Gray Leaf Spot 2004 – Turfgrass Science at Purdue University

Gray Leaf Spot 2004

We’ve received several reports of gray leaf spot outbreaks throughout the state during the past couple of weeks. It may surprise many, because the summer of 2004 has been one of the mildest (zero days where temperatures reached 90 F in West Lafayette) in recent memory. It did not surprise us, however, because our research results suggest that disease will develop over a broad range of environmental conditions (not just hot and wet). We also learned that winter conditions influence pathogen survival and will have a significant effect on gray leaf spot development during the following summer. In other words, the presence of the inoculum may be the limiting factor with this disease, rather than the moisture and temperature conditions that occur during the summer. After all, the pathogen population does not regularly survive midwestern winters. Despite cool nights and moderate daytime temperatures, an outbreak occurred in ryegrass plots at our turf research center this year, but only after we inoculated the plots in mid-August. A report of gray leaf spot development also was issued from the University of Kentucky during the last week in July. Most would admit that the disease was unexpected because of the unusually cool summer.

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 because the pathogen is dispersed by windblown spores. 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 (Cleary’s 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 at Purdue (765-494-4639).

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