Gray Leaf Spot Double Trouble

By Dawn Lyons-Johnson
September 10, 1998

Scientists with the Agricultural Research Service have discovered a "cousin" of the fungus that causes gray leaf spot in corn--a finding that could mean double trouble for growers across the Cornbelt.

The fungus Cercospora zea-maydis causes gray leaf spot. Spores from the fungus land on corn plant leaves and infect the tissues, opening the way to other crop- damaging diseases. A severe infestation can reduce corn yields by up to 25 percent. C. zea-maydis was first observed in southern Illinois in 1925, but since the mid-1980s it has spread across the Cornbelt. Thriving in warm, humid environments, the fungus can be found as far west as Kansas and Nebraska in irrigated corn.

Now, a plant pathologist at ARS' Crop Production and Pest Control Research Laboratory in West Lafayette, Ind., has found a second type of gray leaf spot fungus in the eastern United States. This fungal "cousin" causes the same disease symptoms in corn but has slight genetic differences from the more common type.

The finding could prove crucial as scientists probe the genetic makeup of the disease to learn more about its virulence. Breeders will have to develop new varieties resistant to both forms of the gray leaf spot fungus.

Both types of fungus overwinter in crop residue and in the spring produce spores called conidia. The conidia are blown by wind or splashed by raindrops onto newly emerged corn plants in the spring. The disease became serious in the mid-1980s as farmers switched to tillage systems that leave crop residue on the soil surface.

ARS is the lead scientific agency of the U.S. Department of Agriculture.

Scientific contact: Larry D. Dunkle, Crop Production and Pest Control Research Laboratory, USDA-ARS, Purdue Univ., West Lafayette, Ind., phone (765) 494-6076, dunkle@btny.purdue.edu.