Picture this: flourishing soybean fields. Green leaves, thick canopy, strong stands and roots, an abundance of pods. Optimal moisture, not too hot, not too cold. No disease or pest incidence. The best possible soybean-growing environment you can imagine.
Now picture this: reality. Though the optimal growing conditions may exist, the odds of no pests and disease are pretty small, and chances are you'll deal with a variety of them through the growing season. To alleviate the pain and pressure, though, university and Extension personnel have a wealth of data and advice to help combat those pesky pests and dreadful diseases.
Last growing season certainly saw its share of pests and disease, along with not-so-optimal growing conditions around soybean-producing regions of the U.S. From stink bugs in the south to soybean cyst nematode (SCN) in the north, growers had their hands full of managing beans under reality conditions.
“Minnesota was very dry in much of the soybean growing areas, and diseases did not develop as they normally do,” says Dean Malvick, University of Minnesota Extension plant pathologist. “Though 2007 was not a representative year, disease problems did exist, including SCN, SDS, brown stem rot, charcoal rot — due to dry weather — Phytophthora and Fusarium root rot.”
He expects much of the same this year, including white mold, if Minnesota has near-normal rainfall, except for charcoal rot.
Kentucky experienced plenty of charcoal rot and SCN.
“Charcoal rot is always associated with drought, and much of Kentucky was in serious drought during the second half of the summer in 2007,” says Don Hershman, Extension plant pathologist for University of Kentucky. “SCN is always present, but the effects are worst when plants are under drought stress.”
Missouri fields had an elevated number of stink bugs, along with white grubs, says Wayne Bailey, University of Missouri Extension entomologist. Soybean aphid was also present, and reached economic infestations in a few counties and in fields with potassium deficiencies.
Pathologists and entomologists from across the south and the Midwest have given their predictions about what to watch in 2008. Granted no one can provide a solid outlook, but here's what they recommend you watch for this year.
INSECTS
SOYBEAN APHID
SYMPTOMS: Plant sap is removed via sucking from the small, yellowish, glob-shaped aphid, causing leaves to wilt and curl. If numbers are high, leaves may become yellow and distorted; the plant may be stunted, covered in dark, sooty mold. From seedling to blooming, aphids will colonize tender leaves/branches, later moving down to colonize near the middle/lower underside of leaves and stem.
TIME OF ATTACK: V1-V2 on upper leaves, petioles, stem (scout twice/week); R1-R4 on undersides of mid-canopy leaves, stems.
CONDITIONS FAVORING: Northern states with cooler summer temps; fields with previous aphid populations; late plantings; crops stressed by hot, dry weather; high overwintering populations.
MANAGEMENT: Foliar-applied insecticide when populations reach 250/plant and continue to increase and 80% of field is infested; time treatment to maximize effectiveness. Beneficial lady beetles could help in control.
STINK BUGS
SPECIES: Redbanded, Southern Green, Brown, Green
SYMPTOMS: Flat pod syndrome and delayed maturity; Brown and Green species attack pods and seeds. Feeding punctures cause small brown/black spots. Young seeds can be deformed or undersized; older seeds become shriveled and discolored.
TIME OF ATTACK: R1-R7 reproductive stages.
CONDITIONS FAVORING: High populations of bugs and vulnerable growth stages; common along Louisiana and Texas Gulf coast.
MANAGEMENT: Cultivar selection and planting date. Rescue insecticide treatments when threshold reached while scouting. Populations also partially suppressed by predators and parasitic wasps.
BEAN LEAF BEETLE
SYMPTOMS: Feeding is identified by small, round holes between veins. Entire pods may be clipped and the outside layer of pod tissue entirely consumed. Beetles feed on plants at all stages of development. Overwintering populations feed on cotyledons and leaves. The next generation feeds on leaves and the final generation feeds on leaves and pods.
TIME OF ATTACK: All season; first generation: late V, early R; second generation: pod-fill stage.
CONDITIONS FAVORING: Early planting and above-normal summer temperatures. Late-planted fields and below-normal temps can cause better conditions for second-generation beetle pod feeding.
MANAGEMENT: Timely application of rescue treatment if sampling/defoliation threshold is reached.
SPIDER MITES
SYMPTOMS: Leaves appear sandblasted. Heavily infested leaves turn red-brown and die. Webbing may be present on undersides of leaves. Heavy infestation causes leaves to wilt and die.
TIME OF ATTACK: V2-R7.
CONDITIONS FAVORING: Drought conditions.
MANAGEMENT: Treatment by air or ground application if infested areas increase in size (no established threshold). Cool, wet weather conditions may reduce infestation.
DISEASES
SUDDEN DEATH SYNDROME
SYMPTOMS: Leaves become brown, with veins remaining green; leaf blades drop off but petioles remain attached. Brown to gray discoloration of internal taproot.
TIME OF ATTACK: During pod set and fill.
CONDITIONS FAVORING: Cool, wet conditions throughout summer; well-fertilized fields; early planting/maturity; soil compaction; fields with history of SDS; co-infection with SCN.
MANAGEMENT: Resistant cultivars; avoid early planting — wait until soils are warmer and drier; eliminate soil compaction.
CHARCOAL ROT
SYMPTOMS: Small, black sclerotia form on lower stem and roots, both internally and externally. Early season infection produces red-brown lesions. Premature plant death; plants wilt and eventually die.
TIME OF ATTACK: R3-R6.
CONDITIONS FAVORING: Extended periods of hot, dry weather.
MANAGEMENT: Reduced plant populations; rotation; planting fullest maturity group; irrigation, if available. No way to stave off damage in drought year.
SOYBEAN CYST NEMATODE
SYMPTOMS: Difficult to spot in high-yielding fields or when soil moisture is optimal. Yellowing and stunting of plants; white to yellow lemon-shaped cysts on roots.
TIME OF ATTACK: As soil warms and root systems develop.
CONDITIONS FAVORING: Those that favor soybean growth. May first appear near field entrances, flooded areas, weedy areas, high-pH spots, lower-yielding areas. Moisture and fertility stress can enhance disease.
MANAGEMENT: Crop rotation, resistant varieties (rotate resistant varieties, as well), introduction prevention. Test soil samples.
SEEDLING BLIGHT
SYMPTOMS: Seed rot. Seedlings may grow poorly, turn yellow and die. Dieback of young plants with damping-off of lower stem.
MANAGEMENT: Seed treatments; delayed planting until soils are warmer/drier; plant high-quality seed with high germ and vigor.
Editor's Note:Please keep in mind this is just a general, short list of pests and diseases to keep an eye out for. Others mentioned included corn earworm, white grub, soybean looper, brown stem rot, Fusarium, Phytophthora and frogeye leaf spot. For specific information for your location, contact your local Extension office or university specialist.
