Fusarium blight is a devastating fungal disease of lawns and golf courses. It is a particular problem with Kentucky bluegrass. The classic symptom is a circle of dead grass up to 1 ft (0.3 m) across, with live grass in the center, giving a frog-eye appearance.
This lawn blight is caused by two different species of fusarium, F. tricinctum and F. roseum. They are primarily a problem on cool season grasses, such as Kentucky bluegrass, annual bluegrass, tall fescue, and bentgrass. They will, however, occasionally attack warm season grasses also.
The initial symptoms of fusarium blight are patches of light green that are 2 to 6 in (5.1 to 15.2 cm) in diameter. At higher temperatures, the color changes to reddish-brown, and eventually to the color of straw. As the fungus moves from the leaf blades into the crown and causes crown blight, the grass plants die. The patches usually then get larger, and develop the classic frog-eye pattern with live grass in the center.
Fusarium blight is most severe when day temperatures reach 85 to 95° F (29.4 to 35° C), and night temperatures are at, or over, 70° F (21° C), especially if the lawn is stressed from drought. Humidity exacerbates the problem, and areas in full sun are at the most risk. Over-watering and over-fertilization contribute to the susceptibility of the grasses. This grass blight occurs all over the United States, but is a particular problem in the mid-Atlantic states and the Mid-West, because of their hot and humid summers. In parts of the country with less humid summers, such as much of California, lawn care, such as irrigation practices, play a large role in the development of this disease.
These fungi can destroy a section of a lawn in a week, once spores start being actively produced, and preventative measures are very important to keep it from taking root. It is important to de-thatch the lawn in the fall, if the thatch buildup is greater than 0.5 in (1.3 cm). In addition to surviving the winter in infected grass, these fungi also live in the thatch.
The timing and schedule of watering is also very important. Irrigation should be thorough and infrequent. It should take place in the early morning, so the grass dries out before the day gets very hot. This is generally true for most types of lawn fungus. It is important to both not water the lawn excessively, and to let it dry out.
If fusarium blight is a potential problem in a Kentucky bluegrass lawn, it should be over-seeded with 20% perennial ryegrass. This will minimize the susceptibility of the lawn to this disease. One should keep the lawn well fertilized, but it is important not to over-fertilize with nitrogen fertilizer. Another preventative measure is to mow to a relatively high level of at least 3 in (7.6 cm). Also, there are newer cultivars of Kentucky bluegrass that are less susceptible to fusarium blight.
Once a lawn is infected, it is typically necessary to treat it with fungicides appropriate for the region. Systemic fungicides are most likely to work, but are not guaranteed to succeed. One should act quickly when the symptoms become apparent. These are most likely to start near the edges of sidewalks and driveways, where the grass will be heated more than in the in the center of a lawn.