What disease produces small, purple-to-reddish-brown circular lesions on grass blades that may enlarge and girdle the leaf?

The disease characterized by small, purple-to-reddish-brown circular lesions on grass blades that may enlarge and girdle the leaf is caused by a fungal pathogen known as Helminthosporium. This organism is well-known for its ability to infect a variety of grass species, and the symptoms it produces are indicative of a disease called Helminthosporium leaf spot or brown patch.

The lesions start as small spots but can eventually expand, leading to significant damage to the affected leaves. The girdling effect occurs as these lesions grow, cutting off the sap flow and potentially leading to the death of the affected leaf tissue. This progressive destruction is a key identifying feature of the disease when diagnosing grass problems.

In contrast, red thread typically appears as pink or red fluffy growth on grass and usually does not cause the same circular lesion pattern. Leaf rust is characterized by yellow-orange pustules that eventually produce powdery spores but does not create brown lesions in the same manner. Pythium blight results in a different type of damage, often causing a rapid, wet decay of grass without the specific circular lesions associated with Helminthosporium. Understanding these distinctions is crucial for proper diagnosis and management of turfgrass diseases.