Defend Against Disease

The best offense is often a good defense, and that’s certainly the case when doing battle against corn diseases. An effective game plan starts with early scouting for weeds, followed by a focused and timely fungicide attack.

By: Darrell Smith, Farm Journal Conservation and Machinery Editor

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.  corn lesions

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:

Gray Leaf Spot Five hybrids different resistance

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."

Selecting GLS-resistant hybrids is very important. "Be careful if you decide to switch to shorter-season hybrids because of a late spring," Ferrie cautions. "In that situation, seed companies bring northern hybrids south. You don’t find as much GLS resistance in hybrids sold in northern areas because GLS is less of a problem as you go north."

Fungicides are effective on GLS, but severe infestations might require multiple applications.

"Group 11 (strobilurin) fungicides seem to be more effective on GLS than Group 3 (triazole) fungicides," Ferrie says. "If you have other diseases in addition to GLS, you will need a tank mix. A tank mix of Group 11 and Group 3 fungicides will be more effective on GLS than a Group 3 product alone but less effective than a Group 11 product."

Northern corn leaf blight. As its name suggests, northern corn leaf blight (NCLB) is more common in northern areas. "The disease can show up any time you have cool, moist conditions," Ferrie says. "To become a big problem, it needs a cool, wet spring followed by a cool, moist summer—what many Corn Belt farmers experienced in 2013.

"NCLB causes problems as far south as southern Texas and even Brazil," Dodd says. "I think it is not as much a cool temperature requirement of the pathogen but the genetics of hybrids traditionally used in those areas.

The northern corn leaf blight now harboring in corn residue is a different strain from the one your father fought, Ferrie notes. "The seed industry found hybrids resistant to the HT1 NCLB gene. NCLB almost went extinct, but it came back in the 1980s, became more aggressive in the 1990s and once again is capable of damaging yield. Now we are dealing with the HT2 NCLB gene, and very few hybrids have a high degree of resistance.

"NCLB is most common from central Illinois north," Ferrie says. "Seed produced for those regions contain more built-in resistance because of the environment in which they are bred.

"Seed companies rate their hybrids for NCLB tolerance. In catalogs produced for the south, where the disease is not common, the rating may not be listed, so Southern growers may have to ask their seed rep for the rating."

Since NCLB isn’t usually a problem in the South, Southern growers need to be on guard when conditions are cool and moist. If it shows up, apply a fungicide and shift to a more resistant hybrid next year, Ferrie advises.

NCLB produces lesions that start out small and grow into a longish cigar shape. "When you find NCLB in your fields, manage it aggressively," Ferrie says. "It can do more damage in less time than gray leaf spot because the desiccated leaf area is larger. We sometimes have to spray before and after pollination to control the disease. NCLB can be controlled by Group 3 and Group 11 fungicides."

Like GLS, NCLB builds up over time in residue. "The more continuous corn you grow, the more susceptibility you have, even with tillage," Ferrie says.

Southern corn leaf blight. When the environment is moist—wet leaves and 70°F to 90°F temperatures—southern corn leaf blight (SCLB) is lurking. "The disease is more common in the South," Ferrie says, "But in 2012, we had quite a bit of SCLB in the Midwest because of weather conditions."

SCLB is a mid-season disease, showing up from mid-whorl to maturity. It can show up earlier than gray leaf spot. Like most diseases carried over in residue, it starts in the lower leaves and works its way up the plant.

corn leaf blight

These photos show three stages of southern corn leaf blight. Early on, the lesions can be confused with gray leaf spot, but they are gold in color, rather than gray.

Early season symptoms are similar to gray leaf spot. "But SCLB lesions can be oblong as well as rectangular like GLS, and they can cross leaf veins," Ferrie says. "Its lesions are tan, brown or gold in color."

In inoculation studies, Ferrie found that, once a plant is infected, SCLB symptoms show up within days—no extended warm temperatures required. "Even the resistant hybrids showed early symptoms," he says. "But with susceptible hybrids, the disease really took off and moved up the plant after the second- or third-generation reinfestation as the disease developed."

SCLB also harbors in residue. "But it doesn’t survive well when the residue is incorporated," Ferrie says.

At least one fungicide is labeled for control of SCLB, in addition to some protectant products. Protectants must be applied before the disease strikes.

Goss’s bacterial wilt. Common to Nebraska, Goss’s wilt is on the move. The National Plant Diagnostics Network says it also has been found in South Dakota, North Dakota, Minnesota, Iowa, Wisconsin, Illinois, Indiana, Kansas, Oklahoma, Texas and (most recently) Louisiana, as well as Canada.

"A mild infestation of Goss’s wilt will take 50 bu. to 60 bu. off your yield," Ferrie continues. "But it can take 100 bu. if it hits at the right time."

Unlike the other three diseases, which are caused by fungi, Goss’s wilt is caused by bacteria. "Bacteria can’t enter a plant by themselves," Ferrie says. "They need an entrance created by an injury from hail, sandblasting, insects or wind damage. So a hail storm on top of a moderate infestation of Goss’s wilt can multiply the effects.

"The disease may be present on the plant from the V3 or V4 stage, but if an injury occurs, the plant may suddenly become infected. It can kill an entire field before it matures."

Goss’s wilt can occur from corn’s early vegetative (V) stages through ear fill. Its optimum temperature is 80°F.

"The disease’s development is related to daily temperature and moisture levels in May and June," Ferrie says "It prefers moist conditions. That’s what starts the infection. But you don’t see the results until July, August or even harvesttime. If weather turns hot and dry after infection occurs, progress of the disease slows down, but by then, the damage is already done, and plants may die prematurely. Many growers saw that happen in 2011."

Symptoms include wilting, if the disease is severe, leaf lesions, blighting, leaf necrosis and reduced ear size. "Because the infection is tied to May and June weather when ear size is being determined, you may find ears with only 10 or 12 rows of kernels and only 20 or 30 kernels per row, similar to phosphorus deficiency. Plants may die prematurely. Plants are susceptible at all growth stages," Ferrie says.

The disease survives in surface residue, so you might see the worst infestation in low areas where residue has washed in or where it blows from one field to another. "It also can spread by seed infestation," Ferrie says. "Some grasses serve as hosts. Besides residue, the bacteria can survive for a short time in irrigation water.

"If a continuous corn grower suffers a substantial yield reduction that he can’t account for, he should consider the possibility of Goss’s wilt," Ferrie says. "It’s difficult to identify in the fall after plants have died. So you need to scout and be alert for early symptoms."

DSC 0208   Gosss wilt 1

Because inoculum survives on crop residue, Goss’s bacterial wilt can be most severe in low areas where residue has washed in.

Because Goss’s wilt is caused by bacteria, little chemical control is available. Hybrid selection is the most crucial control measure, followed by crop rotation and residue management.

"Seed companies have different ways of managing Goss’s wilt," Ferrie says. "Some do not sell any hybrids that do not contain a certain level of resistance. Others market susceptible hybrids in areas where Goss’s wilt has not been found. So check with your seed company representative to make sure you select a resistant hybrid." 

Keep Disease Threat in Perspective

Although leaf diseases are scary, don’t let them prevent you from selecting high-yielding hybrids. "For corn breeders, disease resistance and yield sometimes are a trade-off," says Farm Journal Field Agronomist Ken Ferrie. "The hybrid with the highest disease resistance score might not have the highest yield. Select your hybrid based on yield, then manage the disease.

"In our trials, we have seen situations where the hybrid most resistant to a certain disease yielded 30 bu. per acre less than a susceptible hybrid," Ferrie continues. "If you plant the high-yielding hybrid and disease takes 15 bu. off your yield, you’re still 15 bu. ahead.

"If you plant a susceptible hybrid, manage the disease by planting it in fields where that disease has not been a problem. Rotate crops, manage the residue, scout for the disease and apply a fungicide, if needed."

There are situations where disease resistance will take priority over yield, Ferrie notes. Examples include diseases for which there are no effective treatments, such as Goss’s bacterial wilt, and corn planted next to housing developments, where you might not want to apply a fungicide.