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1997, No. 4
"Be on the lookout for" ...
Gray Leaf Spot of Perennial Ryegrass
A new disease of perennial ryegrass, gray leaf spot, has been observed during the last ten years in the eastern and central U. S.
Appearing first in the mid-Atlantic states, it has been moving westward through southern Pennsylvania, Ohio, Kentucky and into Indiana and Illinois.
Gray leaf spot was initially observed in Illinois in 1991 (this was also the year it was first found in southeastern Pennsylvania).
Varying levels of the disease have been noted the last few years, with a major outbreak occurring in the summer of 1995 on the east coast.
So far, gray leaf spot is only a problem on perennial rye golf course fairways, probably due to the intensive management and stress imposed on the turf (high N, irrigation, low mowing).
Gray leaf spot is caused by the fungus Pyricularia grisea which causes leaf spots and blights of several types of warm and cool season grasses, including forages.
The fungus also causes "blast" of rice, a very important disease on one of the world’s most important food crops.
Because of this, you will sometimes hear gray leaf spot referred to as "Ryegrass blast."
Pyricularia grisea produces abundant conidia (asexual spores) on infected leaf tissue.
These spores move with wind and rain and can lead to rapid development of the disease.
Gray leaf spot begins as small gray-to-brown leaf lesions, which may not be noticeable during early stages if infection.
As the disease progresses, reddish brown patches, one-to-two inches in diameter, quickly appear in affected turf.
These symptoms can be easily confused with Pythium blight or early Rhizoctonia brown patch.
However, P. grisea produces no foliar mycelium or "smoke ring," and generally occurs at somewhat cooler temperatures or later in the season than Pythium or Rhizoctonia.
Gray leaf spot is most severe during warm (80°s F) and humid weather, under higher rates of nitrogen, and on ryegrass seedlings.
In Illinois, the disease is most likely to start in late August or early September, once the higher daytime temperatures of summer are past.
This often coincides with the timing for overseeding perennial ryegrass fairways in autumn, and the disease can become chronic and severe on the new seedlings.
Greenhouse Pathogenicity tests have shown that P. grisea causes leaf spots on mature ryegrass, but will kill seedlings outright rather quickly.
So the combination of autumn overseeding, a propensity for the disease to occur at this time, and rapid production of inoculum (spores) can make for a volatile situation on perennial rye fairways.
Control of gray leaf spot has proven difficult, especially where
a late diagnosis or misdiagnosis delays application of effective
fungicides. Timely applications of DMI fungicides (e.g.,
Banner or Bayleton) tank mixed with chlorothalonil (e.g., Daconil)
have provided control for 14-to-21 days in university tests in
Maryland. Tests there and in Kentucky also have shown that
the new fungicide Heritage (azoxystrobin) is effective against
gray leaf spot. If you suspect that gray leaf spot is attacking
your ryegrass, contact either Hank Wilkinson (217-333-8707) or
Randy Kane (630-257-8126) for confirmation.
(RANDY KANE)
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