Outwit Three Soybean Killers

November 1, 2008 07:00 PM
No matter how perfect you do everything else, three diseases can derail even the best soybean crop, cautions Ken Ferrie, Farm Journal Field Agronomist. These three diseases include white mold,
sudden death syndrome (SDS) and Asian soybean rust.
With white mold and SDS, the best option is to avoid getting the disease spores in your field in the first place. Once present, you can't get rid of them. All you can do is manage to minimize the effects. With Asian soybean rust—which arrives on spores blown in by the wind—you can reduce yield loss by scouting and timely treatment with fungicide.

Here are some tips to help you keep the "big three” diseases from throwing your yield train off the track.

White Mold
White mold, or Sclerotinia stem rot, is a threat in most of the North-Central states. A soil-borne fungus, it usually attacks soybeans right after flowering.

"White mold favors cool, moist weather, in which the bottom of the canopy stays wet,” Ferrie says. "The more moisture and the less air circulation through the canopy, the more likely you are to see white mold. That's why you tend to have more disease in narrow rows and high populations.”

"High-yield environments—highly productive fields with tall thick stands of soybeans—favor the development of white mold,” explains Ohio State University (OSU) plant pathologist Anne Dorrance. "It tends to be most severe in areas of fields where moisture collects due to fogs and extended dew periods. Temperatures higher than 90°F arrest disease development.”

You can spot white mold, which looks like frothy white fuzz on the outside of the soybean stem. That is mycelium, the vegetative body of the fungus. Before the mycelium appears, the fungus kills the plant, and the area on the stem looks bleached.

The fungus survives as structures called sclerotia, which are hard, black, irregular in shape and resemble rat feces. The sclerotia are able to germinate directly, or they can produce a small mushroom-like fruiting body, which produces spores.

Although there are no highly resistant white mold varieties, a number of varieties have moderate or partial resistance, Dorrance says.

Topsin M fungicide is labeled for white mold control. "But we recommend spraying only in fields where white mold has limited yields in the past,” Dorrance says. "Successful control requires that the spray penetrate the canopy and reach the flowers.”

Long rotations with nonhost crops, such as corn and wheat, can help minimize buildup of the pathogen, Dorrance continues. But it takes several years to reduce the infestation. "Short crop rotations, such as corn and soybeans, will eventually lead to a buildup of sclerotia in the field,” she says.

If you already have white mold in a field, you should clean your combine thoroughly before moving to the next field.

"Sclerotia are on the inside and outside of the stem; so when you combine they end up in the combine tank,” Ferrie says. "If you fail to clean the combine thoroughly after harvesting an infected field, you can plant the sclerotia in a new field. I also have seen sclerotia spread in manure, when soybean residue was harvested and used for bedding.”

A lesser mode of transmission is airborne spores, Ferrie notes. The spores can be blown about 150' into uninfected fields.

Rotating to nonhost crops decreases the risk when you return to soybeans. "The more sclerotia you can get to germinate in a nonhost crop, the fewer you will have remaining to infect soybeans,” Ferrie says.

Reducing sclerotia populations takes time. "In no-till situations, most sclerotia die over a three- to four-year period between soybean crops,” Dorrance says. "But a short rotation, such as corn/soybeans, will eventually lead to a buildup of sclerotia in the field.”

Burying sclerotia deep in soil actually seems to enhance their survivability, Dorrance notes. "They will survive if you bury them deeply,” Ferrie agrees. "If you bury them only 1" or 2" deep, they will germinate and will die in a nonhost crop. The best option may be to plant reduced-till or no-till corn after the soybean crop.”

Make sure the crops you rotate are not hosts to the fungus. Host crops include sunflowers and canola. "If you must follow sunflowers with soybeans, you can moldboard plow to bury the sunflower residue,” Ferrie says. "Tillage is only a quick fix; eventually, you will chisel the sclerotia to the surface.”

Weeds, such as lambsquarters and pigweed, also serve as hosts, Dorrance adds, making weed control important.

"It's better to keep white mold out of your fields than to have to deal with it,” Ferrie concludes.

Sudden Death Syndrome
"Once you have SDS, you'll understand the name,” Ferrie says. "Once you see the symptoms on the plant leaves, it isn't long until the plants are dead.”

Symptoms include mottling and mosaic of the leaves. Later, the tissue between the leaf veins turns yellow and dies. However, those symptoms can be mistaken for other diseases, especially brown stem rot.

"To diagnose SDS,” Ferrie says, "split the stem and look inside. With brown stem rot, the pith in the center will be brown, but the cortex (the meatier part, outside the pith) will be white or cream-colored. With SDS, the pith will be white, but the cortex will be brown with gray streaks.”

With SDS-infected plants, you also will find root decay and discoloration of the roots and crown, points out OSU's Dorrance.

Expect SDS damage to show up first in stressed areas of the field, Ferrie says—areas with high soybean cyst nematode pressure, poor drainage or compaction. "It sometimes shows up in areas with wheel track compaction, in headlands and corners, and sometimes as streaks across the field,” he adds.

The earlier SDS strikes, the greater the yield loss. "Infections after flowering typically don't have a significant impact on yield,” Dorrance says. "But early infections result in pod abortion and reduced yield.”

SDS is not transported on seed. It results from a soil-borne fungus that enters on machinery or running water. Some soybean varieties have partial resistance to SDS, Dorrance says. "Varieties that are resistant to soybean cyst nematodes tend to be less susceptible to SDS.”

"Watch your neighborhood soybean plots,” Ferrie says. "If SDS is showing up, use that plot to evaluate varieties for resistance.” Good fertility and pH are no help against SDS, he adds.

Grow soybeans no more often than every three years, Dorrance advises. "Continuous soybeans make SDS a lot worse,” Ferrie agrees. "Rotation helps, but we've seen symptoms after five or six years of corn.

"Some farmers in central Illinois plant continuous corn because they can't get soybeans to yield well enough because of SDS. In southern Illinois, many farmers moved more of their soybean productions to double-cropping, following wheat.”

Because SDS is affected by environmental conditions, you can have a disaster one year and no disease the next. Cool, wet conditions during the vegetative stage, when soybeans are growing fast, make SDS worse, "so there's no premium for early planting if you have SDS in a field,” Ferrie adds.

Tilling to promote earlier warming of soil, planting earlier-maturing varieties and reducing soybean cyst nematode populations may help delay the onset of symptoms and reduce the severity of SDS, Dorrance says.

Asian Soybean Rust
Asian soybean rust, a fungal pathogen that blew in from Latin America, primarily affects Southern states, but it has been found late in the season as far north as Indiana, Iowa and Illinois. In one isolated event in 2007, the pathogen was detected in Canada.

"Rust causes severe losses through premature defoliation, poor pod fill and production of low-quality seed,” says University of Florida plant pathologist Carrie Harmon. "Yield loss can range from 10% to 90%, depending on region, the time infection occurs and environmental conditions.”

The disease likes warm, damp conditions, Dorrance says. Long dew periods of six hours or more and temperatures of 59°F to 84°F are ideal. "Any areas of the field that remain shaded, collect moisture or remain wet with dew are good places to scout for initial symptoms,” Harmon says.

Symptoms of rust include tan to brown lesions, volcano-like rust pustules on the lower surface of leaves and chlorotic flecking on the upper surface. Infected leaves become chlorotic and eventually fall off, Harmon explains. Symptoms often begin in the lower canopy and move up as the disease progresses. On heavily infested plants, you may also find pustules on stems, pods and cotyledons.

"Well-developed pustules, containing light tan spores, may be visible with a 20X hand lens,” Harmon says.

Early symptoms of rust resemble water pustules and light yellow flecking of the leaves. "They can be confused with brown spot, downy mildew, frogeye leafspot and bacterial pustule, so a lab test may be required,” she says.

Downy mildew tends to start in the upper canopy, Harmon notes. "Bright yellow spots appear on the upper surface of the leaf, with light-colored tufts of mycelia on the underside. Rust usually begins in the lower canopy, where it produces tiny yellow flecks and volcano-shaped pustules. Bacterial pustule can be differentiated from rust by looking for a crack across the small pustules on the lower leaf surface and by the absence of rust spores.

"Rust pustule openings are circular, and spores are often evident in and around openings,” Harmon says. "Frogeye leaf spot does not have the volcano-shaped pustules.”

You can e-mail Darrell Smith at dsmith@farmjournal.com.

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