Soybean seedling diseases remain problem in Mid-South

The cause of many reductions in stands, seedling diseases remain a problem in soybeans. Stand losses can occur both pre-emergence (seed rotting in the field) and post-emergence (plants dying off soon after they emerge) and can lead to declines in yield.

“It depends on how bad it is and what kind of stand you're left with and how many gaps you have,” said John Rupe, plant pathologist with the University of Arkansas in Fayetteville, at the Arkansas Seed Growers meeting in Brinkley, Ark.

“Soybeans, especially determinate varieties, are good at compensating for gaps. So, stand loss doesn't necessarily mean yield loss. I believe that as seed costs get higher and farmers want to plant fewer seeds per acre, seedling diseases will be more of a factor.”

Seedling diseases are usually associated with environmental stress. Many times that is cold, wet weather during early planting.

“That's especially true with corn and cotton. Soil types — especially clay or poorly-drained soils — also play a part. Usually, high soil moisture and low temperatures are involved. But our research is finding that late-planted crops can benefit from a good seed treatment.”

There are a number of pathogens that cause seedling diseases.

“We know of at least five species of Pythium that attack soybeans in Arkansas. Phytophthora sojae is closely related to Pythium and can also be a concern. Rhizoctonia solani and Fusarium can also (play a role).

“The reason I'm showing you a complex of pathogens affecting seedling diseases is that they have different environmental requirements. Some of them like it cool, some like it hot. Some are more active under wet conditions, others under drier conditions. That makes it difficult to recommend a (comprehensive) seed treatment.”

To control seedling disease, “we look at cultural practices like planting date … Planting depth is also a factor. The longer it takes for the seed to come out of the ground, the more chance for seedling disease.”

Drainage is also very important.

“We usually see seedling problems in poorly drained areas of the field.”

Seeding rates are also a factor.

“Sometimes we can overcome many seedling diseases by overplanting.

“Seed quality can also be a big factor. That's because not only will more seedlings emerge (from quality seed) but poorer quality seed tends to leak more nutrients into the soil when the seed is imbibing. That can stimulate some of the pathogens.”

Seed treatments

Some varieties have resistance to disease, mostly to Phytopthora. Rupe said researchers have also found some resistance to Pythium.

However, the main defense is using chemical seed treatments.

“For the last three years, we've been conducting a fungicide comparison at three Arkansas locations: Keiser, Stuttgart and Hope. There are three planting dates at each location — mid-April, mid-May and a mid-June — to simulate early, conventional and double-cropped soybeans.”

These are planted in 20-foot plots at about 100 seed per row, or 90,000 seed per acre.

“That means we're planting at a low seeding rate. And we're looking at both high-quality and low-quality seed while taking stand and yield data.”

On those, researchers are checking nine seed treatments.

“Some are older like PCNB plus Vitavax, specific for Rhizoctonia. Maxim tends to be effective against Fusarium and also some Pythium. Allegiance is effective against Pythium.

“And then we have a number of broad-spectrum fungicides: Trilex plus Allegiance, Stiletto, Stiletto plus Gaucho (an insecticide), ApronMaxx, ApronMaxx plus Quadris, and ApronMaxx plus Quadris and Cruiser (another insecticide).”

As far as stand goes, “we're seeing significant increases of stand over the control at every planting date and at all three locations. So some seed treatment can improve your stand whenever you plant.”

PCNB plus Vitavax is usually effective only in May and June. But last year, it was warm early in Keiser, Ark., “so we saw some effects.”

Allegiance, which in some previous work showed good effectiveness, hasn't been very effective for the last three years. “That implies that Pythiums, for whatever reason, haven't been (as virulent).”

But the broad-spectrum fungicides — Stiletto and ApronMaxx — seem to be fairly consistent for planting dates and locations for improving stands.

And sometimes improving stands means improvements in yield. This year, at Keiser in April, ApronMaxx plus Quadris and Cruiser was the best. Stiletto and Stiletto plus Gaucho plots also had better yields than the untreated plots.

“We found the same in June. In May, we got an effect, but it wasn't as significant.

“We're seeing this across all seed quality. Where there is a difference, it's usually low-quality seed that benefits from the treatments. What we've found for the last few years is stand improvements at all locations and all planting dates.”

Rupe and colleagues are seeing yield improvements about 55 percent of the time — across all planting dates, but particularly in April and June. “That's probably because the seed are being stressed a bit more then. Usually when we get a significant increase in yield, it's between 6 and 17 bushels per acre over the control.”

Pythium resistance

Along with this work on seedling diseases, “we've come up with something pretty unique with soybeans: resistance to Pythium species. We found it with a cultivar called Archer, a Group 1 out of the Midwest. I believe it was developed in Iowa.”

Tests on Archer show a “pretty dramatic difference” in response to Pythium.

“We know this response with Archer is across the four or five species of Pythium we've looked at.”

Right now, it appears there's a single gene responsible. A graduate student has also found a molecular marker. That discovery, said Rupe, should help with breeding it into future varieties.

Foliar fungicides

A number of foliar diseases — frogeye leafspot and aerial blight among them — remain threats to soybeans. When those are a problem, fungicides can be very effective.

“With those two diseases, an R-3 application with an effective fungicide seems best.

“We've also had the threat of Asian soybean rust. Right now, how many applications of fungicides will be needed to deal with ASR depends on when it's introduced into the state. If it's early, we may need two applications. If it comes in late, we may be able to get away with just one.”

Foliar fungicides can affect seed quality including Phomopsis seed decay and purple seed stain. To preserve seed quality, Rupe said, fungicide applications at R-5 appear better than those at R-3.

“Something you may hear about is a yield increase with foliar fungicides in the absence of disease. In our tests, we've seen a numeric increase — but those haven't been statistically significant.

“Still, there may be something there. Some companies have been promoting their fungicides for plant health. The university doesn't have good data to give you when that could occur and how much.”

Asian soybean rust

ASR was introduced into the United States in the fall of 2004 and was found in eastern Arkansas. Despite fears, it has yet to make a massive impact. But Rupe warned the ASR threat remains.

“Like other rusts, this is a pathogen that must overwinter on living plants. It doesn't survive year-round in Arkansas because of our cold winter. But it does survive on kudzu in Florida.”

In 2005, ASR was mostly confined to the Southeast. In Georgia's commercial fields, “they were getting losses of as much as 20 bushels per acre due to it. In experimental plots, the losses were even higher. But in 2005 we didn't find ASR in Arkansas. We had a very hot, dry summer.”

During the winter of 2005-06, ASR was found in Mexico as well as Weslaco, Texas. And last year, ASR was much more widespread. While it wasn't as severe in the Southeast, it was found across a larger area.

“We found it in Arkansas in late October/early November. Shortly after, it was found across the state. Every year it will be different, so we don't know what to expect in 2007. But it seems like ASR is growing.

“ASR seems to be weather-driven. If we get the right weather, it could move quickly across the soybean-growing area of the country.”

As of Jan. 17, “ASR has been found to be active in Florida, Alabama and Georgia. It's still around. Depending on how much the kudzu was killed back with this latest cool snap, we may have an earlier start with ASR this season.”

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