Cause Drechslera spp. and Bipolaris spp. are fungi that survive as conidia and dormant mycelia in infected plant tissue and debris. Cool season grasses such as tall fescue, perennial ryegrass, bluegrass, fine fescue, bentgrass, and orchardgrass are all affected. This is one of the more common leaf spot diseases of ryegrass and tall fescue. Conidia are spread by wind, rain, irrigation water, or equipment. Cool, wet spring weather favors the disease. Conidia are produced at temperatures from 37°F to 80°F (59°F to 64°F is optimal). Moisture films on leaf surfaces are necessary for infection. Leaf spots generally occur when conditions are cool and wet, but some species are active during warm, moist periods.
Symptoms Leaf spot symptoms vary by host grass. In general, lesions first appear small and water soaked. Later, lesions become uniformly dark, reddish brown to purplish black, and elongated.
Lesions on tall fescue and perennial ryegrass may lie parallel and perpendicular to the leaf axis, resulting in a net-like appearance; hence the name "net blotch." Lesions may coalesce to form dark brown, solid spots. Heavily infected plants progressively die back from the tips, which can result in leaves' turning all brown.
In fine fescue, lesions appear as small, reddish brown, irregularly shaped spots. Lesions girdle leaves, causing yellowing and dieback from the tip. Lesions also attack the stem, killing the head.
Besides leaf blights, Drechslera spp. also cause crown and root diseases. These do not seem to be widespread in grass seed fields.
Chemical control In most years, control is not necessary. Consider applying if upper leaves, especially flag leaves, are heavily infected and if prolonged cool, rainy weather is forecast.
- Chlorothalonil products such as:
- Bravo Ultrex at 0.9 to 1.4 lb/A on 14-day intervals. Do not apply within 14 days of harvest. Do not allow livestock to graze in treated areas or feed treated plant parts to livestock. 12-hr reentry.
- Echo 720 at 1 to 1.5 pints/A or Echo 90DF at 0.875 to 1.25 lb/A on 14-day intervals. Do not apply within 14 days of harvest. Do not allow livestock to graze in treated areas or feed treated plant parts to livestock before harvest. 12-hr reentry.