Cause Septoria spp. (teleomorph: Mycosphaerella) or Stagonospora spp. (teleomorph: Mycosphaerella), fungi that survive winter and summer as mycelia or pycnidia (minute, black, flask-shaped fungal structures) in infected debris. These fungi attack a broad range of hosts, including bentgrass, bluegrass, fescue, and ryegrass. Bluegrass is more susceptible than other grasses. Many species of Septoria exist and may cause leaf spots on other grasses as well.
Conidia reproduce from pycnidia in cool, wet periods in spring and autumn. Rain splashes conidia to healthy tissue. Some of these fungi also have a sexual stage, produced in perithecia on straw in spring and fall. Also, spores from the straw can be airborne for long distances.
The disease is most common in cool, wet weather. Infection requires a film of water on the leaf surface. Septoria spp. have not caused extensive damage on tall fescue and perennial ryegrass seed crops. Serious damage has occurred in bluegrass occasionally.
Symptoms Leaf spots vary in color and could range from gray to dark purple. Septoria spp. can cause leaf spots, streaks, or blotches. As lesions enlarge, they become straw-colored, and leaf tips may become chlorotic and mottled. Small, dark pycnidia appear in the dead areas of older lesions.
Chemical control It is rare to need chemical application on any grass but bluegrass.
- 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.