Target spot on soybean leaves Photo courtesy of Tom Allen

Target spot on soybean leaves. Photo courtesy of Tom Allen.

Soybean yield records fall for some, target spot saps others

Ashlock on possible threat from target spot in soybeans. Hope that susceptible varieties will be identified quickly.

While more producers push past 100-bushel-per-acre soybean yields, a problem disease appears to have kept a lid on the potential of many northeast Arkansas fields. Target spot, once thought a negligible problem, is now making its mark.

Dr. Lanny Ashlock, retired University of Arkansas soybean specialist and current chairman of the Natural Soybean and Grain Alliance, “couldn’t be happier that more and more growers are participating in (the state’s yield contest), and going over, 100 bushels.”

The original idea for the contest “came from some producer-leaders from within the state. Board members of the Arkansas Soybean Association developed a project and submitted it to the Arkansas Soybean Promotion Board. The original ‘Race for a Hundred’ project over 10 years ago and has continued to support this type of project with some modifications up to the present time.”

The state went about five years without any winners. Since then, though, “the growers have figured it out and some of these yields are spectacular.”

Lower yields

Unfortunately in some areas of Arkansas, many growers “are cutting a lot less than they originally thought they would. Some fields were expected to cut 60 or 70 bushels and are only cutting 30 or 40 bushels.

“Often, especially with indeterminate varieties, the crop will yield better than it looks. That just isn’t the case this year with many of our growers in northeast Arkansas. I haven’t heard many complaints south of the Arkansas River so hopefully their yields are closer to what they expected.”

During the summer, Ashlock “was fortunate to spend some time in Cross County and Poinsett County – mostly west of Crowley’s Ridge. Lots of those soybeans looked good during the growing season but tailed off at the end.”

Rick Thompson, Armor Seed consultant and retired County Extension agent, saw some of the problems with target spot last year. This year, “he says the problem has mushroomed.

“I began thinking about it, reading literature and talked to university people. Most of those sources felt target spot had not proven to be a yield-limiting disease in the past. Although target spot has been around a while it wasn’t thought to be very serious until lately. In 2016, it’s at least a partial explanation for the disappointing yields in some northeast Arkansas fields.”

Checking fields

Ashlock recounts visiting with a grower just north of Weiner with “a beautiful field. I saw it around the first of August and thought the field had a great chance to go over 90 bushels. On those soils that’s something because they aren’t the most productive for soybeans. But the grower had done an excellent job.”

A few days later, Thompson called. “He said, ‘Lanny, you need to come back and look at that field again.’ I drove up and thought it still looked great. But when I walked out and opened the canopy it was unbelievable. There had been a massive amount of leaf drop with target spot symptoms. I realized only 10 to 12 inches of the top portion of the plant canopy remained to finish filling out the seed. The plants weren’t near seed fill, they needed another two weeks.”

At that moment, Ashlock says he “became much more aware of what this disease is capable of doing to a soybean variety that is at least somewhat susceptible. I told them if those were my beans I’d spray a fungicide and try to slow down the defoliation of the crop and at least try to save the upper canopy. I understand the grower did spray and the field ended up cutting about 60 bushels per acre -- still much less than I had earlier estimated.”

As the men looked at more and more soybeans, they found several fields and varieties “that appeared susceptible to the disease. Some fields have cut pretty well, it’s true. But as a general rule, they’re cutting about 15 to 20 bushels less than was anticipated. One of the common denominators is the presence of target spot.

“It appears we have some commercial varieties that are more vulnerable to target spot than others, possibly more susceptible than what we’ve had in the past. Mix susceptible varieties with this year’s weather and the pathogen being present and the disease really manifested itself in some very good growers’ fields.”

Hopefully, plant pathologists “will be able to help us identify the more tolerant varieties to target spot very quickly. Also hope that our soybean breeders will be able release varieties with very strong resistance and the problem many of our growers experienced in 2016 won’t be nearly as pronounced in the future.”

Stem canker comparison

The current situation with target spot reminds Ashlock of “what happened when stem canker blew up in Arkansas in 1989. A couple of years earlier, there were some scattered stem canker plants and we confirmed (the disease) was in the state. In 1988, it was in even more fields and there was enough occurrence of the disease that Extension and research personnel could rate the degree of some of the commercial varieties in university trials.”

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That data was ready for 1989. However, “we were too late educating people. We were unable to make all our growers aware of the situation and there were those that thought we were over-exaggerating the problem. That year some leading varieties – including Asgrow 5980, Forrest, etc. – really took it on the chin all over the Mid-South.

“We’ve got a lot to learn about target spot. Jason McGee, (an Armor Seed) consultant and farmer, told me ‘If you’ve got target spot in the field with a pretty susceptible variety and you go ahead and irrigate without trying to stop the disease progression with a fungicide, you’re probably making a mistake. You’ve spent the money and labor on water yet you have possibly increased the potential to aggravate the situation.’”

There were producers who sprayed using a strobilurin or a strobi/triazole combination at the R2-R4 growth stage. They “didn’t believe it was very effective in controlling target spot. In some field it just kept coming. Once a field reaches full canopy, it’s more challenging to get a fungicide down into the plant canopy.

“Apparently target spot is a more common problem in South America and they have settled on a couple of products for control. Here, I’m not sure that we have a good handle how effective some of our labeled products are in helping us deal with the disease, the best volumes, etc.”

Fortunately, there is ongoing individual state research. “I’m particularly thankful that the Mid-South Soybean Board and the United Soybean Board recently funded a comprehensive regional research project entitled, ‘Enhanced Pest Management Project for Mid-South Soybean.’

“This and other research and Extension efforts coupled with cooperative industry support will help us minimize problems like we have experienced in many areas of the Mid-South in 2016.”

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