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
fungi.
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.