You Are Here: Management Updates 2006 Archive August 22, 2006

Date: August 22, 2006
Category: Diseases
Subject: Gray Leaf Spot

The highly destructive disease of perennial ryegrass, gray leaf spot, caused by Pyricularia grisea, has been reported in Kentucky and Indiana. This disease also occurs on annual ryegrass and tall fescues, is favored by warm, humid conditions, and often occurs in late summer to early fall. The initial symptoms often appear to be drought stress; applying more moisture serves to make the problem worse. The pathogen kills the turf by causing a severe leaf blight, partly through the production of phytotoxic chemicals. The optimal temperatures for disease development are 82-90˚ F and epidemics may continue into October, subsiding after a heavy frost. Leaf wetness is also a critical factor in epidemic development.

Gray leaf spot is more serious in newly established plantings, in the shade, and on higher cut turf. Disease severity increases with nitrogen fertility; water-soluble sources of nitrogen promote both rapid growth and disease development. Monitor golf course roughs where disease is most likely to begin. Avoid excess nitrogen fertility and use slow-release forms. Minimize periods of leaf wetness by watering in mid-day and pruning trees and shrubs to reduce shade, improve light penetration, and increase air circulation. Reduce mowing heights. Replace ryegrass species with non-susceptible species. New perennial ryegrass cultivars are being developed with resistance and include Paragon GLR, Panther GLS, Palmer IV, and Repel GLS.

Where gray leaf spot has been diagnosed previously, begin preventive fungicide applications in mid-July. Some of the most effective fungicides are thiophanate-methyl (Cleary’s 3336™, Fungo 50™, T-Storm™), azoxystrobin (Heritage™), trifloxystrobin (Compass™), pyraclostrobin (Insignia™, F 500™), thiophanate-methyl & chlorothalonil (Spectro 90 WDG™), propiconazole (Propiconazole Pro™) & chlorothalonil, and triadimefon (Bayleton™) & chlorothalonil. Be sure to alternate chemical treatments by active ingredient class (FRAC code) to avoid the development of fungicide resistance.

Submitted by: M. Bess Dicklow

DISCLAIMER - As always, it is the responsibility of the applicator to verify the registration status of any pesticide BEFORE applying it. Different states have different regulations as well. The author and the University of Massachusetts are not liable for any consequences of any pesticide "recommendations". Mention of any trade name is not to be considered endorsement of a product.

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