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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