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Soybean Pest Management Highlights

Author: Keith Waldron, Dairy and Field Crops IPM Coordinator, Cornell University.

Soybean production is gaining popularity in New York State. For many producers soybeans fit well in their crop rotations, provide a useful home grown source of livestock feed, and offer a valuable cash crop option. As soybean acreage increases so have questions regarding crop protection.

While our experience with soybean pests is limited, field observations combined with reports from adjacent states and Ontario, Canada are encouraging. Relatively few insects and diseases have been reported to cause significant problems on soybeans in our area. Crop rotations also help break up the life cycles of many common corn and alfalfa insects and diseases.

What soybean pests might you expect and how can you avoid or minimize their damage? Some key soybean pest management considerations for New York producers follow.

SUCCESSFUL SOYBEAN PEST MANAGEMENT HIGHLIGHTS

Use of sound agronomic practices is critical for profitable soybean production. Choose a well adapted, disease resistant variety from a appropriate maturity group. Plant soybeans in a timely manner on a suitable planting site with proper field and seedbed preparation, fertility and soil pH to enhance production success. The use of clean, certified seed is highly recommended. If bin run seed is used have seed checked for germination, seed borne diseases, and presence of weed seed.

An effective weed management program will minimize early season competition and help protect potential yields. As with corn production, spring and fall weed surveys will help identify and assess troublesome weeds and improve information to tailor and time weed management programs. In addition to standard pre- and post-emergence herbicide options, growers should consider banding herbicides in a 10" band over the top of soybean rows at planting and use of timely cultivation(s) to minimize weed pressure. Research studies involving soybeans in 30 inch rows have had very favorable yields with banding of herbicides over the row and timely cultivation's, compared to conventional herbicide or cultivation only weed control programs.

Insects seldom cause significant damage to soybeans in NY. Seedcorn maggot is occasionally a problem under cool conditions on fields with high organic matter content, such as manured fields. Planter box applications with a commercial seed treatment containing an insecticide is recommended. Use of seed treatments combined with high plant populations and soybeans' remarkable ability to compensate for some stand reduction generally minimizes stand losses from this pest. Japanese beetles are a more obvious visitor to soybean fields mid to late summer. Adult beetles feed on soybean leaves causing a skeletonized and brown appearance. While this damage may be very conspicuous, damage is rarely economical. Indeterminate soybean types can tolerate up to 35% defoliation until bloom, about 20% while pods are small and soft, and about 35% when pods are hardening. Treatment for pest damage below the percentages listed are not recommended.

Two Spotted Spider Mites occasionally cause problems during hot dry years. Mites are minute, about 1/60th inch or less in length. Mites typically live in colonies producing a thin web on lower leaf surfaces. Mite injury causes a speckled appearance to leaves, which become yellow, curl, then brown, and plants die. Rain will reduce risk of mite problems.

Disease survey information is limited, but suggests several diseases may pose potential risk to soybeans. White mold caused by Sclerotinia sclerotiorum, downy mildew, brown spot (Septoria Leaf Spot), pod and stem blight caused by Phomopsis spp, Phytophthora root rot, and soybean mosaic have all been reported in New York. Soybean cyst nematode, a major pest of soybeans in many other states and Ontario, Canada has NOT been detected in New York. Crop rotation, selection of disease resistant varieties of appropriate maturity groups, and use of sound agronomic practices help avoid of minimize soybean disease problems. Three diseases bear particular mention: white mold, downy mildew, and pod & stem blight.

White mold may be a problem in soybeans planted directly after dry beans (or other Phaseolus spp), sunflower or canola fields where white mold has been a problem. Rotation to a non-susceptible crop such as corn, wheat or oats is recommended. White mold causes damage under cool moist conditions. Infection typically occurs at stem nodes, usually 4 - 20 inches above the soil line. Frequently a white cotton-like growth and small dark round to elongate shaped bodies can be seen on or within the stem. Stems and pods infected with this fungus are pale brown and appear water soaked. Increasing row width to 30 inches and / or reducing plant density to reduce canopy relative humidity will help reduce risk of white mold. Resistant varieties may be on the horizon.

Downy mildew appears on the upper surface of young leaves as pale green to light yellow spots which enlarge into pale to bright yellow spots of indefinite size and shape. On the lower surface of leaves, particularly in moist weather, lesions are covered with a gray to purplish fungal mass. Severely infected leaves may curl, brown, and prematurely drop. Pod infections may occur without visible external symptoms. Crop rotation and plow down of infected residue are two management options.

Pod and stem blight reduces yield by spoiling seeds which have low test weights or may be lost at harvest. Diseased seeds may have lower viability, a poor appearance, produce lower quality oil or flour and may lead to a grade reduction and lower price. Plant infection may occur in the seedling stage from infected seed, or in larger plants from infested plant debris. Definite leaf or stem lesions are not produced under field conditions. Small black fungal bodies may be apparent often in a series of lines on dead stems, pods, or petioles of dropped leaves. Management of this disease is greatly enhanced by use of certified disease free seed, plow down of infected crop residue and crop rotation with corn or other non-host.

Soybean pest information will continue to improve as experience with this crop in New York increases. For the moment, pest problems appear to be relatively few and can be minimized with sound planning. Producers are encouraged to monitor soybean fields during the growing season to detect potential problems early, particularly during seedling emergence, pod fill stages, and periods of drought. Contact your local cooperative extension field person to discuss any unusual field problems you may observe.