The farmer was concerned early last week when he noticed the top set of trifoliates on his soybeans were specked with light yellow. He's had sudden death syndrome in the past, and that was his first thought. Even after checking with a quick glance at the picture of SDS in the Purdue University Corn & Soybean Field Guide, he was still concerned. SDS also produces yellowing on leaves in early stages.
But the plants were still just flowering, at least two weeks early to see normal symptoms from when SDS typically appears, notes Greg Shaner, Purdue University plant pathologist. So what was going on?
Purdue's Plant and Diagnostic Lab specialist Gail Ruhl received a sample of the leaves the same day. Right away, she knew it wasn't SDS. "The veins are yellow on these leaves- with SDS he veins stay green," she explained.
Turned out she was 100% right, although to a layman worried that this soybeans have SDS, a quick glance a picture in the Guide didn't make it seem so clear, especially when the symptoms he was looking at didn't come close to matching up with anything else in the Field Guide.
Just to be sure, Ruhl sliced the roots and put a sample under her microscope. Typically, the outer part of the inner stem begins to discolor with SDS. If it's brown stem rot, the center of the stem will turn brown. When she sliced these stems open, they were perfectly white and healthy. "Plus, there was good nodulation on the roots, and they appeared to be healthy and working," she says.
Ruhl quickly ruled out almost every possible disease, and after consideration, decided it didn't warrant testing for viruses that could produce such a pattern on leaves, because the rest of the situation didn't fit the description of a typical virus outbreak.
Next stop was Glenn Nice in weed science. "You can get similar symptoms from certain herbicides applied the year before if a dry situation, but those herbicides weren't applied last year in this case," he says. And although Roundup was applied 10 days earlier, it didn't fit any symptoms of Roundup injury he had ever seen.
By the end of the week, the newer sets of leaves coming out looked perfectly normal, the farmer reports. He first saw the symptoms in both irrigated and non-irrigated beans of the same variety, although both received water within a week of each other for the first time since planting, one through irrigation and the other naturally. However, another variety side-by-side never showed the symptoms.
There can be varietal differences, specialists note. Jeff Hinen, agronomist for Monsanto assisting Stewart Seeds, believes it was likely a reaction related to the long dry spell, which ended abruptly. It's possible that it was somehow related to one or more nutrients being impacted by the weather stress and change within the plant, although exactly which ones and how remain a mystery.
The farmer heard what he wanted to hear- it wasn't SDS, likely wasn't any disease, and new growth looked fine.