Soybean growers who effectively control disease in their crop could harvest significantly more beans. According to Iowa soybean disease researchers, “under high disease pressure up to 80 percent yield loss” is possible due to Sudden Death Syndrome (SDS) alone.
Other top yield-robbing concerns include soybean cyst nematode (SCN), white mold (Sclerotinia stem rot), Frogeye Leaf Spot (FLS) and phytophthora root and stem rot (Phytophthora sojae).
Since commodity market experts predict growers will see the greatest net return on soybeans rather than corn, maximizing soybean yield could prove to be a key profit strategy this year.
Disease Control Tactics
Among the most important management tools for soybean disease is resistant variety selection. Thorough knowledge of each field’s disease history, careful timing of planting, and vigilant scouting throughout the growing season are also important to identifying and managing soybean disease.
Daren Mueller, Assistant Professor and Extension Plant Pathologist at Iowa State University, says weather extremes over the last five years have made it challenging to project which soybean diseases might appear in 2015.
“Recently, Iowa growers have experienced weather extremes ranging from floods to drought,” Mueller said. “Climate experts paint a picture of more extreme and dramatic weather events for the future. If that trend continues, Iowa growers will probably encounter SDS, white mold and stem canker because those diseases are more prevalent in extreme weather conditions.”
SDS: A Stealthy Thief
In recent years, SDS has been found in most soybean-growing states and is ranked in the top five of yield robbing diseases in soybeans. The disease, which isn’t easy to detect prior to plant infection, thrives in cool, moist weather conditions. Once it infects a field, it can easily reoccur.
Although SDS infection begins early in the growing season, symptoms are typically not apparent until midsummer. As the disease progresses, leaves and pod abortion are common, resulting in fewer pods and seeds. Affected plants may also produce smaller seeds. Late-forming pods may not fill or mature.
The disease can ravage one area of a field or appear in scattered areas across the field. Once SDS has affected a field, successive infections can more severely reduce yield. If SCN is also present, SDS impact on yield could be magnified.
SCN on the Rise
SCN is present in 31 U.S. states where soybeans are grown and has been found in soybean-producing areas around the world. Yield losses due to SCN can vary each growing season. The pest is influenced by soybean variety, weather and soil conditions, and it is the #1 yield-robbing pest. In heavily infested areas, growers may lose more than 30 percent of their yield.
SCN not only moves through soybean plant roots, it also alters root cells surrounding the nematode. The altered cells produce nutrients necessary for the nematode’s growth, resulting in damage to the plants. An SCN infection can also expose plants to secondary infections by microbial pathogens.
Identifying SCN can involve reviewing year-to-year yield comparisons to reveal yield reductions, observation of uneven soybean heights, slow row closure or expansion and unusual nutrient deficiency symptoms. Soil testing and examination of soybean roots can reveal SCN cysts.
Frogeye Leaf Spot (FLS): Making a Comeback?
Illinois research plant pathologist and professor at USDA-ARS and Department of Crop Sciences’ National Soybean Research Center, Dr. Glen Hartman, said wet, warm weather for the 2015 Illinois growing season could support development of FLS. The disease had practically died out until 1999 when it was identified in Iowa and Missouri, causing significant yield loss to Missouri growers in 2000.
“FLS has typically been seen in southeastern states,” Hartman said. “Over the last few years, though, it’s been found in the southern Midwest, including southern Illinois.”
FLS can be most prevalent in warm, humid conditions. It usually appears during soybean reproductive growth stages but may develop sooner under ideal environmental conditions. In fields where soybeans have been planted for successive seasons, FLS may initially appear in the lower canopy. Prolonged adequate moisture can provide conditions that allow FLS to spread throughout the soybean canopy.
FLS primarily affects soybean foliage but may spread to stems, pods and seeds. Lesions look like dark, water-soaked spots and progress to brown and gray spots surrounded by narrow, dark brown borders. Spots are circular or angular, ranging from 1 to 5 mm in size.
Soybean residue can host FLS, potentially spreading the disease from field to field. Applying a fungicide such as Stratego® YLD provides both preventative and curative activites for controlling FLS, including strobilurin resistant strains. Planting resistant varieties, rotating crops, plowing under crop residue, and applying foliar fungicide also aid in managing FLS.
White Mold: An Aggressive Fungus
Mueller believes cool, wet weather could support development of white mold in Iowa in 2015. The disease can cause yield losses ranging from 10% to 30% in untreated fields.
“White mold, caused by a fungus, is a very aggressive pathogen that can affect large sections of fields,” Mueller said. “There are fungicides available for in-season management of white mold. Not all commonly used fungicides are labeled for use against white mold in soybean. Once white mold is on plants, treatment is far less effective.”
If white mold is found in a field, it’s recommended that the field be harvested last to avoid spreading the disease via harvest equipment.
Early planting dates, narrow row width systems, soil tillage, high plant populations and high soil fertility can increase potential for white mold infection. Many broadleaf weeds host white mold. The fungus can survive in soil for long periods of time.
Frequent rain events coupled with moderate air temperatures can foster white mold development. The disease generally appears somewhere between just prior to flowering and in pod stage development. Periods of low average air temperatures and moist weather that coincide with the soybean flowering stage can initiate white mold infection. Early soybean planting may influence white mold infestation due to taller, denser soybean canopies.
White mold may be first apparent when single plants found in a healthy canopy wilt and rapidly die. The disease causes leaves to turn brown and close inspection of the dead plant’s lower stem may reveal a bleached area. Some stem tissue may survive, but all the leaf tissue is likely to die, even though leaves may not fall from the stem.
Proline® fungicide is a powerful tool that provides outstanding broad-spectrum disease control in multiple crops, including white mold in soybeans, all while helping to preserve quality for enhanced yield and profit.
Phytophthora Root and Stem Rot
Phytophthora root and stem rot is more likely to occur in cool, wet weather conditions. Once it infects a field, little can be done to manage the damage it can cause. Phytophthora root and stem rot is the third top yield-stealing disease in soybeans.
Wisconsin Extension Field Crops Plant Pathologist Damon Smith says the disease was a major concern for Wisconsin soybean growers in the last couple of years.
“It’s been an issue here for many years but we saw a resurgence here when our spring weather was cool and rainy,” Smith said. “The disease has many variables, but the environment is one of the biggest things that can cause it to flourish.”
Use of tiling in fields with poor drainage is helping some growers better suppress phytophthora root and stem rot. Resistant cultivars and seed treatment are the best defense against the disease.
Seed rot (pre-emergence damping off) or yellowing, wilting and death of emerging seedlings are among the first symptoms. During the stem rot stage, leaves yellow, wilt and die but remain attached to the plant. Brown discoloration that progresses 6 to 12 inches up the stem from the soil line is a key diagnostic symptom. If the disease infects roots, plants will be light green and growth may be stunted or uneven.
“Impact on yield can be substantial,” Smith said. “All growers can do once the disease takes hold is mark areas of the field where the disease occurred, provide samples for definitive identification, and develop a plan to manage the disease in the next growing season.”
Strategies for Taking Control
Among the best management practices for reducing the incidence of soybean disease are knowledge of where past disease occurred, selection of resistant varieties, and understanding planting date strategies.
“Seedling diseases aren’t uncommon,” Hartman said. “Growers can reduce them by not getting too anxious to plant in cool, wet conditions. Soybeans need a soil temperature of at least 60 degrees and 65 is even better.”
Mueller notes that, in areas where crop diversity is more common, some diseases may be less prevalent. Soil quality and tillage practices can all impact the presence and severity of soybean disease. Keeping combines clean and free of soybean stems and residue can help avoid spread of soybean disease from one field to another.
While genetic resistance doesn’t guarantee immunity from soybean disease, resistant varieties are a key disease management tool. Hartman advises consulting seed dealers to learn about the up-to-date variety selections.
“The typical life of a soybean variety is three or four years,” Hartman said. “When a grower selects a new variety, he should ensure his dealer knows about any disease or insect problems they have. Dealers are the best source for information on new hybrids and the process for identifying the most appropriate seed variety for specific farms and fields.”
Monty Malone, Crop Science Soybean Agronomist, said Credenz®, a new 39-variety soybean brand introduced by the company, could be a valuable disease and weed management tool for growers in 2015.
“Credenz offers a wide range of choices so growers can find options to address a variety of production needs in a single, national soybean brand,” Malone said. “For example, growers using multiple herbicides for controlling resistant weeds can now leverage one seed brand across all of their soybean acres. And that means they can work with one trusted seed dealer to meet all of their needs, which is a real benefit to growers.”
Another Crop Science product, ILeVO®, is a seed-applied fungicide/nematicide that provides protection for soybean seedlings against SDS and SCN infection. Because it protects against early-season infection, growers using ILeVO have the option of planting soybeans earlier in the growing season. For unmatched control of nematodes, insects and the SDS fungus pair ILeVO with Poncho®/VOTiVO® for improved root and plant protection and higher yield potential. Field trials from 2011 to 2014 indicate ILeVO increased yields by 2 to 10 bushels per acre, depending on disease and nematode pressure.
Hartman advises growers to thoroughly study the history of past soybean diseases on their farm as a first step in managing the current growing season. Since many soybean diseases can easily recur once they’ve infected a field, it’s important to recognize what disease has been present and where it was discovered. A thorough understanding of how various factors have affected a field in the past can also aid in resistant variety selection.
“There is no magic management bullet,” Hartman said. “Neither tillage practices nor rotation plans will eliminate soybean disease. Scouting fields is important and using yield monitors to identify areas with low yield can help determine if yield decreases are due to disease, fertility, insect infestation, or other problems.”