Bipolaris & Exserohilum (Helmintho-sporium) Diseases.
Texas Cooperative Extension
Text and images copyright © Richard Duble.
The Bipolaris and Exserobilum spp. of fungi were once grouped
under the name Helminthosporium. Warm season turfgrasses are susceptible
to various diseases caused by Helminthosporium spp. Leaf spot, crown
rot, root rot and other diseases are incited by Helminthosporium
Symptoms. On bermudagrass, small purplish to black spots may appear on the leaves, stems or crowns of infected plant. On leaves, these spots increase in size, the centers turn brown and fade to a light tan with purplish borders. Severely infected leaves may die and appear light tan to straw-colored. During warm (60-85°F), moist conditions leaf blighting may occur as a rapid collapse and drying of the leaves. Under favorable conditions, the disease may complete this whole cycle in a period of several days. The overall pattern appears as brownish fading-out of turfgrass areas of various sizes.
On bermudagrass stems, symptoms begin as small purple spots on stems and crown. Leaves close to the stem lesions become chlorotic, but may not show any spots or lesions. Diseased areas become stunted and spindly.
On bentgrass, the disease first appears as irregularly shaped, smoky blue areas varying from 1 to 4 feet in diameter. Soon after these areas appear, yellowing and complete killing of the grass occurs. Finally, these areas appear water-soaked and matted down. On leaves, the first symptoms are small yellow flecks that progress to irregularly shaped water-soaked blotches.
Disease Cycle. The disease-causing fungi survives the winter as mycelium in the tissue of infected plants and as spores in the thatch layer of the turfgrass. As the temperatures warm in the spring the disease first appears as small spots on the leaves or stems. The severity of the disease increases with temperature and humidity. At 78°F a period of 8 to 10 hours with 100% relative humidity is all that is required for a high level of infection to develop.
The severity of these diseases seems to be greater when grass is maintained at low levels of nitrogen and potassium. Also, any stress situation such as drought, herbicide injury or heavy traffic seems to increase the severity of the disease.
Control. Healthy turfgrass is the best protection against this turf disease. Maintain vigorous turf through proper fertilization. Give special attention to maintaining adequate levels of nitrogen and potassium. Avoid excessive applications of nitrogen.
Try to reduce stress during periods favorable for disease development. Do not allow the turf to become extremely dry during warm weather. At the same time, good surface and internal drainage is important during wet conditions. Avoid herbicide applications during critical periods of disease activity. Disperse traffic in high traffic areas such as golf greens and soccer fields by moving the cup and goals frequently. Also, raise mowing heights slightly to reduce stress.
During periods of high disease activity, fungicides can protect the turf from severe injury by Helminthosporium diseases. Preventive applications of fungicides on a regular schedule is much more effective than treating a severe outbreak of any Helminthosporium disease. During periods of high disease activity, applications of fungicides at 7 to 14 day intervals may be required.