Gray leaf spot is a foliar disease that affects perennial ryegrass and tall fescue. It is caused by a fungal pathogen (Pyricularia grisea) that readily infects and kills leaf blades. Leaf infections can progress into the crown area, resulting in death of individual plants. Moderate outbreaks of gray leaf spot result in clusters of thin, off-colored turf. However, severe outbreaks will result in the death and decay of extensive areas and ruin the entire turf stand.
Gray leaf spot poses less of a threat than 10-15 years ago, primarily because of the reduction in the acreage of perennial ryegrass in the Midwest and the introduction of cultivars with some gray leaf spot resistance. However, there are cases where turf managers must be vigilant in addressing the threat of this year.
At this time of year, other leaf spot diseases can be confused for gray leaf spot. If you manage perennial ryegrass or tall fescue, obtaining an accurate diagnosis is essential. Otherwise, you may bear an unnecessary expense with costly fungicide applications. Observing a few leaf spots or the “fish hook” symptom is not enough to draw the conclusion that gray leaf spot is present. The only sure sign is the presence of very characteristic conidia (spores) that can be viewed only microscopically.
For individuals interested in chemical control, the most effective fungicides are QoI (strobilurin) products such as Heritage, Insignia and Disarm. Thiophanate-methyl (Cleary 3336) also is very effective. Chlorothalonil and DMI fungicides have limited efficacy but may be useful when disease pressure is low.
Details on gray leaf spot are available online at www.extension.purdue.edu/extmedia/BP/BP-107-W.pdf.