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Publication #SS-PLP-9

`Helminthosporium' Leaf Spot1

M. L. Elliott and G. W. Simone2

`Helminthosporium' Leaf Spot

  • Pathogen(s): Bipolaris, Drechslera and Exserohilum spp. (previously classified as Helminthosporium fungi)

  • Turfgrasses Affected: All warm-season turfgrasses, but it is usually most serious on bermudagrass. Different species of these fungal pathogens affect different species of turfgrass.

  • Occurrence: This disease includes a group of fungi that are active over a wide range of temperatures. At any given time of the year, at least one species within this fungal group can be isolated. Thus, diseases caused by these fungi can occur at any time of year. However, as a general rule, the leaf spot disease occurs during mild, wet periods in fall through winter. Again, these diseases occur most frequently on bermudagrass.

  • Symptoms/Signs: Leaf spot symptoms tend to vary with each pathogen/host pair from very small (pinhead size), solid brown to purple color lesions or spots to expanded lesions with bleached centers that girdle the leaf blade (Plate 18). Severely infected leaves turn purple or reddishcolor, giving the turf an overall purple cast. Severely infected leaves will eventually wither and dry to a light tan color. Distinct patches or patterns to the disease are usually not obvious. "Meltingoccurs under severe infections as turf areas thin and die. Lesions on stolons are dark purple to black.

Plate 18. 

Example of 'Helminthosporium' Leaf Spot symptoms on bermudagrass.

  • Cultural Controls: Avoid excess nitrogen during potential disease development periods. The nitrogen level must be balanced with potassium. In areas that are affected routinely by this disease, increase the potassium level prior to the time the disease normally occurs. A ratio of 1:2 (N:K) would be recommended. Use slow-release potassium sources or apply quick-release potassium sources more frequently.

  • Overuse of certain herbicides (MCPP, 2,4and dicamba) have been shown to enhance 'Helminthosporium' disease development on coolgrasses. Note if there is a correlation between disease outbreaks and the use of certain pesticides. If a correlation is determined, then avoid using that particular pesticide or treat preventively for 'Helminthosporium' disease prior to the use of that pesticide.

  • Chemical Controls: azoxystrobin, chlorothalonil, iprodione, mancozeb, myclobutanil, PCNB, propiconazole, trifloxystrobin, vinclozolin.

  • Mancozeb can be applied to a residential lawn only by a professional pesticide applicator. Chlorothalonil and vinclozolin cannot be applied to a residential lawn, but it can be applied to turfgrass in a business or industrial landscape.

Footnotes

1.

This document is SS-PLP-09, a series of the Plant Pathology Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. This information is included in the Florida Lawn Handbook, SP-45. For a copy of this handbook, request information on its purchase at your county extension office. First published: May 1991. Revised: April 2001

2.

M. L. Elliott, Associate Professor, Plant Pathology, Fort Lauderdale Research and Education Center, and G. W. Simone, Emeritus Professor, Plant Pathology Department, Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, FL 32611.

The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For more information on obtaining other extension publications, contact your county Cooperative Extension service.

U.S. Department of Agriculture, Cooperative Extension Service, University of Florida, IFAS, Florida A. & M. University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Millie Ferrer-Chancy, Interim Dean.