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Stem canker hiding behind rust?

Mar 14, 2005 8:33 AM, By David Bennett

LITTLE ROCK, Ark. -- Since last November, Asian soybean rust has justifiably captured many headlines, says Terry Kirkpatrick. But remember there are other diseases, he warns: don’t barricade the front door against Asian rust and then let another thief tiptoe through the back entrance.

“We’ve been dealing with many pathogens in Arkansas for a long time,” said the Extension plant pathologist, located at the Southwest Research and Extension Center in Hope. “I’m concerned that with all the hoopla on Asian rust, growers will focus on that to the exclusion of the proper practices that have allowed good soybean production over the last decade.”

Stem canker

An example of Kirkpatrick’s concern was seen when soybean stem canker showed up last year. Until then, the disease had been relatively quiet for years with the last bad outbreak in the early 1990s. To avoid another stem canker debacle, farmers switched to resistant varieties. But as memories grew long and weather kept the disease down, stem canker ceased being as big a worry. In recent years, susceptible varieties began slipping back into rotation.

“Last year, it finally caught us. The long, rainy period into the early summer promoted stem canker. In many cases, we found where susceptible varieties were grown, stem canker surfaced. Most of the time, the pathogen was present without the growers’ knowledge.”

Arkansas had some “devastating” losses, with stem canker problems most severe in the southeast. But there was stem canker and SDS (sudden death syndrome) throughout the state to varying degrees.

“I saw fields of susceptible varieties with 50-plus bushel yield potential harvest 25 bushels. A lot of soybeans were lost.”

On one operation, Kirkpatrick said, a grower began planting a susceptible variety. Half-way across the field, he ran out of seed. The grower finished planting the field with a resistant variety.

“When he harvested, the difference between the two sides was incredibly dramatic. His susceptible beans harvested 20 bushels. Right next to them, his resistant variety harvested 55 to 60 bushels.”

In the face of so much uncertainty with Asian rust, Kirkpatrick implores producers to play it safe.

“If you had — or suspect you had — stem canker last year, it would be tragic to ignore it and plant a susceptible variety. Imagine if you plant a susceptible variety and Asian rust does become a factor. The rust requires, as near we can tell, a fungicide treatment or two to the tune of $15 per application, per acre.

“So the producer, to save his crop from Asian rust, sprays fungicides twice. Then, a couple of months later, stem canker shows up and takes out the field. At that point, not only has the producer lost his crop but he’s also wasted $30-plus per acre on pointless fungicides.”

Producers needn’t be intimidated, though. Asian rust should be recognized “as just another production constraint. It needs to be dealt with from that perspective since it can be controlled in-season. Diseases like SDS, stem canker and soybean cyst nematodes aren’t controllable in-season to a great degree. The day you plant seed is the last day you have to address those diseases for the season. We must not forget about them.”

Stem canker infection is sneaky and occurs very early in the growing season. The pathogen grows down the petiole into the stem and then stays there, latent, with no symptoms for several months. A producer will often apply inputs, believing he’s raising a clean crop. Then, sometime mid-reproductive stage, stem canker shows itself.

“That’s what’s so frustrating. You don’t know you’ve got the problem until you’ve already spent a bunch of money dealing with other things. It would be such a shame if, while dealing with Asian rust properly, you lose the crop to something else entirely.”

Scouting for stem canker, it’s a good idea for growers to check fields shortly after the reproductive stage begins.

“But even if it’s found, it’s too late to deal with. What you’re scouting for is next year’s crop — the scouting is prophylactic.”

Nematodes

The soybean cyst nematode — another target of Kirkpatrick’s research — is “very much alive and well” in Arkansas. The cyst is a soil-borne disease problem, but is also a true parasite. Where fungi tend to infect plants and may eventually kill them, the nematode depends on the plant being alive in order to complete its life cycle. “The analogy I like is mistletoe: if it kills the tree, the mistletoe dies too. So it isn’t in the mistletoe’s interest to wipe the tree out.

“With stem canker, you’d know you have a problem by the end of the year because a large percentage of the field would be dead. Nematodes, though, just divert many of the inputs farmers are putting out for their crops. Instead of those inputs helping fill pods, the nematodes are busy making lots of new nematodes on the back of a producer’s back account.”

Nematodes are subtle in their thievery — they tend to siphon off yield without too much alarm. Unless populations are very high, producers aren’t likely to see a lot of dramatic symptoms.

“We know through nematode diagnostic lab records for the last few years that nearly 60 percent of Arkansas’ soybean acreage has detectable populations of cyst nematodes. They’re widespread, although many fields aren’t at levels where economic problems result. However, monoculture soybeans — even soybean/rice rotations that are one-year-in, one-year-out — tend to increase populations over time.”

Early beans/root-knot

Kirkpatrick said more growers are looking at early-maturing soybean varieties: Group 4s, mostly, although some consider late Group 3s. The early beans finish earlier in the fall and fit well in many operations.

“These varieties have really found a place here. There has been some contention that early-maturing varieties will be better against Asian rust. That’s debatable, at best.”

The idea being pushed in some circles is that by planting Group 3s and Group 4s, the crop can be made before Asian rust develops. Kirkpatrick said the success of that approach hinges on when the rust spores actually blow in.

“Whatever the reason, more producers are growing these early varieties. The fact is the vast majority are susceptible to another nematode we have in Arkansas: the southern root-knot. That makes sense because the varieties were developed in the Midwest where the southern root-knot nematode isn’t a problem.

“Regardless, we’ve brought these susceptible varieties into Arkansas. Invariably, some of the fields they’re planted in have root-knot nematodes, which can do considerably more damage to a soybean crop than cyst nematodes can. If root-knot is present, the crop will be hurt. Farmers don’t need to set themselves up.”

e-mail: dbennett@primediabusiness.com