There have been several reports of gray leaf spot outbreaks throughout Indiana and the lower Midwest during the past two weeks. The disease will develop over a broad range of environmental conditions (not just hot and wet). Our research results showed that the presence of inoculum is the disease-limiting factor north of the Ohio River. After all, the pathogen population does not regularly survive midwestern winters, unless they are very mild (just like last winter). Outbreaks should be expected in southern Indiana, but the threat to perennial ryegrass in the northern third of the state is limited. In between, disease occurrence will be sporadic. Those stands of perennial ryegrass that had significant gray leaf spot problems in 2005 should be watched most closely, because it is quite likely that the pathogen population survived the winter in numbers sufficient to threaten turf this summer. Once the disease is identified in a given area, other perennial ryegrass stands (mostly juvenile rye) are at risk. Newly seeded ryegrass will be under the greatest threat. Several fungicides may be effective in suppressing disease development including the strobilurin products (Heritage, Compass, Insignia) and thiophanate methyl (Cleary’s 3336 and others). Chlorothalonil and the DMI fungicides a somewhat less effective.
Extensive information on gray leaf spot can be found at http://www.extension.purdue.edu/extmedia/BP/BP-107-W.pdf. If you suspect an outbreak of gray leaf spot has occurred on your course, please contact Rick Latin at rlatin@purdue.edu or 765-494-4639.