Many homeowners are observing patches of dying grass that have suddenly appeared on their lawns. Although these symptoms can have many causes, the most common one is fungal disease. The diseases occur when the environmental and cultural conditions are favorable for their development. The wet cool weather of recent weeks has provided the ideal conditions for these diseases to become problematic.
The most common disease is brown patch, which afflicts cool season grasses such as tall fescue. The same fungus also attacks warm season grasses, such as bermudagrass and zoysiagrass, where it is called large patch. Both diseases are active when the nighttime temperatures are above 60 degrees, and the daytime temperatures are 80 degrees or more with high relative humidity.
Brown patch has rings of blighted turfgrass that vary in diameter with thin brown borders and a smoky appearance around the diseased area. The disease is most problematic in the summer. It affects the leaves and not the roots so the grass blades will come back up from the crowns starting in the center giving a doughnut shaped appearance. Lesions with tan centers and dark margins will appear on the individual leaf blades.
Large patch on warm season turf occurs mostly during the spring and fall, but can affect a lawn at any time when conditions are right. The main symptom is the development of large circular patches. Grass blades at the edge of the diseased area may exhibit an orange to gray color.
Large patch infection occurs on the leaf sheaths and the lesions have a water-soaked, reddish-brown or black appearance. The dieback starts at the tips and moves downward towards the base. The fungus thrives under certain cultural conditions. Excessive amounts of high nitrogen fertilizer promote the growth of soft, succulent grass blades that are more susceptible to the fungus.
Avoid or limit the applications of high nitrogen fertilizer to turfgrasses, especially during periods when the grass is most susceptible. On lawns where the foliage stays wet more than 10 hours a day for several consecutive days, these diseases can be more of a problem. Apply water early in the morning so the grass will dry out before evening. Make sure the lawn has adequate drainage.
Periodic mechanical dethatching or core aerification is needed to reduce thatch levels. Mowing the turf below the recommended height can increase the lawn’s susceptibility to the disease. Also, the mower blade should be sharpened before using it. Dull mower blades shred the ends of the grass blades increasing the likelihood of infection.
Using fungicides to control brown patch and large patch is difficult, but they do offer some level of protection. These chemicals suppress the development and spread of the disease, but they do not reverse the damage that is already visible. Continue applications at the recommended label intervals until the disease is controlled. As with any chemical pesticide, follow all label directions and safety precautions. Although chemical applications may help to suppress the spread of the disease, the control is only temporary. Long-term control requires altering the cultural conditions that favor its development.
Timothy Daly is an Agricultural and Natural Resource Extension Agent with Gwinnett County. He can be contacted at 678-377-4010 or firstname.lastname@example.org.