Gray leaf spot of perennial ryegrass

Ned Tisserat   Extension Specialist, Plant Pathology     

Symptoms

Gray leaf spot, also called blast, was first observed on perennial ryegrass throughout the eastern half of the United States in 1992. The epidemic resulted in extensive damage to golf fairways and athletic fields. The disease now is established in most areas of the country where perennial ryegrass is grown. Disease development in our region is sporadic with little or no disease development in some years. Nevertheless, the potential destructiveness of gray leaf spot forces most turfgrass managers to apply preventive fungicide applications every year.

Gray leaf spot symptoms initially develop in early- to late-August. Small, chocolate brown spots approximately 1/16 inch in diameter develop on leaves. During early disease development these leaf spots may not be numerous and can be easily overlooked. The spots expand slightly with age and develop a gray center surrounded by a yellow halo (Figure 1). Depending on weather, multiple coalescing leaf spots will girdle and kill both leaf and crown tissue. Dying leaves tend to develop a characteristic twist at the leaf tip that helps differentiate this disease from brown patch or Pythium blight.

Figure 1. Gray leaf spot of perennial ryegrass (photo courtesy D. Settle).

Gray leaf often develops first in heat or drought prone areas such as the tops of small mounds or on steep slopes (Figure 2). The diseased turf first appears droughty even though soil moisture is sufficient, but soon collapses and turns a dull brown. The fungus may quickly spread from these diseased patches and cause extensive blighting provided weather conditions are favorable for infection (Figure 3). In some years the disease never progresses beyond blighting in these 'hot spots'. Other turfgrass species including Kentucky bluegrass, annual bluegrass, and creeping bentgrass that may be mixed in with the ryegrass are not affected by the disease. Blighting may continue through late summer and into early fall and cause significant damage to young ryegrass seedlings emerging from fall overseeding.

Figure 2. Gray leaf spot of perennial ryegrass.

Figure 3. Gray leaf spot on a perennial ryegrass fairway.

Gray leaf spot may also occur on tall fescue although it is rare in Kansas and Missouri. Symptoms are similar to those on perennial ryegrass but disease progression is slower and damage is much less severe. Leaves initially develop small dark brown spots that eventually expand and develop a gray center (Figure 4). The spots rarely coalesce to kill leaf blades or plants.

Figure 4. Gray leaf spot on tall fescue.

Conditions

Very little is known about the disease cycle of the gray leaf spot. The fungus probably survives the winter locally in leaf debris or perhaps on living perennial ryegrass plants although influx of spores from distant sources may also occur in some years. The fungus apparently infects perennial ryegrass plants at non-detectible levels in early summer then builds in intensity later in the growing season. Factors that eventually trigger epidemics are not completely understood, but high temperatures, extended periods of leaf wetness interspersed with intermittent dry periods favor disease development. The fungus sporulates profusely from leaf lesions and vast quantities of spores can be spread by wind, splashing water and equipment to infect new leaves (Figure 5). New leaf symptoms appear within a few days of infection. The combination of quick symptom development and massive spore production are reasons why gray leaf spot epidemics progress rapidly and are so destructive.

Figure 5. Spores (conidia) of the gray leaf spot fungus.

Control

Certain cultural practices may help reduce gray leaf spot severity. Avoid excessive nitrogen fertilization during the late spring or summer months. Apply no more than 0.1 to 0.25 pounds of nitrogen per 1,000 square feet from spring greenup through Labor Day. Time irrigations to minimize leaf wetness periods but avoid letting the turfgrass go under drought stress. Mowing height apparently does not have a significant effect on severity of gray leaf spot.

Cultural practices alone are unlikely to control gray leaf spot. Fungicide applications beginning in early- to mid- August and continuing through September at regular intervals may be required to prevent damage on golf course fairways and athletic fields. Although thiophanate methyl and the QoI fungicides (azoxystrobin and trifloxystrobin) provide the best control of gray leaf spot, their use should be limited to one or possibly two applications per season to prevent the selection of fungicide resistant strains. Instead, rotate with other fungicides listed in the table.

14-28 days

excellent

7-14 days

good

10-14 days

good

14 days

 fair

14 days

good

14-21 days

excellent

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Web updated 9/01/06
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