Gray Leaf Spot Update – Turfgrass Science at Purdue University

Gray Leaf Spot Update

There have been numerous reports of gray leaf spot outbreaks throughout Indiana and the lower Midwest during the past three weeks. We generally expect that hot humid conditions will favor gray leaf spot development. However, you may recall that outbreaks also occurred during the summer of 2004, a year in which temperatures reached 90 F in West Lafayette only twice. I just returned from California and observed gray leaf spot symptoms on ryegrass and kikuyu grass where daytime temperatures were mild and nights were cool. This does not surprise us because our research results suggest that gray leaf spot will develop over a broad range of environmental conditions (not just hot and wet). It is likely 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. The pathogen is dispersed via airborne spores, so it is quite possible that inoculum was introduced into the Midwest this year by the remnants of hurricane Dennis in mid July.

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 the past should be watched most closely, in the event that the pathogen population survived the winter in tact. Once the disease is identified in a given area, other perennial ryegrass stands (mostly juvenile rye) are at risk. At this time, newly seeded ryegrass is under the greatest threat. Several fungicides may be effective in suppressing disease development including the strobilurin products (Heritage, Compass, Insignia) and thiophanate methyl (Cleary¹s 3336 and others). Chlorothalonil and the DMI fungicides a somewhat less effective.

If you suspect an outbreak of gray leaf spot has occurred on your course, please contact Rick Latin <http://www.btny.purdue.edu/Faculty/Latin/> at Purdue (765-494-4639).


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