Gray Leaf Spot

Magnaporthe oryzae

Gray leaf spot is a destructive disease of perennial ryegrass turf in southern Pennsylvania.This disease is usually only a major problem on irrigated golf course fairways and athletic fields but can occasionally occur in home lawns.

Symptoms and signs

Early symptoms of gray leaf spot.

Early symptoms of gray leaf spot.                  

Gray leaf spot is most damaging on seedling and young ryegrass plants, but it can also kill mature plants. Symptoms first appear as small, tan or gray spots on leaf blades and quickly progress to blighting of all foliar portions of the plant. Individual spots may be surrounded by purple and/or yellow halos. Blighted leaf tips often appear twisted and hook shaped and sometimes exhibit a gray, feltlike appearance due to massive amounts of spores covering the leaf tissue. Spores are pear shaped and can only be distinguished through the use of a microscope. Under conditions conducive for disease development, large, irregular areas of gray leaf spot-affected ryegrass may appear severely thinned, dehydrated, or dead. Resistant species (bluegrasses and bentgrasses) that may be present in affected stands appear healthy. Gray leaf spot damage is often confused with drought-stressed turf.

Disease cycle

The causal fungus, Magnaporthe oryzae, survives cold periods as dormant mycelia in infested plant debris. When environmental conditions favor growth of the fungus, it begins to produce spores, which can be disseminated by wind, water droplets, or turf equipment. Spore germination and infection of plant tissue occurs in warm weather and under high humidity. In Pennsylvania, gray leaf spot usually occurs in late summer (August through September).

Cultural control

Spores of Magnaporthe oryzae

Spores of Magnaporthe oryzae.                  

Management practices aimed at reducing stress and extended periods of leaf wetness should help reduce disease severity. Irrigation frequency should be reduced to allow foliage to dry between irrigation events. Excess nitrogen and applications of some herbicides have been shown to promote gray leaf spot. In cases where gray leaf spot is a chronic problem, converting fairways from perennial ryegrass to creeping bentgrass can eliminate the problem. Using perennial ryegrass cultivars that have improved tolerance to gray leaf spot can also help reduce disease damage.

Chemical control

Preventative applications of fungicides are generally the most effective means of controlling gray leaf spot on perennial ryegrass fairways. Application timing may vary from year to year, depending on environmental conditions.

Blighted and twisted leaf tip symptoms of gray leaf spot.

Blighted and twisted leaf tip symptoms of gray leaf spot (photo courtesy of Dr. Wakar Uddin)

Perennial ryegrass rough affected by gray leaf spot

Perennial ryegrass rough affected by gray leaf spot (photo courtesy of Dr. Wakar Uddin)