Corn, Soybean, Wheat, and Alfalfa Field Guide

Bulletin 827-05

SOYBEAN MANAGEMENT

DESCRIPTION OF VEGETATIVE AND REPRODUCTIVE STAGES

INSECT SCOUTING CALENDAR FOR SOYBEAN

Defoliation Assessment: To estimate defoliation collect at random 30 or more trifoliates and estimate average level of defoliation (see examples). Avoid samples from field perimeter.

Maximum levels of defoliation tolerated by soybeans without significant loss in yield.

Pod Injury Assessment: To evaluate pod injury inspect 10 or more whole plants, selected at random. Pod injury is highest in the upper canopy. When 10 to 15% or more of pods exhibit injury seed damage is evident, especially if high moisture infection periods trigger mold development.

Sweep Net Sampling of Insect Pests: Trends of increasing or decreasing pest activity on soybeans can be determined by periodically taking 10 sweeps from 3 or more locations in a field.

Assessing Soybean Aphid: Count the number of aphids per soybean plant at 30 locations throughout the field, and calculate an average number of aphids per plant.

SOYBEAN DEFOLIATION LEVELS

SEEDCORN MAGGOT

Larva & Damage

Identification & Incidence: Soybean emergence failure due to feeding by small, yellowish-white, legless fly larvae (maggots) on germinating seeds. Damage is likely to occur in fields having high organic matter (especially fields that have had green material, such as cover crops or weeds incorporated into the soil) and when cool and damp soil conditions delay emergence.

Sampling: Maggots may be detected by inspecting areas in the field exhibiting lack of emergence. Check seed to see if maggot damage has occurred.

Economic Threshold: No economic threshold exists for this insect and no rescue treatment is available.

Management Options: Seed maggot injury may be prevented by use of seed treatment at planting. See page 81 for insecticides labeled for SCM.

SLUGS

Damage

Identification & Incidence: Slugs are soft-bodied animals belonging to a class called Gastropoda (not insects). They range in color from pale cream to gray to shiny black and range in size as adults from less than an inch to over two inches in length. They can damage seeds and seedlings reducing stand and may defoliate established stands that may delay plant development. Slugs damage soybean by destroying the germinating seed prior to emergence and causes significant defoliation.

Sampling: Inspect several areas of the field and determine percentage of plants being fed upon and percent defoliation. Slugs are nocturnal animals and it may be necessary to inspect fields at dusk or after dark to see slugs and determine population densities.

Economic Threshold: Treatment may be necessary if defoliation is greater than 40% on slow growing plants or if plants are being killed.

Management Options: Several bait formulations of metaldehyde are labeled for use on soybean for slug control.

BEAN LEAF BEETLE

Adult		Pod Injury

Identification & Incidence: Damage to soybeans by bean leaf beetle (BLB) extends from feeding of over wintering adults on emerging seedlings to pod feeding by 2nd generation adults in the fall. Life cycle of BLB includes 2 generations with the peak of the 1st generation in July and the peak of the 2nd generation in late August or September. BLB impact on soybeans tends to increase when most soybeans are planted early.

Sampling: Early in the season sample 5 plants in 5 locations for defoliation. Late in the season sample for both defoliation and pod damage.

Economic Threshold: Treatment is warranted when BLB adults are actively feeding and the defoliation is 40% pre-bloom, 15% bloom to pod-fill and 25% pod-fill to harvest. Treatment may be needed late in the season for pod damage if 10% or more of the pods are being fed upon and BLB is still present.

Management Options: Timely application of rescue treatment will reduce losses in yield and seed quality. Late plantings of soybeans will escape defoliation by over wintering BLB and limit establishment of first generation. See page 81 for insecticides labeled for BLB.

MEXICAN BEAN BEETLE

MBB Adult		MBB Larvae & Damage

Identification & Incidence: Both the bronze adult Mexican bean beetle (MBB) having 16 black spots and the spiny larvae stage cause skeletonized foliar injury of soybeans. Egg and pupae stages are also found on the foliage. MBB infestations in Ohio occur primarily in the eastern and southern regions. Severe defoliation generally occurs in late summer when late instar larvae of the 2nd generation reach peak activity.

Sampling: Inspect 5 plants from 5 locations and determine the percent defoliation. Determine the predominant growth stage of MBB.

Economic Threshold: Treatment is warranted when MBB larvae and/or adults are actively feeding and the defoliation is 40% pre-bloom, 15% at pod fill and 25% at full pod to harvest.

Management Options: Rescue treatments should be applied in a timely manner to prevent peak injury caused by late instar larvae. Treatments applied after larvae begin pupating achieve limited results. See Soybean Insecticides Table for insecticides labeled for MBB.

OTHER INSECT DEFOLIATORS

Japanese Beetle Adult

Metallic colored adult Japanese beetles (JB) emerge in early July and remain abundant through summer. JB adults tend to aggregate and are often more abundant in pockets in the field. Thus, random sampling of foliage is important.

Grasshopper

During hot summers, various species of grasshoppers may be abundant, especially along grassy borders of a field.

Green Cloverworm

This pest is often found in soybeans in low numbers and economic infestations warranting treatment are uncommon.

The occurrence of one or more insect defoliators in abundance may lead to unacceptable defoliation. Defoliation levels should be monitored closely, especially from bloom to pod-fill. Treatment is warranted when defoliation is 40% prebloom, 15% bloom and 25% pod-fill to harvest. See Soybean Insecticides Table for insecticides labeled on these insects.

TWO-SPOTTED SPIDER MITE

Damage

Identification & Incidence: Drought conditions initiated during the spring may lead to the development of two-spotted spider mite (TSSM) outbreaks by early or mid-summer. Initial signs of infestations will be evident where drought stress is most severe. Foliage discoloration and mites will be detected on the underside of the foliage. Use of a hand lens is advised to detect presence of adults and immature stages. Mite eggs appear as clear or yellow marbles under magnification. Recently hatched nymphs have 6 legs compared to later nymphs and adults, which have 8 legs.

Sampling: Inspect plants especially along the field margins but also farther inside the field for mites and damage. Record the stages of mites that are present in the field.

Economic Threshold: Economic thresholds have not been established for two-spotted spider mites. Treatment is warranted if infested areas begin to increase in size and weather forecasts continue to call for hot, dry weather.

Management Options: Treatment of TSSM may be applied by air or ground equipment. Aerial treatments require the use of a miticide with systemic activity. Onset of cool and wet weather conditions may reduce TSSM infestations. See Soybean Insecticides Table for miticides labeled for TSSM.

SOYBEAN APHID

White objects are aphid cast skins

Identification & Incidence: The soybean aphid is a small, yellow aphid with distinct black tailpipes, or cornicles, on the abdomen. The adult aphid is the size of a small potato leafhopper nymph. Adults lay live young rather than eggs. Early in the season the aphids will be found on the upper leaves. As populations increase, they can be found on the upper stems and later on the lower leaves.

Sampling: Sample plants from 30 locations throughout the field and make an estimate of the average number of aphids per plant.

Economic Threshold: The economic threshold for soybean aphid has been established at 250 soybean aphids per plant, with an increasing population density. Thus, at least two samples are needed to determine whether the population size is rising.

Management Options: There are several predators, including lady beetles, that may help take care of soybean aphid. If populations reach 250 aphids per plant and the density is rising, then the use of an insecticide might be needed. See Soybean Insecticides Table for insecticides labeled for soybean aphid.

TIME OF DISEASE SYMPTOM OCCURRENCE ON SOYBEANS

PHYTOPHTHORA DAMPING OFF AND ROOT ROT

Description: Phytophthora damping-off is recognized as a pre- or post-emergence damping off. Affected seedlings have brown, water-soaked lesions on the hypocotyl. Phytophthora root rot is recognized on adult plants as a severe root rot with stem discoloration that may extend several nodes up the stem.

Location: Phytophthora root rot has been a problem in the heavy clay soils throughout Ohio, particularly in the northwest, with the increase in the use of reduced tillage. The disease has been reported in most soybean growing counties of the state, due to adaptations by the Phytophthora population to the most commonly deployed Rps genes in commercial soybean varieties.

Time of Infection: Seedling damping off occurs when rainfall saturates the soil soon after planting. Saturated soils also favor root rot of adult plants later in the season. Soybeans planted in compacted soils or soils with poor drainage are particularly vulnerable to seedling and adult plant kill.

Management:

• Resistant varieties
• Crop rotation
• Improve drainage
• High rates of fungicide seed treatment

SOYBEAN RUST

Description: Lesions first appear as small chlorotic and irregularly shaped spots, turning tan to brown or reddish as the disease progresses. Lesions are usually confined to the veins or close to the veins, and are very, very small, less than the size of a leaf hair. Spores form predominately on the underside of the leaf and can be abundant. Soybean rust causes premature defoliation leading to yield losses, fewer seeds per pod, decreased number of filled pods per plant and early maturity. At later stages, the key diagnostic feature of soybean rust is the presence of egg-shaped pustules that are filled with cream to tan colored spores.

Time of Infection: The environmental conditions that favor disease development are extended periods of leaf wetness accompanied by temperatures of 59° to 82°F. Temperatures above 86°F retard the development of disease. Spores are generally produced 10 days after infection, and are released after about three weeks. Spores are continually released, as long as moist and moderate temperatures are present. The pathogen is not spread by seed. Soybean plants are susceptible to rust at any stage, but disease severity depends on the developmental stage of the plant at the time of infection.

Management:

• Early detection
• Fungicide application

Here are some specific recommendations to help achieve the best coverage and control when spraying for soybean rust.

• Keep spray volume above 15 gpa for best results.
• Choose the appropriate size and type of nozzles and operate them at a pressure that will allow them to produce small to medium-size droplets (200-300 microns).
• Nozzles producing a flat-fan pattern seem to provide a better coverage than the nozzles producing cone pattern when spraying for soybean rust when there is full canopy.
• Use twin nozzle/pattern technology. Research has shown that two spray patterns, one angled forward and one angled backward, perform better than single nozzles spraying in one direction.
• Flat fan nozzles require about a 50% overlap of adjacent spray patterns. Improper overlap will yield to streaks of untreated areas in the field. Make sure the boom height is adjusted properly so that the proper overlap is achieved.
• Avoid drift of very small droplets. You may want to use some of the new “low-drift” nozzles which allow a grower to increase the pressure without increasing the number of small, drift-prone droplets (air-induction low-drift nozzles are not recommended). Operate these nozzles at slightly higher pressures (60-70 psi) than usual. Higher spray pressures usually help the droplets penetrate the canopy better.
• Very few soybean fields are planted in a 30" row in Ohio. If your beans are planted on 30" rows, and if there is sufficient open space between the rows at the time of spraying, take advantage of directed spraying to cover the plant with more than one nozzle from different angles (from top and both sides).
• If economically feasible, use air-assisted spraying. Research has shown that air-assisted spraying, which uses air to help small droplets reach inside of the plant canopy and underside of leaves, consistently provides the best coverage and droplet penetration, especially when beans are at or near their full-growth stage. One should match air flow to canopy density when using air-assisted sprayers.
• For current fungicide recommendations, refer to the Ohio Field Crop Disease web site www.oardc.ohio-state.edu/ohiofieldcropdisease.

RHIZOCTONIA SEED ROT AND STEM ROT

Description: Rhizoctonia produces a reddish brown lesion on roots emerging from the seed. On emerged seedlings the fungus produces a brick-red, sunken canker on the stem at the soil surface. Plants that are not killed as seedlings will appear yellow and stunted.

Location: Rhizoctonia is found throughout Ohio.

Time of Infection: The specific conditions that favor attack by Rhizoctonia range from dry to very wet soils. Herbicide injury can increase the severity of this disease. Rhizoctonia is a soil-borne fungus present in most soils. In general, dry soils after seedling emergence favors seedling blight and stem rot of infected plants.

Management:

• Crop rotation
• Seed treatment
• Improved drainage

BACTERIAL LEAF BLIGHT

Description: Bacterial leaf blight appears as small angular, water-soaked dark spots surrounded by a narrow yellow halo. The centers of lesions may fall out or tear leaving the leaves with a ragged appearance.

Location: Bacterial leaf blight occurs throughout Ohio.

Time of Infection: Bacterial leaf blight is generally an early season disease. Bacteria are spread by driving rain from infected cotyledons to newly developing leaves.

Management:

• Crop rotation
• Resistant varieties

BROWN SPOT

Description: Brown spot is a fungal disease that produces irregular, dark brown spots that vary from minute flecks to lesions 3/16" in diameter. Lesions coalesce, turn dark brown and may have a yellow border surrounding the blotchy brown areas.

Location: Brown spot occurs throughout Ohio and can be severe in continuous soybean production fields.

Time of Infection: Spread of brown spot occurs early in the season on young seedlings and again late season as the crop begins to mature. Wet conditions with heavy rain spreads the fungus upwards in the plant canopy.

Management:

• Crop rotation
• Tillage to bury residues

DOWNY MILDEW

Description: Downy mildew appears on the upper surface of young leaves as pale green to yellow spots that enlarge into bright yellow lesions of indefinite size and shape. During moist weather lesions on the lower leaf surface are covered with tufts of grayish mold growth. Seeds may be encrusted with dull, whitish mold growth.

Location: Wherever soybeans are grown in the state.

Time of Infection: First symptoms may be seen on trifoliate leaves of young plants in June during periods of high humidity and cool temperatures. Disease spread continues if temperatures remain cool. Most commonly found in years with cooler than average temperatures and above normal rainfall.

Management:

BUD BLIGHT

Description: Bud blight is a virus disease transmitted by thrips. Damage is usually confined to the perimeter of the field. Seedling infections result in severe stunting, shortened internodes, curving of the terminal bud to form a crook, and a brown discoloration of the pith near the nodes. Buds become brittle and fall off and leaves become rugose and bronzed. Plants stay green and do not mature. Do not confuse with herbicide injury.

Location: Throughout Ohio.

Time of Infection: Seedlings through adult plants.

Virus symptoms vary on different soybean varieties and more than one virus can infect a plant. Laboratory analysis is necessary for determining which virus is responsible for the disease.

YELLOW MOSAIC

Description: Yellow mosaic is a virus disease transmitted by aphids from legume weeds (clover). Symptoms are recognized as a conspicuous yellow mottling of the leaves during periods of cool temperatures. Yellow areas are scattered over the leaf surface and rusty, tan spots may appear in them.

Location: The most common and widespread virus disease of soybean in Ohio.

Time of Infection: Symptoms generally show up late in the season and damage is minimal.

Virus symptoms vary on different soybean varieties and more than one virus can infect a plant. Laboratory analysis is necessary for determining which virus is responsible for the disease.

SOYBEAN MOSAIC

Description: Soybean mosaic is seed-borne and spread by aphids in the field. Leaves of affected plants are generally misshapen and puckered, occasionally with green venations. Leaflets are asymmetric and curl down at the margin. Less severe symptoms include a mild mosaic.

Location: Throughout Ohio.

Time of Infection: Seedlings or adult plants may show symptoms. Symptoms are most evident during the cooler growing periods and the symptoms are masked during hot weather (above 85°F).

Management:

• Resistant varieties

Virus symptoms vary on different soybean varieties and more than one virus can infect a plant. Laboratory analysis is necessary for determining which virus is responsible for the disease.

SCLEROTINIA STEM ROT OR WHITE MOLD

Description: Sclerotinia stem rot is identified as a white cottony mold growth on soybean stems after flowering. As the cottony growth ages, hard, black, oval to elongate resistant structures (sclerotia) form on the stem surface or inside the stem. The sclerotia are gray to pinkish white on the inside with a black outer surface.

Location: Sclerotinia stem rot occurs throughout the state, but has been more severe in fields surrounded by woods, with high plant populations, and high fertility soils.

Time of Infection: Wet weather before and during flowering of the soybean plant is important for infection. The duration of the wet period during flowering and the capacity of the soybean canopy to keep stems wet for extended periods of time contribute to disease severity. Symptoms first appear in late August or early September.

Management:

• Resistant varieties
• Seed treatment
• No-till
• Lower plant populations

STEM CANKER

Description: Stem canker begins as a reddish-brown lesion on the leaf scar after the petiole on the stem has fallen. The lesion enlarges rapidly forming a slightly sunken, reddish-brown to black girdling canker. The stem above the canker generally dies.

Location: Stem canker can occur anywhere in Ohio.

Time of Infection: Lower leaves may be affected early in the season, but girdling occurs after mid-season such that dead plants can be found in August.

Management:

• Resistant varieties
• Crop rotation

SOYBEAN CYST NEMATODE

Description: The most common symptom of soybean cyst nematode is reduced yield with no above ground plant symptoms. Yellowing and stunting of plants is generally only detected in fields with high populations. The cyst nematode can best be seen on roots in the field during late July or August. Plants should be dug to keep roots intact. Gently remove soil from around roots and examine root surfaces for very small (about the size of a pin head) lemonshaped, white cysts. Later in the season cysts turn brown and are difficult to see. Soil samples should be collected in the fall after harvest for disease management strategies.

Location: Soybean cyst nematode has been found in all of the soybean growing counties in Ohio, especially in the western half of the state.

Time of Infection: Soybean cyst nematodes become active as the soil warms and young soybean plants produce root systems. The nematode is capable of several generations during the growing season.

Management:

• Crop rotation among crops (wheat-corn)
• Rotation among sources of resistance (PI88788, Peking; PI437654, Hartwig)

BROWN STEM ROT

Description: Leaf symptoms of brown stem rot do not always occur in Ohio but consist of interveinal chlorosis and necrosis where the veins remain green. The most reliable characteristic is the dark, reddish-brown discoloration of the stem pith in the lower stem. Leaves may suddenly wilt and plants die 3-4 weeks prior to maturity.

Location: Brown stem rot is more prevalent in fields with a long history of soybean cultivation. Especially those in continuous soybean production have a greater probability of having economic losses to brown stem rot.

Time of Infection: The brown stem rot fungus resides in the soil and generally attacks plants during the first half of the growing season. No obvious symptoms develop on plants until after flowering then symptoms develop rapidly during periods of dry weather.

Management:

• Crop rotation
• Resistant varieties

CHARCOAL ROT

Description: Charcoal rot has been occasionally diagnosed in Ohio. Early season diagnosis is very difficult, but during the later stages of plant development very small, black resistant structures (sclerotia) become evident as a grayish-black discoloration in the root and lower stem tissues. Cutting the lower stem with a knife will expose the discolored tissues in comparison to normally white tissues of healthy plants.

Location: Charcoal rot is more prevalent in southern and western Ohio than in the north central region.

Time of Infection: Infection occurs early in the season on seedlings and the fungus grows slowly in the plant until hot dry weather occurs after flowering. Premature death of plants suffering from drought stress is an indication of charcoal rot.

Management:

• Crop rotation

POD AND STEM BLIGHT		PHOMOPSIS SEED ROT

Description: Linear rows of small black fruiting bodies (pycnidia) on stems of plants and scattered over the surface of pods at or before harvest are signs of pod and stem blight.

Description: Phomopsis seed rot can be detected in harvested grain as badly cracked, shriveled and moldy seed.

Location: Pod and stem blight and Phomopsis seed rot occur throughout Ohio, but are more prevalent in the southern and western regions of the state.

Time of Infection: The fungi that cause these diseases reside on residues or on seed. Infection occurs during pod formation, but symptoms do not develop until plants begin to mature and pods turn yellow. Under wet conditions that prevent timely harvest of the crop, the pod and stem blight fungi enter the developing seed and cause Phomopsis seed rot.

Management:

• Crop rotation
• Seed treatments
• Tillage

SUDDEN DEATH SYNDROME

Description: Leaf symptoms start as bright yellow, small spots which grow to necrotic areas. The pith is white but roots are rotted and the tissue in the crown is discolored.

Location: Sudden death syndrome has been confirmed in a few isolated fields in western Ohio. In Ohio it has always been found in association with soybean cyst nematode.

Time of Infection: The sudden death syndrome fungus generally attacks plants prior to flowering when soils are wet.

Management:

• Reduce SCN populations
• Improve soil drainage
• Plant earlier maturing varieties
• Crop rotation

BEAN POD MOTTLE

Description: Bean pod mottle is a virus that is transmitted by bean leaf beetles. Symptoms can range from a mild mosaic to severely distorted leaves with raised areas. Symptoms are dependent on host genetics and environment.

Location: Confirmed in six counties but probably occurs statewide.

Time of Infection: Seedlings to adult plants may show symptoms.

Virus symptoms vary on different soybean varieties and more than one virus can infect a plant. Laboratory analysis is necessary for determining which virus is responsible for the disease.