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Do’s and don’ts for controlling soybean disease

Working in a hot spot for soybean diseases keeps University of Tennessee plant pathologist Melvin Newman on his toes most of the time. If he’s not at the microscope distinguishing spores from spots, he’s in front of farmers talking about preventive practices or at the computer screen tabulating disease studies.

Over the last half dozen years, Newman and his fellow scientists have compiled a significant amount of data on varietal responses to common soybeans diseases and fungicides. Here are Newman’s observations on diseases common to the Mid-South:

Charcoal rot — Disease loss to frogeye leaf spot, one of the Mid-South’s most prevalent diseases, crashed in 2007 due to hot weather. But that same hot weather created a paradise for charcoal rot, Newman said. “That year, we had a record 6 percent loss to charcoal rot across the state. Thankfully, it came down quite a bit in 2008.”

Symptoms of charcoal rot include the presence of a charcoal-like substance which is actually tiny seeds of the fungus, ready to break out the following year.

Irrigation is a good way to control the disease, but irrigation is not possible in many parts of west Tennessee. If you can, avoid high seeding rates, Newman says. “The more plants per row foot, the drier the soil. Plenty of fertility will also help, especially potash. Chances of having wet weather are better in the spring, so plant early-maturity varieties early.”

Stem canker — This is probably the most destructive disease in Tennessee, according to Newman. “It can go across the field and kill every plant, from pod set on. But we have good resistant varieties. Later planting will help. No. 1 in all our recommendations is rotation with other crops. Be sure to treat seed with fungicide.”

Sudden death syndrome — The disease loves moisture, good fertility and high organic matter. Plant resistant varieties and keep potash levels high. Rotating with corn will not help. Higher plant populations will. The presence of the soybean cyst nematode will increase the severity of SDS. Delayed planting with early maturity beans might help.

Phytophthora root rot — most likely to infect soybeans planted in clay soils. The disease starts at the bottom of the stem and progresses upward. Some varieties have resistance. Improving field drainage and soil compaction will help as will crop rotation. Treating the seed with Apron will help early. High potash may increase the disease.

Foliar diseases such as frogeye, brown spot, anthracnose and soybean rust — Frogeye is more prevalent in Tennessee than most other states, Newman says. “We could have as many as 100 races of this disease.”

Testing of varieties for resistance to frogeye indicates that varieties with moderate or higher disease ratings should be sprayed. Variety ratings from six years of data are available at

Brown spot will often look like soybean rust, Newman says. Late in the season the disease will result in premature defoliation, which will cut into yield. A fungicide spray is a good control measure for brown spot as well as anthracnose.

Newman and other scientists at UT have also compiled years of data on the benefits of fungicides on soybeans.

Four years of data on various fungicide options on irrigated, no-till research plots in Milan indicated significant yield benefits from applying a fungicide, Newman says. The plots were in continuous soybeans and planted in a susceptible variety. The highest yield increase over the check for a single application was 9.2 bushels, with a single application of Stratego applied at R3. The highest overall increase of 12.8 bushels occurred with two applications of Headline at R3 and R5.

“It is possible to get an economic increase with a second spray,” Newman said. “But remember, we can’t spray enough fungicide to keep every spot off the plant.”

Some general rules of thumb on foliar diseases, according to Newman: “Rotate if you can. Find resistant varieties. Keep fertility high and pH somewhere around 6.5. Treat every seed you plant with a fungicide.”

Soybean rust has turned out to be more of an annual potential disease rather than one that strikes every year, notes Newman. Temperatures cold enough to eliminate the disease’s host will also eliminate the disease. There are no resistant varieties at this time.

The disease’s movement is monitored in the United States, and an infection usually happens somewhere in the Mid-South. “Use a triazole or triazole mix,” Newman says. “But the triazoles are not very good on our other diseases. So you don’t want to go with a triazole if you don’t have a soybean rust threat.

“Since most infections of soybean rust occur late in the season, it helps to plant soybeans early. If soybean rust is in a soybean field at R-1 to R-3, we have a problem, especially if we have moisture with it.”

A new kit for identifying soybean rust “is so easy even a caveman can do it,” Newman said. The Soybean Promotion Board is providing enough funds for any county office that requests a kit, from QuickStix.

Soybean cyst nematode — Newman says this pest “is a train wreck about to happen. The races are changing. We do not have any Roundup Ready varieties that are resistant to soybean cyst nematode, race 2. Anand is resistant, but it is not Roundup Ready.”

Newman says to sample fields in the fall. “If you have 100 cyst nematodes per pint, that’s economic threshold. If you’re at that level, you need to know what race is out there. If you have race 3 or 14, there are resistant varieties.

Rotation helps control cyst nematodes. “In fact, you lose about 75 percent of the nematodes the first year in a rotation. Keep your potash level high. No-till and later plantings may mean lower cyst nematodes in the long run. Nematicides also help, but will give you only about half of the yield increase you would expect from using resistant varieties.”

Seventy-two percent of the samples pulled in west Tennessee farmer fields last year were infested with cyst nematode, according to Newman. Eighteen of the fields tested were at economic threshold.

Newman says that most years, disease will worsen as soybean yield potential rises. “That’s because favorable weather for soybeans means favorable weather for most diseases. An exception held true in 2008 when we had a 32-bushel average yield in the state and one of the lowest estimates in recent time on disease losses. That’s because in 2008, most areas across the state had really dry weather during the late stages of soybean maturity, which held diseases down. But we just see that every once in a while.”


TAGS: Soybean
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