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Corn+Soybean Digest

White Mold Threat To Soybeans More Widespread

Due to cool, wet weather last growing season, much of the nation’s Bean Belt saw a fair share more white mold disease problems than normal, says Paul Esker, University of Wisconsin field crops Extension plant pathologist. More frequent white mold outbreaks last year mean more disease inoculum are in the Midwest to cause problems for soybean growers, both this year and next, he adds.

“Normally, we wouldn’t see white mold as far spread across the region like we saw last year,” says Esker. “However, white mold’s frequency and range of occurrence has increased quite a bit during the 1990s. For most of the 1970s and 1980s, it was mainly a northern-range disease. Now, you’ll see it more often in some southern regions, especially in high-yielding or irrigated fields.”

Kip Cullers, the world-record soybean yield champ from Purdy, MO, says he’s had his share of problems from white mold, especially last year. Over the winter, Cullers traveled to Brazil, in part, to help him find answers to controlling the disease.

“White mold is the biggest disease issue down in Brazil,” says Cullers. “It’s even bigger than Asian soybean rust, because there isn’t a good cure for it yet.”

In addition to trying some experimental fungicides that show promise for white mold control, Cullers says this year he’ll be focusing more on optimal timing for both fungicide and weed-control applications to ensure yield-robbing weed and disease pests fail to take hold.

Timing can make a real difference when it comes to controlling white mold with a fungicide, agrees Esker. “The best results for an application in soybeans is about the R1 or early R2 growth stage, when one or two flowers just start to pop out, up until about half the field is flowering,” he says. “Fungicide applications can also penetrate the canopy better at this stage than when soybeans grow larger, with a fuller canopy.”

A rotation to corn and good weed management can also be helpful to keeping white mold at bay, says Shawn Conley, University of Wisconsin Extension soybean specialist. “Weeds such as common lambsquarters and pigweed can be a host to white mold as can susceptible crops like soybeans, dry beans and some vegetable crops,” says Conley. “So, a good, early season weed-control program in your fields can help drop the inoculums’ presence in soybeans for the next few years.”

Whether their fields have been prone to white mold infestations in the past or not, soybean growers should still educate themselves on ways to stop the disease from proliferating, says Conley. To help soybean growers take the necessary steps to control white mold, Conley and Esker have developed the following top-five, white mold management tips list:

1. Know your field history. “Farmers should scout their fields regularly from the get-go all the way through harvest so that you can see the problems as they develop and react in time with a control treatment, if necessary,” says Esker. “Also, knowing which fields have a history of white mold will help you select the best-suited varieties or to know how long to wait before planting a soybean crop there again.”

2. Check variety tolerance. Farmers who’ve had a history of white mold should be especially careful to select soybean varieties that have a good tolerance rating, says Conley. However, not every seed company has the same scale to rate a variety’s tolerance, he cautions.

“A tolerance rating of seven might be pretty good for one company’s variety, but just the opposite for another,” says Conley. “Also, the perception among a lot of growers is that all Roundup Ready 2 Yield soybean varieties are susceptible to white mold. We did see a few that were susceptible, but mainly Roundup Ready 2 Yield varieties were very tolerant to white mold. Just be sure to check the rating.”

3. Ease back on plant population. Growers who have had problems with white mold in the past should think about reducing their soybean seeding rate, says Esker. “In fields with a white mold history in Wisconsin, we recommend a seeding rate of no more than about 160,000-165,000 seeds/acre,” he says.

The goal is still to maintain above about a 100,000 plants/acre population at harvest for optimal yield potential, but not too much more above that, which would make fields more vulnerable to white mold, says Conley.

4. Anticipate control needs and read labels. “Farmers should be prepared to apply a fungicide if they are planting a high-value, soybean seed production field or if fields have a history of 25% infestation levels or more,” says Esker. “Also, if you have a really well-developed canopy, when compared to previous years, and conditions are cool and wet as flowering begins, then you may be at an increased risk of a white mold infestation.”

One reason soybean growers in the Midwest had so many white mold problems last year was the cool, wet weather that prevailed in July, when soybeans typically flower and are most susceptible to the disease, adds Esker. Iowa, Illinois, Indiana and Ohio all experienced record-cold temperatures last July, and surrounding states had near-record cold for that month, as well, he points out.

However, if conditions look like a fungicide is warranted, be sure to read the label before buying. “Some fungicides are approved for use in soybeans, but not for white mold control,” says Esker. “Others are approved to control white mold, but not in soybeans. So, be sure to pick one that is labeled for both.”

5. Reconsider row spacings. Farmers may need to change their row spacing on a field-by-field basis, says Esker. In fields that have had bad white mold infestations in the past, drilling beans in 7½-in. row spacings is probably a bad idea, he says. Yet, fields that have never had a problem with white mold may yield better when more densely planted with drills or narrow-row spacing.

Soybean growers who want more information for managing white mold can readily find it online. Here are a few suggested Web links:

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