Necrotic Ring Spot (fungus – Leptospheria korrae)
Host Grass: Bentgrass(Agrostis palustris), Fescue, Poa Series(Poa sp.)
Cause and Symptoms: Circular patches over a foot in diameter appear in the spring, fade with warmer temperatures and then reappear with heat and drought stress. Initially, leaves are purplish colored and wilted. Plants die and turn straw colored. Because roots, crowns and rhizomes are rotted, plants are easily removed from turf. Two- to three-year old patches may have a frog-eye appearance where plants have survived or recolonized the middle of the affected area.
Control and Management: The fungus is active in the cool wet weather of spring and fall. Adequate fertilization is required. Complete fertilizers containing phosphorus and potassium as well as slow release nitrogen carriers are the most effective. Daily irrigation, applied midday, cools the turfgrass and allows infected plants with depleted root systems to survive the late afternoon heat stress. Early spring applications of fungicides (See the section Chemical Controls for Turfgrass Diseases) reduces severity but may not completely control the disease. Fungicides need to be drenched into the soil before they dry on the foliage because this is a root disease. DMI fungicides like fenarimol and propiconazole, which when used at the high rate necessary to control necrotic ring spot, slow down plant growth. So while they may control the fungus, recovery of the turf is not evident.
Content edited by: Young-ki Jo, firstname.lastname@example.org , Assistant Professor and Extension Specialist, Dept Plant Pathology & Microbiology, Texas A&M University, Texas AgriLife Extension Service, May 31, 2013