Gray Leaf Spot

Disease

Pyricularia grisea


Gray leaf spot is considered a problem on primarily perennial ryegrass and St. Augustinegrass. The disease first appears as tiny brown spots that enlarge and become oval or elongated. The mature spots usually have depressed gray centers with irregular brown margins, and a ring of chlorotic tissue surrounding. Severe infection shows burned or scorched blades of turfgrass. Lesions are also found on the leaf sheath, spike and stems. Stem lesions tend to be brown to black.

The disease is most severe during warm, humid weather. The pathogen overwinters as spores and dormant mycelium in the lower leaves of infected plants and thatch. When temperatures rise, the spores are spread by wind and water to healthy plants. Under conditions of high humidity and high temperature (80° to 90° F [26° to 32° C]), the disease may develop rapidly. Newly established stands of St. Augustinegrass are more susceptible to gray leaf spot than mature turfs.

  • Crops Affected: Turfgrass

    Damage

    The location of this disease is home lawns and golf courses. General symptoms are wilting and irregular coloring. Foliar symptoms are browning, and spotting. This can occur during the months of June, July, and August. Perennial ryegrass and St. Augustinegrass are highly susceptible to gray leaf spot.

    Management

    Avoiding excessive nitrogen applications before and during the time of year when gray leaf spot is a problem can aide in suppression of the disease. Applying irrigation in the middle of the day will allow foliage to dry quickly and thus help reduce the severity of the disease.

    Certain cultivars of St. Augustinegrass are more resistant than others; however, there seems to be no cultivars of perennial ryegrass resistant to gray leaf spot.

    Gray leaf spot can be managed with the QoI, DMI, benzimidazole, and chlorothalonil fungicides. The QoI fungicide azoxystrobin is the best curative fungicide for the control of gray leaf spot once it has become a problem. The other fungicide classes work best if applied preventively. Applications of chlorothalonil must begin in the late spring and continued on a biweekly basis throughout epidemic period to be effective.