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Corn+Soybean Digest

More Maize Malaise?

Expect more leaf diseases, stalk and ear rots, and rootworm in corn after corn fields.

“Growing corn on corn means that one of the fundamental disease management practices — crop rotation — is not being implemented,” says Alison Robertson, Iowa State University Extension plant pathologist.

The same applies to pest management. “You're not breaking the life cycle of the insect by planting an alternate crop,” says Christian Krupke, Purdue University Extension entomologist.

Hybrid genetics, tillage and weather will also affect the incidence of disease and insects in second-year cornfields.

Leaf diseases, stalk rots and ear rots are caused by pathogens harbored in corn residue. Planting corn on corn — especially in reduced or no-till systems — builds up inoculum. “As we increase soil residue, disease risk goes up,” says Dean Malvick, University of Minnesota Extension plant pathologist. Residue makes the seedbed colder and wetter, too, which slows plant growth and lengthens seedlings' exposure to soilborne diseases.

Along with the increase in corn acreage this year, “We expect increased foliar disease pressure, even in areas where these diseases haven't been a problem in the past,” Robertson says. Two serious diseases to watch for are gray leaf spot and northern corn leaf blight.

Gray leaf spot, common in the central Corn Belt, occurs between silking and maturity. Yield losses are unpredictable, Robertson says, but can reach 50% in susceptible hybrids under heavy disease pressure.

The first symptoms are yellowish-tan, rectangular lesions on lower corn leaves. Mature lesions are gray and range in size from 0.2 to 2 in. long, with distinct parallel edges. Infection is fostered by extended periods of warm, humid weather and prolonged leaf wetness, as well as lingering heavy dews, morning fog and low-lying, weedy fields.

Northern corn leaf blight, more common in the cooler northern Corn Belt, usually appears at or after silking and is caused by several races of the pathogens. Grain losses can be significant if the disease becomes established before silking, Malvick says.

Symptoms are gray-green, cigar-shaped lesions 1-6 in. long. Infection is encouraged by mild temperatures between 64 and 80F, and prolonged wet periods or extended night dews. The disease was widespread in the Midwest in 2004, says Greg Shaner, Purdue University Extension pathologist, “and it's one that could be severe again in 2007.”

Scouting for leaf blights should begin in late June or early July if weather conditions have been favorable for disease development, Shaner says. Effective foliar fungicides are available, “but historically, we haven't recommended them,” because it usually doesn't pay, he says. Chemical and application costs average $16-22/acre.

What's changed this year is the price of corn: “You don't need to get back as many bushels” to offset spraying costs. There's also been a lot of talk about yield increases in corn sprayed with strobilurin fungicides (Headline, Quadris), Shaner adds, but “we don't have good data on that yet.”

Growers who are thinking about spraying for leaf blight should consider not only corn value and yield, but also hybrid susceptibility, field disease history and weather, Shaner says. Frequent, well-spaced rains, high relative humidity and morning dews all favor leaf blights. On the other hand, “If it's early to mid-July and you can see some disease in the crop, look at the 30- to 40-day weather forecast,” he says. “If they're predicting dry weather, you may not need to worry.”

Stalk rots also increase when corn follows corn. Stalk rot fungi, like leaf blight pathogens, overwinter in corn residue. The risk is intensified by foliar disease, which hinders photosynthesis, making plants “more susceptible to stalk rot,” Shaner says. “It's an indirect effect.”

In addition, environmental stresses such as drought, hail, prolonged wet weather, cloudiness and insect injury foster stalk rot diseases. “Corn rootworm feeding injury facilitates entry of stalk and root rots,” explains Ken Ostlie, University of Minnesota Extension entomologist. “Similarly, corn borer tunnels in the stalk and feeding in the ear facilitate stalk and ear rots.”

Stalk rots cause the internal stalk tissue to die and may kill plants prematurely, shutting off grain production. But most losses result from weakened stalks that break or fall over. With more corn to harvest in 2007, the crop will be left standing longer than normal, Robertson notes. “That could lead to more lodging and more harvesting problems,” she says.

Several common fungi cause stalk rot, including Colletotrichum (anthracnose), Gibberella, Fusarium and Diplodia. Although the pathogens differ, stalk rot disease symptoms look quite similar, Robertson says. Look for gray-green, rolled-up leaves; spongy stalks; and brown patches on the outer stalks at the lower nodes. Inner stalks look shredded and sometimes discolored.

Anthracnose, both a leaf and a stalk disease, is frequently seen in the eastern and central Corn Belt. Early in the season, oval, water-soaked spots appear on seedling leaves. Lesions are tan with reddish-brown borders, Malvick says. In mid-summer, during vigorous vegetative growth, the disease fades. But if weather conditions are favorable during ear fill, plants may become susceptible to the top die-back or stalk rot phases of the disease.

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Top die-back begins two or three weeks after pollination. The uppermost leaves turn yellow or reddish-purple, then brown. Symptoms of the stalk rot phase appear late in the season, just before crop maturity. Shiny black streaks and blotches develop on the outer stalk. The inner stalk may also be discolored or shredded.

Gibberella and Fusarium stalk rots usually occur after pollination. Gibberella, the same pathogen that causes wheat scab, is favored by warm, wet weather. Fusarium stalk rot is favored by dry weather before silking and warm, wet weather after silking. The pith of diseased stalks disintegrates and has a pink or red discoloration. With Gibberella stalk rot, black specks can be seen on the outer stalk, near the nodes.

There's no treatment for stalk rots. To reduce losses, growers should scout for stalk quality before black layer, about 40-50 days after pollination, Robertson says. Look for visible symptoms and test stalk firmness at the lower internodes by squeezing the stalk between the thumb and forefinger. If more than 15% of plants in a field are affected, significant lodging is possible and early harvest should be scheduled, she says.

Ear rots also occur with more frequency in multiple-year corn as inoculum levels increase. Three common ones to watch for are Gibberella, Fusarium and Diplodia ear rots. In Indiana, for example, all three diseases were reported in 2006, Shaner says. Another damaging ear disease, Aspergillis, “may or may not be more common under corn-on-corn conditions,” Malvick says.

Gibberella ear rot is favored by cool, wet conditions after silking. Reddish mold appears first at the ear tip and spreads downward. Usually, the ear is only partly covered with mycelium, Malvick says, but early infection may cause the entire ear to be covered.

Fusarium ear rot occurs during or after flowering and is favored by hot, dry weather. Symptoms vary according to hybrid genetics, environment and disease severity, Malvick says. Individual infected kernels or groups of kernels may be covered with a whitish-pink to lavender mycelium. Kernels may also have a white starburst pattern on the dent end. The fungus is most often found at the ear tip. Kernels may also be infected at the embryo, making symptoms hard to see. Both Gibberella and Fusarium ear molds produce grain mycotoxins harmful to humans and animals.

Diplodia ear rot, which is common in the southern Corn Belt, doesn't produce harmful mycotoxins. But it's just as destructive as Gibberella or Fusarium ear rots. The disease causes lightweight kernels, reducing yields and the nutritional value of the grain. Diplodia ear rot occurs after flowering and is fostered by warm, wet weather, especially during the three weeks after silking, Malvick says. Husks appear bleached or straw colored. Infection usually begins at the base of the ear and white or grayish mycelium develops on the kernels or husk. If the infection occurs early, the entire ear may be shrunken and the kernels glued to the husk.

Aspergillis ear mold produces the dangerous grain mycotoxin, aflotoxin. The disease occurs in mid to late season during hot, dry years “and can be of much concern,” Malvick says. Patches of green to yellow mold appear on or between kernels. It's most common on the ear tip and a few scattered kernels.

Growers should scout for ear rot symptoms at black layer and again a couple of weeks before harvest, Robertson says. Look for pink, white or greenish-yellow molds growing on or between kernels. Grain with significant ear mold should be harvested as soon as possible and dried to 15% moisture or less within 48 hours, she says.

Insects also survive in corn residue and previous corn ground. The most damaging is corn rootworm (CRW). Other corn pests, such as wireworm, seed corn maggot and white grub, thrive in cornfields planted back to corn, too, says Krupke, the Purdue University field crops entomologist. And planting into corn residue “means seedlings are slower to emerge and are vulnerable to early season pests for a longer time.”

Growers who have used crop rotation only to control CRW will need to use other strategies in their second-year cornfields, Krupke says. “If you're in an area with high corn rootworm pressure, you can't afford not to protect corn.”

But not all second-year corn will necessarily see higher rootworm pressure, he notes. In some parts of the Corn Belt, CRW has adapted to defeat crop rotation. A variant of the western corn rootworm deposits its eggs in soybeans and other field crops, damaging first-year corn the following spring. It's a threat in Illinois, Indiana, northwest Ohio and southern Wisconsin, and has been found in southern Michigan and eastern Iowa.

The northern corn rootworm extended diapause enables some eggs to remain dormant for two or more years. This variant is a threat in parts of South Dakota, Minnesota, Iowa and Nebraska, says Ostlie, the Minnesota entomologist. In extended diapause regions, only a portion of the eggs laid in 2006 will hatch into second-year cornfields this season, he says. “But in the long run, continuous corn will have greater rootworm pressure than rotated corn.”

CRW management options include resistant transgenic hybrids, soil insecticides and pre-applied insecticide seed coatings. The best choice will depend on how much CRW damage you expect in each field, Ostlie says. “Seed treatments are less effective in heavy corn rootworm pressure,” he notes. However, he discourages planting Bt-rootworm corn as “an insurance” strategy, without any idea of rootworm pressure. Instead, “Ask yourself the key question, ‘Do rootworms need to be managed in this field?’ If so, choose management options based on risk. Target where to plant Bt-RW corn,” he says.

Growers should scout to get a handle on rootworm risk, Ostlie adds. To estimate the threat in fields that will be planted to corn in 2008, monitor the abundance of adult rootworm beetles weekly from late July to early September. For continuous corn in the Midwest, ¾-1 adult rootworm beetle per corn plant in a random sampling of 20 or more widely separated plants per field indicates a potential for significant damage, Ostlie says.

Monitor and control European corn borers, too, he adds, although second-year corn won't have higher populations than rotated corn. “Corn borer is still with us. In 2006, I saw some fields with the highest infestations of corn borer I've seen since our last outbreak in 1996,” Ostlie says.

As always, weather will be a huge factor in disease and insect pressure. “A lot of the story will be told early,” Krupke says. Adds Ostlie: “Every year is different, so pay attention to what you're hearing about pest problems in your area. Every year throws us a surprise.”

Should You Spray For Corn Leaf Blights?

These questions can help you decide.

  • Did you plant a variety with good leaf blight resistance?
  • Are weather and environmental conditions conducive to leaf blights?
  • Does the field have a lot of corn residue on the soil surface?
  • Does the field have a history of leaf diseases?
  • Are there symptoms of leaf blight on the first five lower leaves?
  • Does the 30-day weather forecast favor leaf disease development?
  • Are corn values high enough to provide an economic payback to spraying?
  • Can you spray in time to realize an economic return? (For many foliar diseases, two to four weeks after tasseling is too late to make a fungicide application pay.)

Sources: Greg Shaner, Purdue University; Alison Robertson, Iowa State University; Dean Malvick, University of Minnesota

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