaves and cause wilting.
In addition to red leaf spot, two other species of Drechslera, D. catenaria and D. gigantea, also cause crown rot of Agrostis. Infected areas may be as small as 1 inch at first, but may coalesce as they spread and cover a large area. D. catenaria is more active at cooler temperatures, while D. erythrospila and D. gigantea occur in warmer weather.
Drechslera poae causes melting out in Kentucky bluegrass, although many modern KBG cultivars are resistant. D. poae causes leaf spot and melting out at cooler temperatures and melting out, root, rhizome, and crown rot at warmer temperatures; infection of stems, roots, and rhizomes occurs spring through autumn and produces a dry rot. Conidia are produced at 38-80° F, with an optimum of 58-64° F. Free moisture is needed for infection. Leaf spots therefore occur primarily when weather is cool and wet.
D. poae causes leaf lesions that range in color from yellow to purple to black. Lesions may girdle leaves and cause wilting. The pathogen invades leaves and travels into the crown.
Regardless of the causal species, prevention and management of Drechslera and Bipolaris is the same. Raise mowing height if possible. Reduce thatch to a maximum of 0.5". Removal of excess thatch is crucial because it serves as a reservoir for these pathogens. Supply adequate nitrogen, but avoid overfertilization, especially in spring and early summer. Water deeply and infrequently. Many modern cultivars of Poa are resistant to Drechslera, so it is more likely to be seen on sites with older cultivars. There are no resistant cultivars of Agrostis. The herbicides prodiamine and pendimethalin have been reported to exacerbate diseases caused by Drechslera. Avoid overuse of DMI fungicides as their growth regulating effects can hinder turf recovery. For more information on fungicides for prevention and treatment of Drechslera, please see the Disease Management chapter of our Professional Guide for IPM in Turf.
Submitted by: Dr. Angela Madeiras