If you farm in the north-central U.S.—white mold country—the better a soybean manager you are, the more likely you are to get whacked by the disease. That's because you are more likely to plant soybeans early, in narrow rows and in soils with high fertility. The disease tends to be most severe in fields with dense fast-closing canopies.
Technically called Sclerotinia stem rot, white mold is caused by a fungus called Sclerotinia sclerotiorum. In extreme cases, the disease can cut soybean yields by 50%.
Whether or not white mold hits a given field primarily depends on the weather at the time soybeans are planted and during the summer months. In 2009, many areas experienced cool wet weather during the soybeans' early reproductive stages—ideal conditions for the disease.
"Many of my clients as far south as Tuscola, Ill., saw white mold for the first time in their fields in 2009,” says Farm Journal Field Agronomist Ken Ferrie. "Until now, we saw it mostly from I-80 north.”
Actually, there have been outbreaks of white mold before—just not for a number of years. "The last time we saw white mold this widespread in the Midwest was the late 1990s,” says Don Schafer, senior marketing manager for Pioneer Hi-Bred.
"During several years through the 1990s, there was plenty of white mold south of I-80,” says Glen Hartman, a plant pathologist for the USDA–Agricultural Research Service and the
University of Illinois.
Now, farmers throughout the area are asking whether the likelihood of white mold justifies preventive measures for 2010.
Symptoms. You first notice white mold when individual plants in a healthy field wither and die. The stems of infected plants contain a bleached area, radiating several inches from the leaf axil. The most telling sign, which makes white mold easy to identify under wet conditions, is fluffy, stringy, white mycelium along the stem. Mycelium is the vegetative body of the Sclerotinia sclerotiorum fungus.
"The fluffy mycelium is not commonly seen if the stems are dry, but it rapidly becomes fluffy again when the stems get wet,” explains University of Minnesota plant pathologist Dean Malvick. "On dry sunny days, mycelium can go through a cycle of wet and fluffy in the mornings and dry and appressed to [lying flat against] the stem in the afternoon. At this stage, it looks similar to thin white paint on the stem.”
"Farmers tell me that when you have a lot of white mold, you can roll down your truck window and actually smell the disease,” Hartman says.
Black oblong structures, from 1⁄8" to ¾" long, called sclerotia, form inside the infected stem. They are the fungus' survival structures. Sclerotia can live in the soil for many years, leading to the release of spores later on, which infect new crops of soybeans. Although the number of sclerotia in soil declines over time, it takes only a few sclerotia to produce enough spores for an epidemic, according to Purdue University plant pathologists.
Sclerotia can be spread by combines, by tillage and by water flowing across fields. "An Illinois grower planted soybeans in a field that had been in corn for 27 years,” Ferrie says. "That field developed white mold where water flowed across it, carrying sclerotia from a neighboring, infected field.” It can also be spread in seed, Ferrie adds. (See "Should You Treat Seed for White Mold?” sidebar below.)
If sclerotia are within an inch or so of the soil surface, fruiting bodies called apothecia—consisting of cuplike structures on a short stalk—emerge from the soil. They release spores, which are carried by wind currents to soybean plants. The spores infect senescing flowers or areas where flower parts stick to the stems and then colonize other tissue. The fungus grows down through the dead flower parts and infects the stem of the plant.
Weather is key.
Fungicides and Biological Control
If 2010 conditions are conducive to white mold, consider applying Topsin M or Domark fungicides. Both products work best as preventive sprays: Topsin at the R1 or R2 growth stage and Domark between R1 and R3.
Applied later, the products help suppress the spread of white mold from plant to plant, says Phil Robinson of United Phosphorus Inc., which manufactures Topsin, and Eric Ott of Valent, which manufactures Domark.
Both companies have research trials under way to document the benefits of later application in reducing plant-to-plant spread. For now, plant pathologists Dean Malvick of the University of Minnesota and Alison Robertson and X. B. Yang of Iowa State University recommend using the products only as preventive treatments.
University of Illinois plant pathologist Carl Bradley is among the researchers studying fungicides for control of white mold. "Although my research this year focused on preventive-type applications, I understand several farmers in Illinois applied Domark after symptoms were observed. We hope to be able to glean some data from these late-applied farmer fields,” he says.
If Bradley is successful, the information will appear in his Department of Crop Sciences' Bulletin in late in 2009. You can find it at http://ipm.illinois.edu/bulletin
Valent proposes a white mold management program that includes an application of Cobra herbicide just prior to the R1 growth stage. "Cobra induces a systemic resistance in soybeans, acting like a vaccine against white mold,” Ott says. The program should also include crop rotation, resistant or tolerant varieties and a fungicide if weather conditions favor the onset of disease, he adds.
Contans WG, which contains a fungus that parasitizes white mold sclerotia in the soil, "definitely has potential as an additional weapon in your arsenal,” Robertson says. The product, from SipcamAdvan, is sprayed onto the soil of infected fields after they are harvested.
Bradley cautions: "In fields with a high load of sclerotia in the soil, enough sclerotia may survive to still cause a substantial level of the disease.”
Bearing in mind that weather and soybean planting date determine disease severity, if you had white mold this year, how should you prepare for 2010?
Plant pathologists warn that no single measure ensures control. "If you're experiencing white mold for the first time, plant the most resistant varieties you can,” Malvick says. "They are worth trying; but, under severe disease conditions, white mold may still develop.”
Some farmers have gone to 30" rows to combat white mold. But many pathologists believe that plant population is much more important than the row width. "For example, if you're planting 150,000 seeds per acre, in some areas you may be able to cut back to 125,000 without reducing yield potential,” Malvick says. "That's easier than changing planters.”
Ferrie concurs. "Row spacing doesn't make much difference in the amount of white mold in fields I've looked at in Illinois,” he says. "But where farmers doubled up with the planter and you have higher populations, such as on point rows, you see more disease.”
Tilling infected fields to bury the sclerotia may help. (Clean soil off your tillage tool to avoid transporting sclerotia to other fields, Ferrie cautions.) If sclerotia are buried 2" or so deep, they won't germinate. But if you do any tillage in the future, you will uncover some of them.
No-till helps. In some Wisconsin studies, a 100% no-till regimen, which left sclerotia on the surface, resulted in fewer apothecia when the field was again planted to soybeans. Ferrie has observed that white mold often is worse in tilled soil than where soybeans were no-tilled. "In one field, where farmers tilled the headlands, you could see the difference to the row,” he says.
"No-till may help, but the data is not always consistent,” Malvick says.
Rotation to nonhost crops may help, but one year of corn isn't enough to reduce disease pressure. When rotating, keep in mind that the Sclerotinia fungus survives on many crop hosts, including alfalfa, clover, sunflowers and canola. Weed hosts, such as dandelion, lambsquarters, ragweed and velvetleaf, can negate the benefits of rotation. (Some estimates place the number of white mold hosts as high as 300 species of plants.)
Some Wisconsin studies indicate that adding wheat, or some other cereal crop, to a corn–soybean rotation reduces white mold severity.
Fungicides can help prevent white mold if you apply them in time and get plants thoroughly covered. (See the "Fungicides and Biological Control” sidebar below.)
To avoid spreading the disease, harvest infected areas last and clean your combine thoroughly, Ferrie advises.
"Since the odds of white mold hitting again next year aren't that high, I won't recommend clients make any major changes in their soybean management program,” Ferrie says. "That will change, of course, if we begin to see the disease occur more often. Meanwhile, it's a good idea to keep white mold limited to the fields it is already in and try to avoid spreading sclerotia to others.”
Should You Treat Seed for White Mold?
White mold sclerotia can be transported in seed as well as on tillage and harvesting machinery and by water. So it is possible to introduce the disease to a new field by planting sclerotia contained in a bag of seed.
Glen Hartman, a USDA–Agricultural Research Service and University of Illinois plant pathologist who searched for sclerotia in fields across Illinois in the 1990s, believes any field that has ever had the disease likely harbors sclerotia in the soil.
"Probably 99% of white mold infestations come from sclerotia in the soil, rather than in seed,” he says. "If your neighbors' fields have ever had white mold, you probably have it in your fields, as well.”
Even so, the prevalence of white mold along the I-80 corridor, in both commercial and seed fields, has farmers wondering about the danger of white mold sclerotia making its way into 2010's soybean seed.
Don Schafer, senior marketing manager for Pioneer Hi-Bred, says seed companies have the experience and equipment to keep seed supplies virtually sclerotia-free, gained by dealing with the disease in northern
areas where it occurs more often. "White mold is not an unusual disease for us to deal with, and the seed industry is conscious of the issue this year,” he says.
"Pioneer has many growing locations along the I-80 corridor, and white mold is not present in every field,” he says. "Having many seed fields of the same variety provides us plenty of options to choose from.
"If a field is heavily infected, we'll reject it during our final field inspection,” Schafer continues. "If there are isolated spots of disease, the operator will harvest around them. If there are only individual diseased plants here and there, they will be harvested; but we do bin inspections and quality sampling to find sclerotia before they progress any further through the chain.
"Inside our seed plants, we have multiple tools, including air screen cleaners, gravity separators, spiral separators and stoners to separate sclerotia out of seed,” Schafer says.
"So we have a very good chance to separate out the sclerotia. As the seed goes through the plants, we analyze samples frequently, and we sample again from the final seed lots.”
Ready for 2010. Schafer says if your 2010 seed comes from a reputable company, you shouldn't see white mold's small black sclerotia in your seed. If you do find any, you should contact your seeds salesperson.
Will a seed fungicide treatment help? "Unfortunately, the seed-applied fungicides currently in the marketplace can not stop white mold from occurring,” Schafer says.
"If there is sclerotia in a seed lot, a [seed-applied] fungicide will have no impact,” Hartman says. "The fungicide will be gone by the time the sclerotia start growing, about mid-July. A fungicide will kill white mold fungus that has infected the seed or seed coat. But if you have the fungal mycelia in the seed, you probably have sclerotia in the seed lot also.”