The lesions start out small (left), but in a matter of weeks, northern corn leaf blight grows into a longish cigar shape (right). Northern corn leaf blight lesions will cross veins in the leaf, unlike gray leaf spot lesions.
A defensive strategy should include understanding corn diseases, scouting fields and following a management plan
If corn growers compiled a list of their greatest corn disease threats, gray leaf spot, northern corn leaf blight, southern corn leaf blight and Goss’s bacterial wilt would be right at the top. Gray leaf spot would earn its ranking because it’s so widespread. Northern and southern corn leaf blight can sneak in and take you by surprise. Goss’s wilt is moving into new areas at an unprecedented rate.
All those diseases can slash yield. "They attack corn leaves, which function like a plant’s solar panels, taking in energy that is used to produce food," says Farm Journal Field Agronomist Ken Ferrie. "When leaves are damaged or killed, it affects ear size, stalk quality and standability because when the plant can no longer manufacture food, it cannibalizes itself."
Knowledge is power. You can defend your corn fields from the big four yield threats by understanding the diseases, scouting fields and following a disease management plan. Information Ferrie gleaned from his own disease demonstrations in 2013 in the field and in the laboratory can help you. (See Tailgate Talk on page 104 for more.)
Here’s a closer look at the big four corn diseases:
Varying degrees of gray leaf spot resistance are evident on leaves from five hybrids.
Gray leaf spot. Gray leaf spot (GLS) roughly spans from Minnesota to Louisiana and from Texas to Pennsylvania. "It can range from a moderate infestation to a devastating yield robber," Ferrie says.
GLS symptoms don’t show up until just before tasseling. "You probably won’t be able to identify it with your scouting manual until corn is moving into the reproductive stage," Ferrie says.
"Depending on your locality, GLS has been described as both a cool- and warm-season disease. We’ve found it’s tied more to humidity than to temperature. GLS needs high humidity and wet leaves—wet nights, rainy days and heavy dews—to start the process."
Researchers at Iowa State University found that, once gray leaf spot spores germinate on a leaf, it grows on the leaf surface after the humidity reaches 95%, says Jim Dodd of Professional Seed Research in Sugar Grove, Ill. At lower humidity, growth declines, but it resumes again if humidity passes 95%.
"It might continue to do this for at least 96 hours (cumulative) of 95% humidity before it actually penetrates the leaf," Dodd says. "That makes gray leaf spot an excellent fit for most of the Corn Belt, especially in river bottoms."
GLS lesions are gray and rectangular in shape. "The lesions remain between the leaf veins—they don’t cross them," Ferrie says. "That’s important for diagnosis. There’s a yellow halo at the end of the lesion.
"Early on, it is difficult to tell symptoms of GLS, eyespot and southern leaf blight apart," Ferrie says. "As you get farther along, they separate out."
GLS inoculum harbors in residue. "It can be in the stalk or leaves, buried or on the surface," Ferrie says. "The inoculum builds up over time, so the threat of GLS is worse in continuous corn. Yields might seem OK for awhile because infected plants might not die until late in the season. But if you plant a susceptible hybrid after inoculum has built up to a high level, the disease can devastate the crop.
"Anything you can do to reduce the amount of residue helps manage GLS. Crop rotation can help," Ferrie continues. "But a corn/soybean rotation may not eliminate the buildup of residue over time, especially if you do not till between corn and soybeans."
- January 2014