One of the biggest hurdles to stopping brown stem rot (BSR) is overcoming a case of mistaken identity.
Too often, the browned leaves on prematurely dying soybeans in mid-August are confused with sudden death syndrome (SDS). But splitting a stem open and seeing chocolate brown pith confirms BSR, a disease whose mild infections can take 2-3 bu from yields and whose severe infections claim 8-10 bu. With SDS, the stem pith will be white or tan.
Mistaken identity may be one reason BSR claimed 24 million bushels in 1997 and 14.6 million in 1998. Another reason may be its reputation as mainly a northern disease - north of Interstate 80. It's a major problem in northern Iowa, southern Minnesota and Wisconsin, yet it's more common and more severe than most farmers think.
"It occurs someplace in the state every year," points out Wayne Pedersen, University of Illinois research plant pathologist. "In the right year - a cool, wet year - you can get BSR in central Illinois."
Pedersen conducted a 1998 field survey that found severe BSR infection in 30-40% of the fields in the northwestern corner of Illinois and into the second tier of counties.
BSR is caused by a soilborne fungus that produces toxins called gregatins. These toxins damage the plant's vascular system, slowing the water and nutrient movement. Infected plants produce fewer and smaller beans.
Damage is greatest in weather ideal for growing soybeans: cool and wet during pod fill, followed by dry. The disease develops best in temperatures from 59 degrees to 81 degrees but stops in heat above 90 degrees.
First signs can be plants dying early, but the disease comes in two types - one that shows both leaf symptoms and pith damage and one that only shows pith damage.
The fungus lives in the soil for years, which means normal two-year corn-soybean rotation (or rotations with other non-hosts such as sorghum or small grains) won't eliminate the inoculum.
The best defense, Pedersen says, is resistant varieties. He cites two reasons why, even with the availability of resistant varieties, BSR is still a problem.
"We believe there are new strains able to cause susceptible reactions. Over time, we have selected for new strains."
The other reason is that many farmers don't realize they have BSR and are planting susceptible varieties. However, Pedersen points out, "Farmers who are concerned about BSR are selecting varieties with resistance."
Meanwhile, U.S. researchers are widening the search for more genes that offer resistance. During the last five years, they've gathered several hundred soybean lines from China, screening them for new sources of BSR resistance.
Pedersen says some of the Chinese soybean lines are promising and need to be because of the development of new strains for BSR.
"Some resistant varieties are showing BSR symptoms. That may indicate new strains. We've never had a single-gene source that would last forever."