Soybean growers who effectively control disease in their crop could harvest significantly more beans.
Losses to Sudden Death Syndrome (SDS) can range from 20 percent of a crop to more than 70 percent. 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).
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 SDS 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, leaf and pod loss 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 the No. 1 yield-robbing pest in soybeans, claiming more than 30 percent in heavily infested areas. 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 Remains a Risk
Frogeye leaf spot 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, frogeye may initially appear in the lower canopy. Prolonged adequate moisture can provide conditions that allow the disease to spread throughout the soybean canopy.
Frogeye leaf spot 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 frogeye leaf spot, potentially spreading the disease from field to field. Applying a fungicide such as Delaro™ or Stratego® YLD provides both preventive and curative activities for controlling frogeye, including strobilurin-resistant strains. Planting resistant varieties, rotating crops, tilling crop residue and applying foliar fungicide also aid in managing frogeye leaf spot.
White Mold: An Aggressive Fungus
White mold can cause yield losses ranging from 10 to 30 percent in untreated fields. 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.
Delaro offers a unique solution for white mold. The ideal time to first apply Delaro is prior to disease development, at R1, followed by a second treatment at R3 or R4. Disease-susceptible cultivars and seed soybeans are most likely to benefit from a Delaro application. For cost efficiency, Delaro can be tankmixed with insecticides and applied in the same trip across the field.
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-most yield-stealing disease in soybeans.
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.
Strategies for Taking Control of Soybean Diseases
Among the most important management tools for soybean disease are resistant variety selection, seed treatments and timely fungicide applications. 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.
Know Your Fields' Disease Histories
Growers should thoroughly study the history of past soybean diseases on their farm as a first step in managing the current growing season. Since many 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.
Consider Cultural Strategies
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 the spread of soybean disease from one field to another.
Selection of Resistant Varieties
While genetic resistance doesn’t guarantee immunity from soybean disease, resistant varieties are a key disease management tool. Monty Malone, Crop Science Soybean Agronomist, said Credenz® offers growers a wide range of choices to address their specific needs.
“For example, growers using multiple herbicides for controlling resistant weeds can now leverage one seed brand across all of their soybean acres," Malone said. "And that means they can work with one trusted seed dealer to meet all of their needs, which is a real benefit to them.”
Get Added Protection with Seed Treatments
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. Field trials from 2011-2017 indicate ILeVO increased yields by 2 to 10 bushels per acre, depending on disease and nematode pressure
Apply Fungicides Early in the Season
Timely fungicide applications, including foliar fungicides, help protect soybean plants from fungal diseases through the season.
Delaro™ fungicide is formulated with two heavy-hitting modes of action, together maximizing control of even the toughest diseases, including frogeye leaf spot, brown spot and white mold. With both strobilurin and triazole components, Delaro delivers unmatched, long-lasting protection and fights development of disease resistance. Delaro promotes healthy, dark green leaves for improved photosynthesis and increased stress tolerance, helping you realize the full potential of your soybeans.
Before purchasing seed, selecting a seed treatment or applying any fungicide, please read the entire label for the best possible results and to confirm that the product is effective on the disease you need to control. Every product is not suitable for every situation, and correct application technique will ensure the best results.