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Corn+Soybean Digest

Anthracnose Poses Double Threat

Described by plant pathologists as the most consistently damaging stalk rot in the eastern U.S., anthracnose also carries the threat of leaf blight. And not just in the East.

In '98 and '99, anthracnose was very prevalent in several Midwestern states where it previously hadn't been widely reported. Cornfields over large areas of Iowa and Nebraska were infected, and symptoms also were reported in parts of Illinois, Minnesota and North Dakota.

"Anthracnose has caused premature death of plants in whole fields over the last two years," says Gary Munkvold, Iowa State University extension plant pathologist. "It's important for producers to notice if this is happening, because if the fungus has rotted the stalk base, these plants are likely to lodge."

Anthracnose-causing fungi overwinter in corn residue. Consequently, it's most common in continuous cornfields. And not surprisingly, as the popularity of reduced tillage has increased, so has the incidence of anthracnose. But there's no cut-and-dried case for increased tillage, either.

Fields are infected when wind-borne fungi attach to seedlings in raindrops or dew. New spores develop on infected plants, providing a source for season-long foliar infection and later-season stalk infection.

"We could probably find that fungus in almost any field that's had corn growing there," says Munkvold. "It's a fungus that commonly can be found in corn residue, but it causes disease only under certain conditions."

Research indicates the fungus favors cloudy conditions, high humidity and rain. And, like many diseases, it's opportunistic - taking advantage of stressed plants. Corn borers and other parasites, foliar diseases and moisture or nutrient stress can make plants more susceptible.

Telltale signs of anthracnose leaf blight usually appear initially on lower leaves and may move up the plant later. Stalk rot may affect the lower internodes, the entire stalk or only the top.

"It has different phases," explains Munkvold. "It will attack the leaves - and that's usually not a major problem. It will rot the stalks from the base of the plant, more like most stalk rot fungi do. But then it also has this top dieback symptom."

While top dieback doesn't create the lodging of stalk rot, it creates other problems.

"Plants die prematurely so their grain-fill period is shortened," he explains. "In some fields last year it caused yield loss."

Burying crop residue sometimes is recommended for anthracnose control. But Munkvold says tillage can have both a beneficial and detrimental effect on the fungus.

"Tillage affects the survival of the fungus, but it also changes the moisture-holding capacity of the soil," he says. "If a field is moisture-stressed during the latter half of the season, you're better off to have more residue on the surface instead of less.

"At Iowa State, we do not recommend tillage for stalk rot control, unless there is some other disease contributing to the problem." The following control measures may be more effective:

* Keep corn borers in check.

* Plant resistant hybrids. Most have at least moderate anthracnose resistance.

* Rotate crops. It can significantly reduce the amount of the fungus in a field, although sorghum and some weeds also act as hosts.

* Maintain balanced soil fertility. Unstressed plants are generally stronger and more disease-resistant.

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