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.”

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