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How to curb soybean seedling diseases

Agriculture.com Staff 06/20/2007 @ 8:42am

There are lots of seedling diseases out there waiting to stymie soybeans. How to deal with them was the focus of a Syngenta seed care field tour at the University of Illinois in mid-June.

Carl Bradley, University of Illinois Extension plant pathologist, ticked off several soybean diseases that cause problems in the seedling stage.

  • Pythium seedling blight loves cool and wet soils, such as those under no-till. Stand loss most often is its calling card.
  • Its trademark reddish brown lesions on the soybean hypocotyl area and roots peg Rhizoctonia. The disease can cause postemergence damping off.
  • Fusarium seedling blight and root rot. Fusarium also leaves lesions on the plant. A difference between Fusarium and Rhizoctonia infections is that Fusarium affects secondary lateral roots.
  • Phytophthora root rot (PRR) leaves dark lesions on the stem coming up from the soil line. Although PRR can strike at other times, it does attack during the seedling stage.

Bradley says seed treatments are good ways to control these seedling diseases. In the case of PRR, Ohio State University research shows a combination of variety field tolerance and seed treatments work well.

Earlier planting, conservation tillage, and higher seed prices have fueled the rise of soybean seed treatments, says Mark Jirak, Syngenta crop manager for seed care. In 1996, eight percent of U.S. soybean seed received a seed treatment, says Jirak. That number in 2007 rose to 40%, with 50% projected for 2011. This year, 32% of soybean seed was treated with a fungicide, with another eight percent treated with an insecticide/fungicide combination.

Fungicide-treated seed can fend off seedling diseases like pythium and early-season PRR, says Jirak. Insecticide-laced seed treatments can control insects that plague early-season growth like white grubs. They also can curb early infestations of soybean aphids and bean leaf beetles. Jirak says in six site years from 2004 to 2006, soybeans treated with CruiserMaxx Beans (Syngenta's insecticide-fungicide seed treatment) in company trials outyielded untreated soybeans by 5.5 bushels per acre.

"One nice thing about controlling insects with the seed is it goes easy on the beneficial insects, unlike a topical insecticide," says Jirak.

It spares insects like Asian ladybird beetles that naturally prey on soybean aphids. CruiserMaxx Beans cost hovers around $12 per acre. With soybean prices hovering around $8 per bushel, it would take a 1.5 bushel per acre yield increase to pay for the cost, says Jirak.

Wayne Pedersen, retired University of Illinois Extension plant pathologist, has seen a change in attitudes regarding crop production technology in the last couple years compared to the previous 20.

"The thinking used to be if we increased yields tremendously, the price would decrease," he says. No more. The dynamic demand led by biofuels is swallowing increased production. "We are now looking for ways to increase yields," he says.

One of those ways are seed treatments. They aren't a panacea for all production cases. For example, Pedersen referred to a soybean field that had been tilled several times before planting in the first week of May. In this warm seedbed, a seed treatment payoff wouldn't be likely as soybeans planted into a cold no-till continuous corn field.

Pedersen is using digitized readings of roots to study damage inflicted by seedling diseases. He's observed damage to root hairs caused by early-season corn and soybean diseases. He believes root hairs play a key role in boosting corn and soybean yields. Seed treatments are one way to retain root hairs.

"I think root hairs (in soybeans) are what allows pods to make seed and not fall off," he says. "The answer isn't in getting more pods, it's getting ones that are there not to fall off."

This tour also covered a corn topic that up until the past few years was cornered by soybeans: Nematodes. You've probably heard about soybean cyst nematodes (SCN) until you're blue in the face. There's good reason: SCN costs soybean growers millions of dollars each year in lost yields.

But corn has nematodes that are now inflicting yield damage. "Typically, half my calls are about corn nematodes," says Terry Niblack, University of Illinois Extension nematologist.

Corn nematodes are native to the United States, and have been present in soils where corn is grown. So why haven't you heard about them before?

That's primarily due to three factors, says Niblack.

  1. An increase in continuous corn acres. Corn-on-corn doesn't disrupt the nematodes' life cycle as crop rotation does.
  2. An increase in no-till acres. Corn nematodes hate soil disturbance. That's why tillage controls them. With no tillage, corn nematodes thrive.
  3. A reduction in the use of carbamate ad organophosate insecticides. These corn rootworm insecticides also controlled corn nematodes. With these controls now used less due to the popularity of corn rootworm resistant hybrids and nicotinoid and pyrethroid insecticides, corn nematodes have an opening.

One thing that makes corn nematodes so frustrating is diagnosis. Corn nematodes are often confused for other maladies like herbicide injury. What to do? A good start is soil sampling.

"If you want to find out who's at home and how many, three to four weeks after planting is the best time to sample," says Niblack. "You can't sample for corn nematode in the same way you sample for soybean cyst nematode."

Corn nematodes are sensitive to mishandling. One idea is to put sampled soils in a cooler wrapped in newspapers flanked by ice to ensure getting a good count. And don't drop the samples en route to the lab.

"If you drop them, it can cause corn nematodes to burst," says Niblack.

One control measure on the way is Avicta from Syngenta. This seed treatment nematicide is currently labeled for cotton. If all goes according to plan, Syngneta plans to launch a full-scale launch for corn in 2009.

There are lots of seedling diseases out there waiting to stymie soybeans. How to deal with them was the focus of a Syngenta seed care field tour at the University of Illinois in mid-June.

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