2023 Soybean Insect & Disease Control
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Information on Insect and Disease Control in Soybeans

Information on Insect and Disease Control in Soybeans

We have several sites in the area that monitor corn earworm moth flights from corn to cotton and soybeans. As expected, numbers started to climb in late July. Please see the trap data for more information.

I have been sweeping fields this past week and have not found corn earworms at threshold levels.  Some of our early marrying varieties are far enough along that earworms may not be a problem. Scout to make sure, but, blooms attract moths. Research has shown that pods need to be present for insecticide sprays to be effective against podworms in soybeans.

I have caught stink bugs and kudzu bugs in the sweep net and on the beat cloth. As pictured, the Kudzu Bug eggs on the left leaf with a corn earworm on the leaf. Stink bugs were at the seed threshold in a couple of those fields. On the right. A corn earworm on a leaf with frogeye leafspot.

Soybean Podworm Complex and Resistance

We have several effective insecticides for corn earworm in North Carolina soybeans. One of these is the insecticide chlorantraniliprole, which is the active ingredient in Coragen, Prevathon, and Besiege. Because we rely entirely on this insecticide to control corn earworm (bollworm) in cotton and Coragen is used in tobacco, Dominic and Anders are encouraging growers to avoid this insecticide in soybeans. In addition, soybean looper is resistant to this insecticide and generally shows up in the biggest numbers a few weeks after earworms.

Therefore, growers should use products like Blackhawk, Intrepid Edge, or Steward to control corn earworms in soybeans. These products will pick up most of the soybean worm complex. Proper Identification of the soybean worm complex can help growers make better insecticide choices. Thresholds for podworms vary so please use the podworm calculator.

Stink Bugs

Stink bugs are seed feeders and are very destructive to yield at R4 and R5, destroying pods and aborting or shriveling seed. They can still cause yield loss at R6 by sucking away nutrients the plant needs to fill out seed weight and can cause stay-green. Use these thresholds to know when you should treat stink bugs.

You can double those thresholds once you hit R6.5 (where the seed is starting to separate away from the pod wall), and don’t treat past R7.

Kudzu Bugs

Growers should treat for Kudzu bug when numbers hit one nymph per swoosh of the net (15/15 sweeps) until R6. Once reaching R6, growers can relax the threshold greatly and tolerate more kudzu bugs. I’d be hard-pressed to recommend a number without more information, but consider the cost of the spray, including drive-down loss over the beans and yield potential. Also, check fields to see if any kudzu bugs are infected with a fungus. This fungus can provide 50-100% control; if it’s present, consider saving the spray.

Disease Control

It is always highly recommended to plant soybeans with resistance to as many soybean diseases as possible for the area you are planting in. With that stated, wet and humid weather and cool and humid weather can often lead to foliar and stem diseases on soybeans. I have included the latest update from the Asian soybean rust site on 7/27/2023 and a link to the updated maps.

Soybean Rust
(Phakospora pachyrhizi )

Although soybean rust can cause defoliation and significant yield loss, the pathogen does not overwinter in North Carolina. The alternate host for soybean rust is kudzu; however, environmental conditions during the winter months in North Carolina generally prevent overwintering from occurring. Each year, inoculum disperses northward, and inoculum is tracked using the Soybean Rust IPM Pipe. Cool, wet weather with prolonged leaf wetness (more than 6 hours) allows for spore germination, and conditions with high humidity encourage disease development after establishment.

Symptoms. Rust lesions are tan to reddish-brown and may occur on any green surface of the plant. Lesions contain one to three pustules that are raised on the leaf surface.

Lesions may have an angular shape and may be confused with bacterial pustule lesions. Placing leaves in a bag with a moistened paper towel for 24 hours may cause pustules to erupt and expose spores. Infected plants may defoliate early and have smaller seeds. If you suspect that rust may be present in your field, contact your county Extension agent to send samples to the North Carolina State University Plant Disease and Insect Clinic for confirmation.

Management. There are no rust-resistant soybean varieties commercially available in North America. When rust is found within 100 miles of a field, foliar fungicides may be necessary to manage the disease. To adequately control rust, fungicides must be applied before or soon after the first infection. Foliar fungicides applied between R3 and R5 may reduce the level of disease.

Based on data from NC State, making a fungicide application can produce a profit, but choosing the right product at the right time can make or break profits in any given year.

Please see the 2023 Ag Chemical Manual for recommendations.

Other foliar diseases in our area that cause issues are Frogeye Leaf Spot, Cercospora Blight, Bacterial Blight, and Brown spot. It is always RECOMMENDED to check for variety resistance before spraying.

Frogeye Leaf Spot
(Cercospora sojina)

Frogeye leaf spot is primarily a foliar disease of soybean but is capable of being seed-borne. Disease epidemics can become severe if infected seeds are planted or if soybeas are planted following a disease epidemic in the previous growing season. The fungus survives in infested crop residue, and continuous soybean fields may be more affected due to the persistence of inoculum.

Symptoms. Lesions on leaves may be circular or angular, gray spots with reddish- purple margins. These spots are first visible on the upper surface of the leaf. As lesions age, the centers become gray or light brown, sometimes with small dark spots in the center. Lesions may coalesce to form larger, irregular spots. Heavy infestations may lead to premature defoliation of the plant. While young leaves are most susceptible to infections, lesions may not be visible until leaves mature because it takes several weeks for lesions to develop.

Under persistent heavy rainfall or humidity, stems and seeds may also become infected. Stem lesions appear as narrow, brown lesions that become light gray with dark margins as the lesions mature. Lesions on pods are initially circular and red-brown and become gray with a dark margin.

Management. Planting resistant varieties or certified disease-free, high quality seed are the best mechanism for preventing disease outbreaks. Continuous soybean rotations also promote significant buildup of residues, and should be avoided. If planting soybean following a year with frogeye leaf spot infestation, plant a resistant variety. Foliar fungicides may be warranted where there are favorable environmental conditions and susceptible varieties. Strobilurin-resistant Cercospora sojina populations have been detected in several counties of North Carolina, and using a pre-mix fungicide with multiple modes of action may help prevent losses of control.

Cercospora Blight and Purple Seed Stain
(Cercospora kikuchii)

Cercospora blight accounts for 0.3% of yield losses caused by foliar pathogens in North Carolina and can cause significant damages under warm, wet conditions. When seed become infected, causing purple seed stain, the value of the crop may be reduced.

Symptoms. Initial signs of infection are found in the upper canopy on young leaves during pod-filling stages and into maturity. Affected leaves show irregularly shaped patches of purple to bronze discolorations, typically only on the leaf surface, resembling sunburn. Severe infections may result in premature defoliation of the uppermost leaves, while lower leaves remain green and attached to the plant.

When seed are affected, a purple to pink discoloration may occur in small spots to the entire surface of the seed. Infected seed may also show no symptoms of infection.

Resulting seedlings from infected seed will show symptoms of the disease, causing cotyledons to turn purple, shrivel and drop. Stem lesions may also form on seedlings, which can girdle and kill the seedling. Seedlings that survive produce stunted plants and limit yield potential.

Management. A combination of cultural practices and host resistance will provide the best control of this disease. Promoting air flow and good soil drainage may help reduce incidence of Cercospora blight. Crop rotation and residue management, such as tillage to promote rapid decomposition of crop residue, will also help reduce inoculum build-up from previous soybean crops. Choose more-resistant varieties and plant certified, disease-free seed. Fungicides are not generally recommended, and no fungicides are labeled with high efficacy.

Bacterial Blight

(Pseudomonas syringae)

Pseudomonas syringae causes angular leaf spot and bacterial blight, which usually cause minimal damage in North Carolina. More severe outbreaks are associated with heavy rainfalls and winds. Infections occur through natural openings in the plant or wounds, such as those caused by hail or cultivation. High temperatures often slow or stop the development of the disease.

Symptoms. Young leaves are most susceptible to infections, and leaf spots may appear on any portion of the plants. Lesions are often small, angular, and brown to red-brown with a water-soaked margin. A yellow halo often surrounds margins. Lesions will continually grow together to produce large, irregularly shaped necrotic areas of the leaf, the centers of which frequently fall out. On stems and petioles, lesions are large and black but do not share the angular margins of leaf infections.

Management. While complete resistance to bacterial blight is not available commercially, the difference in tolerance has been observed in the field. Selecting a variety with some tolerance to bacterial blight may reduce incidence where there is a history of angular leaf spot and bacterial blight. Crop rotation and tillage to reduce the survival of inoculum may aid in limiting infections.

Septoria Brown Spot
(Septoria glycines)

Septoria brown spot causes 0.5% of yield losses in North Carolina soybean production and can be found throughout the growing season. Warm, wet weather favors disease development, and dense plantings favor disease development in the lower canopy.

Symptoms. Lesions are small, irregularly shaped, and dark brown and can be found on all leaf surfaces. Adjacent lesions can merge to form large, irregularly shaped brown spots. Infected leaves quickly turn yellow and drop. Disease typically begins in the lower portion of the canopy and, under favorable conditions, progresses into the upper parts of the canopy.

Management. Crop rotation and tillage reduce inoculum availability. Other legume species are hosts, so crop rotations should include non-leguminous plants like corn or small grains. Foliar fungicide applications may be warranted under conducive environmental conditions, and the efficacy of different modes of action is essential to rotate each production season. Applications made between R3 and R5 may slow the rate of development to protect yield.

NC State University and N.C. A&T State University work in tandem, along with federal, state and local governments, to form a strategic partnership called N.C. Cooperative Extension.

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