Screening for soybean diseases

Pick up the annual soybean update from the University of Arkansas and you’ll find almost any popular variety and learn whether it’s resistant to rootknot, stem canker, frogeye, and other diseases. Scott Monfort, Arkansas Extension plant pathologist was on hand at the recent Arkansas Soybean Research Conference in Brinkley, Ark., to update how those disease determinations are made.

A majority of the information “comes from screening efforts (by a large group, including) county Extension agents, station directors and others. We also use company data on some of the varieties we haven’t been able (to screen) — although we’re now doing what we can to make all the data used Arkansas-based. We’re trying to enhance screening projects and get consistency from year to year.”

The longest-running disease project in the state, which kicked off in 1984, deals with stem canker. “It was a problem in soybeans and, of those varieties available, researchers wondered which ones had resistance.”

Part of the screening method involves raising the stem canker pathogen on grain sorghum. “It’s put in-furrow with the plants to ensure infection. This screening method is less labor-intensive than other methods and allows screening of many varieties.”

The plant pathologists normally take University of Arkansas variety test coordinator Don Dombek’s variety screening project — about 280 early-, late-, and full-season varieties — and see what all of them offer in terms of resistance.

“Sprinkler systems are set up and must be kept running throughout the growing season. This test can yield good results. However, there are disadvantages, including the specialized equipment and season-long management.”

Even though the tests are on an experiment station “doesn’t mean things can’t happen. We’ve had things like hurricanes, extreme rains, and other issues that can lead to inconsistent results.”

Terry Kirkpatrick, state Extension plant pathologist, is using a new testing method “to help speed up the process. That way, they can work in the field or the greenhouse and look at a lot more varieties — instead of 280 varieties, maybe 400 — over a shorter amount of time. It will allow for a quick analysis and evaluation.”

While a bit more labor-intensive, the data is more consistent. “You can get that consistency by being able to control environmental conditions. One thing is it doesn’t require and additional updating of equipment or resources.”

Varieties are also rigorously screened for nematode resistance. By using small, clay pots in greenhouses, resistance or susceptibility can now be ascertained in only about 40 days. Screening in the greenhouse is easier than doing field inoculations. If a variety is found that shows good resistance in the greenhouse, “it will be moved to field evaluations under more severe rootknot problems to get a better idea of the resistance benefits, or yield loss.

“One thing we implemented this year, which hasn’t occurred in years past except in Don Dombek’s variety screenings, is a frogeye nursery. The Soybean Board provided assistance and allowed us to go in and pick a spot around Newport. We hope this will be a yearly thing because we really need some good, Arkansas-based data.”

The plant pathologists can inoculate to provide more consistency and there’s a pivot to keep the tests watered. To get up and going, the test must be irrigated every second or third day with misting water — two-tenths of an inch. The small pivot available allows that.

Since this is the first year of research, “we weren’t expecting a lot of good information. In the test, we had Group 3s all the way to late 5s and early 6s. The late 3s and early 4s showed little disease, it didn’t progress quickly enough. Once in the late 4s, the disease showed up with 5 percent to 20 percent severity. So, even in the first year, the test provided some useful knowledge on frogeye leafspot.

“In the Group 5s, some varieties showed disease severity of 40 percent. But a lot of them showed pretty good resistance to frogeye.”

Next year, the researchers will return to the same spot and not rotate to another crop. “We want to build the inoculum in the soil and residue and build disease pressure.”

Also under consideration: nurseries for SDS and cercospora leaf blight. “We don’t really know yet how well that’ll work. We’re working with agents and growers to find spots suitable to do variety screenings. Late-season problems with cercospora were seen nearly all over the northeast and central Delta.”

Rick Cartwright, another Extension plant pathologist, “has also been a big help in developing an inoculation technique for aerial blight. We need consistent data for aerial blight. He’s also working on chloride toxicity and looking at new techniques for screening to get more effective, efficient results.”

Cartwright has also conducted fungicide studies — “and those are also going on in nursery tests with other diseases — including Quadris and Headline at different rates. All seemed to do very well compared to the untreated checks. On aerial blight, Quadris seemed to do a bit better. There was a decent yield increase from controlling the disease — 74 bushels versus 65 bushels.”

Emerging diseases

Several new up-and-coming diseases are emerging in Arkansas soybeans.

“We don’t know how big an issue they’ll be overall, but where they’re found they’re quite problematic. (One is) blackroot rot, Thielavopsis basicola, something anyone that’s grown cotton is aware of. We’ve found this is in a Phillips County field where it was causing quite a bit of damage.”

Black root rot is often evident in cotton acreage. However, it had never been confirmed in Arkansas soybeans.

“Symptoms show it makes the field uneven, with a lot of skips. Plants don’t grow off well, there are chlorotic leaves and even seedling death. When we pulled the plants up, they looked (affected by) blackroot rot, but I’d never seen that. It was interesting when it was confirmed. A large part of the field that was affected was on a type of sand blow.”

A lot of initial screening was conducted to determine if, indeed, the malady was blackroot rot. “We planted 100 seed in both sterile soil from the field along with black root rot inoculated soils and waited to see if infections occurred.”

About 38 plants emerged from the inoculated soils while, in the sterile soil, 72 emerged. “So, there was something going on causing problems with emergence. Of the 38 plants, 31 showed evidence of black root rot. That’s a pretty hefty stand loss.”

Another newly-confirmed disease is Neocosmospora vasinfecta, soil-borne pathogen proving to be a major player in some of root impairment. “It’s causing problems late into the growing season.”

This growing season also had plenty of interesting cases of green bean syndrome or “some sort of hormonal imbalance within the plants (for more, see Such cases were in the central and northeast parts of the state. We’re (studying) those trying to determine what (level) of problem that may be in the future.”

The problem is quite new. “We had a number of calls on this in the fall. Plants in some areas of fields are staying green while the rest dry down, as expected.”

At least one Prairie County field was totally affected. Although it ended up yielding 30 bushels per acre, from a distance “it looked like it should have gotten 60 bushels.

“Affected fields have a lot of bud proliferation. We don’t yet know if it’s related to a pathogen or insect damage. Something is causing hormonal imbalance, though.”

Monfort was asked if the problem could be fungicide related.

“We just don’t know. Some of the fields did have a fungicide and others didn’t. Not every (affected) field had the same insecticides. Some areas of the field had bud proliferation, others didn’t. There were degrees of severity.”

What about a virus? “Everything that’s been looked at so far has not detected a virus. They’ve looked for a microplasm but haven’t found that either. If it is a microplasm, insects — a sucking type — would be a player. But, at this point, we don’t know what that might be, what’s initiating it, whether it’s early in the season, or late.”

As for the future, Monfort is set to begin screening varieties for blackroot rot. “We need to do some surveys to determine how big a threat these new soybeans diseases are. We need to develop new management strategies to deal with them — perhaps involving fungicides and seed treatments.”

The screening efforts are a big project that utilizes many people. “We’re grateful to the Arkansas Soybean Board for helping us as well as the producers. We also appreciate the soybean seed industry — they give us the seed to work with. Again, we want more and more Arkansas-based data. We need information from this state’s environments.”

e-mail: [email protected]

TAGS: Management
Hide comments


  • Allowed HTML tags: <em> <strong> <blockquote> <br> <p>

Plain text

  • No HTML tags allowed.
  • Web page addresses and e-mail addresses turn into links automatically.
  • Lines and paragraphs break automatically.