This is either one of the fungal leaf spots or a rust disease on grasses. Without a lab culturing it’s hard to know for sure which is, and impossible from just a photograph. But what promotes these diseases, and how they are treated once you have them, is the same. Frequently splashing of grasses from automatic irrigation systems is the most common cause of leaf spot diseases on Cape Cod. If you are watering your plants, water deeply less often. A deep soaking every five to seven days so that a rain gauge fills to 3/4 to an inch is best. Water in the morning so that the foliage dries off during the day.
At the end of the fall or in early winter cut these grasses down to about a foot tall and either burn or throw the blades away – don’t toss them in the compost or you’ll be keeping more of the spores on your property.
Next year you might want to spray the new foliage early in the season with one of the organic fungicides to protect your plants from getting re-infected. Ask at your local garden center for a product recommendation.
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