IPM Identification and Mangement Of Turfgrass Diseases

Author(s)

Gray Leaf Spot

Pathogen: Pyricularia grisea
Hosts: Perennial ryegrass, tall fescue

Symptoms and signs

Gray leaf spot often develops first in heat or drought prone areas such as the tops of small mounds or on steep slopes. The diseased turf first appears droughty even though soil moisture is sufficient but soon collapses and turns a dull brown (Figure 1). The fungus may quickly spread from these diseased patches and cause extensive blighting if weather conditions are favorable for infection (Figure 2). In some years the disease never progresses beyond blighting in these “hot spots.” Other turfgrass species that may be mixed in with the ryegrass, including Kentucky bluegrass, annual bluegrass and creeping bentgrass, are not affected by the disease. Blighting can continue through late summer and into early fall and cause significant damage to young ryegrass seedlings emerging from fall overseeding.

Small, chocolate-brown spots about 1/16 inch in diameter develop on leaves (Figure 3). During early disease development, these leaf spots may not be numerous and can be easily overlooked. 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, which helps differentiate this disease from brown patch or Pythium foliar blight (Figure 4).
Gray leaf spot also occurs infrequently on tall fescue. 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 5). The spots rarely coalesce to kill leaf blades or plants.
Pyricularia grisea sporulates prolifically in the center of lesions during periods of high relative humidity (Figure 6). The conidia (spores) are spread by wind, splashing water and equipment to infect new leaves. New foliar symptoms appear within a few days of infection.

Conditions

P. grisea probably survives the winter locally in leaf debris or perhaps in living perennial ryegrass plants, although influx of conidia from distant sources occurs in some years. P. grisea infects perennial ryegrass plants at non-detectible levels in early summer, then builds in intensity later in the growing season when conditions are more favorable for disease development. Prolific sporulation and relatively short disease cycle are reasons why gray leaf spot progresses rapidly and destructively.

Factors that trigger epidemics include high temperatures or extended periods of leaf wetness interspersed with intermittent dry periods. Symptoms develop under hot (82 to 90°F), humid conditions. At these temperatures, infection occurs with a leaf wetness period as brief as 9 hours. The lower the temperature, the longer leaves must remain wet for infection to occur.
Seedlings are more susceptible than older plants to P. grisea, especially under high levels of available nitrogen. Gray leaf spot is often more severe in roughs than on fairways. This is probably related more to microclimate differences in duration of leaf wetness within the turfgrass canopy rather than to height of cut.

The information herein is supplied with the understanding that no discrimination is intended and that listing of commercial products, necessary to this guide, implies no endorsement by the authors or the Extension Services of Nebraska, Colorado, Wyoming or Montana. Criticism of products or equipment not listed is neither implied nor intended. Due to constantly changing labels, laws and regulations, the Extension Services can assume no liability for the suggested use of chemicals contained herein. Pesticides must be applied legally complying with all label directions and precautions on the pesticide container and any supplemental labeling and rules of state and federal pesticide regulatory agencies. State rules and regulations and special pesticide use allowances may vary from state to state: contact your State Department of Agriculture for the rules, regulations and allowances applicable in your state and locality.