Leaf Spot

Disease

Bipolaris sorokiniana


Symptoms first appear as small dark purple to black colored spots on the leaf blade. As the spots enlarge, the centers often turn light tan. In warmer temperatures (> 85° F), the entire blade often appears dry and straw colored. The disease is mostly confined to the leaf blades during the cool weather, but can infect leaf sheaths, crowns and roots during hot, humid weather.

Leaf spot is a warm-weather disease, but the pathogen overwinters as dormant mycelium in infected plants and dead grass debris. Leaf spot can become evident when temperatures reach 70° F (21.1° C). The disease is most severe when temperatures are above 90° F and humidity is high. Conditions of drought stress followed by rewetting intensify the disease.

  • Crops Affected: Turfgrass

    Damage

    The location of this disease is home lawns and golf courses. General symptoms are spotted or wilted grass. Foliar symptoms are browning and spotting. This can occur during the months of May, June, July, August, and September. Hosts of the disease are Kentucky Bluegrass, Perennial Ryegrass, Tall Fescue, Fine Fescue, Creeping Bentgrass, Annual Bluegrass, and Colonial Bent.

    Management

    When leaf spot is active, nitrogen levels should be kept moderate to avoid overstimulation of growth. Light, daily, midday irrigation applications, avoiding drought stress when the pathogen is active, will help to reduce or even prevent development of the disease. Avoid irrigation during the late afternoon and early evening during summer months. Genetic resistance to leaf spot exists in newer varieties of Kentucky bluegrass. These cultivars can be used to overseed damaged areas.

    Both contact and systemic fungicides can be used to manage leaf spot. Contact fungicides like chlorothalonil and maneb, must be applied every 7-10 days to be effective. Systemic fungicides like iprodione and the QoI's, can be applied on 14-21 day schedules. Timing is crucial, and preventative fungicide applications should be made just before the disease season begins, or at the onset of early disease symptoms.