Last year's weather stalled gray leaf spot damage and minimized farmers' losses. But with favorable weather, the disease is poised to steal up to 30 percent of Indiana's corn yields, Purdue plant pathologist Don Scott said. A third of the state's corn crop is worth about $475 million.
"The most effective way to deal with the problem is going to be to plant resistant varieties," Scott said, although he characterized currently available varieties as ranging only from "susceptible' to "super-susceptible."
Farmers must assess the risk for each field and plan strategies accordingly, Scott said. Reduced tillage fields where corn follows corn face the greatest risk of infection, especially if the field is in a low spot that collects fog or dew. For those fields, farmers should ask seed dealers to recommend less-susceptible varieties.
"They need to talk to a couple of seedsmen and to get to know their rating systems," Scott said. "Then they'll need to match that with their degree of risk. A corn variety that has a four- to five-bushel-per-acre lower yield potential could still outyield a more susceptible variety in a high-risk field."
All this concern about gray leaf spot is surprisingly new. In the late 1960s, the disease was a major concern only in the mid-Atlantic and southern areas, primarily in Virginia and North Carolina, according to Larry Dunkle, a U.S. Department of Agriculture-Agricultural Research Service plant pathologist at Purdue.
But since the 1970s, Indiana farmers have parked their plows and put about 70 percent of the state's fields into reduced tillage systems. Their counterparts across the country did likewise. Reduced tillage provides economic and environmental benefits, but one of its drawbacks is that it promotes gray leaf spot. When the fungal spores were tilled under, they died, Dunkle said. Now, reduced tillage leaves them alive in residue on the soil surface -- and they're spreading.
"Gray leaf spot has swept across the Midwestern Corn Belt and beyond," Dunkle said. "I took a survey trip this October and found infected fields in eastern Colorado and western Kansas."
The fungus just gained a foothold in Indiana in the last couple of years.
"In 1994 the fungus was found throughout the state for the first time, but it came in late and didn't do much damage," Scott said. "In 1995 it came in very early, statewide, and I estimate it took 15 percent of the crop." Last year's weird weather slowed fungal development.
Scott predicted that 1997 will be more like 1995, and that the fungus isn't going to go away. Farmers will have to plan around it from now on, he said.
Corn breeders are working to develop more resistant hybrids, but it will be at least a couple of years before truly resistant seeds are available. Breeders are scrambling to learn more about the upstart fungus as they plan their defense strategy.
Dunkle, an expert in host-pathogen interactions, jumped into gray leaf spot research when he heard that some corn hybrids in some parts of the country react differently to the fungus than those same hybrids in other parts of the country. He wants to find out if the difference in the corn's reaction is due to environmental factors or genetic differences in the infecting fungus. If the fungus itself is different, Dunkle wants to know how it changes as it moves.
So little is known about the gray leaf spot fungus that Dunkle is getting back to the basics of fungal reproduction. He's trying to determine if genetic changes in the gray leaf spot fungus are happening by mutation, sexual reproduction or vegetative recombination (two fungal threads meet, fuse and exchange genetic material).
If most changes arise by mutation, a relatively slow way to change, breeders can assume that a few resistant varieties will keep the fungus at bay for several years. If most changes happen through sexual or vegetative recombination, the breeders face a much bigger challenge -- the fungus may rapidly overcome any resistance they introduce.
Dunkle's preliminary data suggest that gray leaf spot fungus changes by all three mechanisms. If he's right, corn breeders will be scrambling to keep ahead of the fungus.
Sources: Larry Dunkle, (765) 494-6076; e-mail, email@example.com
Don Scott, (765) 494-4627; e-mail, firstname.lastname@example.org
Writer: Rebecca J. Goetz (765) 494-0461; e-mail, email@example.com
Purdue News Service: (765) 494-2096; e-mail, firstname.lastname@example.org
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