Gray leaf spot
Host range
Both Cercospora zeae-maydis and Cercospora zeina infect only maize. Pyricularia grisea has been known to only infect perennial ryegrass, and tall fescue as well as St. Augustinegrass in places with warm and rainy climates such as Florida.
Symptoms
Early symptoms of gray leaf spot can be seen on leaves as small, spherical lesions with a yellow halo around them. These first lesions may be tan or brown before fungal sporulation begins. The initial disease can be hard to identify as gray leaf spot at this stage because it looks similar to eyespot and common rust. However, as the lesions mature, they elongate into rectangular, narrow, brown to gray spots that usually develop on the lower leaves and spread upward on the plant during the season. The lesions elongate and expand parallel to the leaf veins and can be 1.5 to 2 inches long. With favorable weather the lesions can rapidly merge and kill the entire leaf. Mature gray leaf spot symptoms can also be confused with symptoms of anthracnose leaf blight.
Management
If available, choosing a maize hybrid that is resistant to gray leaf spot is the most cost-effective way to control the disease. No maize hybrid is completely immune to gray leaf spot, but there are varieties that have very good levels of resistance. Most seed companies have a gray leaf spot resistance rating for their hybrids that are available. As far as controlling Pyricularia grisea, ryegrass, tall fescue, and St. Augustinegrass are all fairly susceptible to gray leaf spot and neither of them are considered resistant. Planting other types of grasses such as Kentucky bluegrass, bentgrass, or annual bluegrass would be a lot more beneficial as they are not affected by gray leaf spot. Managing crop residue is another important thing to keep in mind when trying to control gray leaf spot. Continuous corn and a no-till or reduced tillage system are at a very high risk for disease development because of the amount of residue being left on the soil surface. Rotating crops away from corn followed by tillage is recommended to prevent disease in the subsequent crop. In fields with a history of grey leaf spot with a no-till system, a two-year rotation may be needed. Using fungicides effectively can also reduce the amount of diseased crops and help stop the spread of gray leaf spot. Some research has shown that strobilurin and strobilurin/triazole fungicides are the most effective at preventing yield loss from the disease. Fungicides should be applied when lesions are first seen on basal leaves. A secondary application may be needed 30 days after the first application depending on the weather conditions.