When the 2015 planting season began, soybean disease experts like Heather Young-Kelly at the University of Tennessee thought they were seeing another year of conditions favorable to the development of sudden death syndrome or SDS.
It didn’t turn out that way – the further growers got into the 2015 season the hotter it became in the Mid-South. As a result, sudden death syndrome wasn’t as pervasive as it was in 2013 and 2014, but it was still there, she says.
“This year we started off very similarly with a cool, wet spring,” says Dr. Young-Kelly, Extension plant pathologist for west Tennessee. “But compared to last year we definitely have some hotter weather and also some dryer weather in certain areas. So we haven’t seen nearly as much SDS this year as we have in the past two years because of those differences.”
It’s not that unusual to see SDS every year in higher-yielding varieties and in fields where there might be leftover nitrogen from other crops or soybean cyst nematode. “If you have soybean cyst nematode and SDS in your field, you’d have a greater impact on yield. ILeVO helps reduce that impact and reduce the overall severity of SDS but it doesn’t completely eliminate it.”
On the day she was interviewed at the West Tennessee Research and Education Center in Jackson in early September, Dr. Young-Kelly was rating soybean variety trials on the back side of the station.
“When I’m not in a sandy spot, I can see some SDS,” she said. “In other trials where I have the exact same varieties, I’m seeing stem canker, and I’m not seeing any SDS. Depending on where you are and what you have in your soils, you might be seeing more stem canker this year.”
Waiting for an explosion
The changing climactic conditions this year have been disappointing to some observers, such as Chip Graham, technical development specialist with Bayer CropScience who has been working on ILeVO, Bayer CropScience’s new seed treatment for SDS.
“In the Mid-South, we’re not seeing SDS at the level we’ve seen it in the last two years,” said Graham, who is based in Grenada, Miss. “We are seeing it in pockets in some fields. We had excellent conditions for it early season; we had cool, wet weather, and we thought there might be an explosion of SDS this year. But it just hasn’t materialized.”
Graham said Bayer CropScience has side-by-side comparisons of ILeVO-treated seed and untreated seed from Louisiana up through Arkansas, Mississippi and Tennessee. “So we hope we will have a lot of good data that maybe will show some differences in nematodes in some of these fields where we have nematodes.
“The Midwest is another story,” said Graham. “They have areas where they have had tremendous levels of sudden death syndrome, and ILeVO is performing very, very well. The Farm Progress Show was last week (in Decatur, Ill.), and we had a lot of growers wanting to know more about ILeVO.”
Dr. Young-Kelly was asked which was preferable – SDS or stem canker?
“With having ILeVO registered on soybeans, I think maybe you would rather have SDS,” she said. “Sometimes with stem canker it means you have a higher yielding crop because of the left-over nitrogen in the field. Seeing a little SDS isn’t the worst thing in the world, on the other hand.”
Paralyzing the nematode
Besides reducing the severity of SDS, ILeVO has also shown some nemastatic properties; that is, it paralyzes nematodes. “We’re investigating both SDS and soybean cyst nematodes with ILeVO and other treatments and different varieties because that would be your other line of defense for both soybean cyst nematode and sudden death syndrome,” says Dr. Young-Kelly.
SDS is a soil-borne fungus that affects the plant early in the season. “The foliar symptoms you see later usually in the reproductive growth stages is a toxin that the fungus produces,” she said. “So there’s no foliar spray that is going to reduce or manage that foliar symptoms because it’s a toxin that is being produced by the fungus in the soil infecting the roots.
“That’s why applying ILeVO as a seed treatment at planting can protect the root system and reduce the infection of the fungus and then reduce that foliar symptom seen later in the season.”
Sudden death syndrome can look similar to stem canker, says Young-Kelly. Chlorosis (an abnormal yellow, mottling color and crinkling) between the major veins in the leaves is common to both types of diseases.
“So when trying to differentiate between the two you want to examine the stem of the soybean plant. Stem canker will have a canker, and if it’s southern stem canker it will be on one side of the stem. The other thing is you can split the stem open and look for the center pith. In sudden death syndrome that center pith will be white and in steam canker, underneath that lesion it will be brown.”
Growers should be aware, however that if Dectes stem borer has gotten into the soybean plant the center pith will be brown because the larvae has eaten its way through the center pith.
“Later in the season, in sudden death syndrome plants, the leaves will fall off, but the petioles will stick on to the plants, defoliating the plant prematurely,” she notes. “With steam canker the leaves will stick on the plant and turn brown, and they won’t fall off. So that’s two ways you might be able to differentiate between the two.”
The disease symptoms can also mimic a chemical burn, whether it be a triazole burn or surfactant burn. “You want to look at that newest growth to differentiate whether it is a disease or a burn. So if that newest growth has no symptoms, you need to look back in your book and see what was sprayed. If the symptoms are on the new growth, you need to examine the plant to determine whether you have SDS, stem canker or a virus.”
For more on SDS and other soybean diseases, click on http://www.apsnet.org/edcenter/intropp/lessons/fungi/ascomycetes/pages/suddendeath.aspx.