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Corn+Soybean Digest

No-Till Hikes Soybean Diseases

If you started no-tilling soybeans recently, you may have noticed an increase - maybe a big one - in phytophthora root rot, brown stem rot or other soilborne diseases.

That's in part because, with no-till, soilborne disease fungi are no longer buried but survive in crop residue on the soil surface. So say X.B. Yang and Greg Tylka, Iowa State University plant pathologists.

"Data show phytophthora and brown stem rot significantly increase in no-till compared with conventional," says Yang.

Although Yang and Tylka are gathering some negative evidence on no-till in relation to disease incidence, there's still hope for its enthusiasts. Yang suggests using resistant varieties, crop rotation and delayed planting as ways to reduce disease in no-till. And there's always the option to do some tillage to reduce disease pressure.

The plant pathologists have completed a regional no-till project, funded by checkoff dollars, that involved six North Central states. They found, in two years of data, that no-till fields had 7-11 percentage-point increases in phytophthora root rot (PRR) incidence over conventionally tilled fields.

Intensity of the disease also increased in no-till fields where PRR was found. In 1995, average PRR intensity across the region was 20% in no-till fields compared with only 13% in conventional fields. The next year, the level was 45% for no-till fields and 34% for conventional fields.

Brown stem rot (BSR), too, was more prevalent in no-till. In 1995, 82% of the no-till fields had BSR fungi, compared with 63% of the conventional-till fields. In '96, the no-till vs. conventional score was 74% to 63%.

Surface residue doesn't increase disease just by harboring fungi. It also reduces soil temperature and increases soil moisture - two factors favorable to many soilborne diseases. Another side effect of no-till: increased use of herbicides, providing more potential for herbicide injury, Yang says.

Excessive herbicide stress will enhance sudden-death syndrome (SDS), according to a study by Scott Abney, USDA plant pathologist at Purdue University. Even though SDS occurs in August, its pathogen infects soybeans during the seedling stage.

Abney and colleagues suggest that growers move toward a two- or three-crop rotation to reduce SDS pressure. And delay planting until the soil warms, when disease fungi are less active.

But the best way to control SDS is to switch to ridge-till or use resistant varieties. "The disease will be four times higher using no-till than ridge-till," Yang reports.

Use a seed treatment, such as Apron, to reduce incidence of PRR in soybeans, Yang suggests. PRR-resistant varieties are also fairly widely available. Or minimum-till in early spring to churn the soil.

BSR, like SDS, can infect plants as early as the seedling stage and fields should be scouted in August or early September.

"A lot of growers don't recognize that brown stem rot is a problem because symptoms that show up in late August or early September are mistaken for early maturity," Yang cautions. Using a BSR-resistant variety and rotating with corn are the options.

Not all the news is bad when it comes to no-till and soybean diseases.

Soybean cyst nematode (SCN) is less likely to be a problem in no-till beans than in conventional, according to an Iowa State study led by Tylka. That's because tillage moves the disease around. The more static the soil, the less of a problem with the spread of SCN, Yang explains.

In another study, no-till is being examined for its effect on white mold. In Iowa, 10% of no-till soybean fields have at least a 5% incidence of white mold - the disease infects one out of 20 plants. In Minnesota, 20% of fields have at least a 5% incidence. Wisconsin and Michigan are other states with a very significant white mold problem.

White mold sclerotia can survive in soil for five to seven years. Tillage that buries the sclerotia under more than 2" of soil keeps them from germinating. But sclerotia left on or within 2" of the surface can germinate and cause.

Yang and his colleagues have high hopes for a white mold control method that will be tested this year.

"If you have white mold bad next year, grow no-till corn the following year. If you leave residue on the surface, sclerotia can germinate, pop off and die," he says.

"Then do moldboard or deep tillage to bury the remaining sclerotia. We don't know how effective this is, but, theoretically, it should work."

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