• Continuous rain during spring months can saturate fields, causing severe delays in the timeliness of planting.
• As crop planting is postponed, the development of the crop is set back, making the management of some field pests crucial.
• Smaller crops are more vulnerable to pests, making scouting very important.
• In this Crop Focus article, we will display a brief overview of a selection of pests that might pose a risk to late planted soybeans, as well as management practices when applicable.

Before using pesticides, consider the following:

• Percent of leaf area affected/damage inflicted
• Corn growth stage
• Cost of treatment
• Expected value of the crop

Defoliating Insects

• Insects that defoliate small and vulnerable plants may pose more of a threat in growing seasons with late planting.
• Defoliating can cause significant damage to plants that are already behind on vegetative growth (Figure 1).
• Bean leaf beetle pod feeding can also cause significant damage.
• If insects are present and feeding, and defoliation exceeds 30% of the leaf surface area, treatment may be necessary (Hunt, Jarvi, Ohnesorg, & Mueller, 2016).
• Common pests that defoliate soybeans are bean leaf beetles, Japanese beetles, Mexican bean beetles, a variety of caterpillars, etc.

Figure 1. Bean leaf beetles feeding on soybean, can vector bean mottle virus.

Figure 2. Skeletonization of soybean leaf due to Japanese beetle feeding.

Soybean Aphid

• Aphids pose a threat to soybeans from May – August.
• Piercing sucking mouthparts damage already stressed soybeans and can vector viruses.
• Females are parthenogenic, meaning they can reproduce without mating, causing infestations to progress rapidly.
• There are many beneficial organisms that are natural enemies to aphids and can suppress their numbers.
• The economic threshold for aphids is 250 per plant, monitoring their numbers is crucial to proper management.

Figure 3. Ladybird beetle predating on aphids.

Figure 4. Close up of soybean aphid.

Stink Bugs

• Found throughout the temperate and tropical areas of the world.
• Stink bugs are most problematic when appearing in soybean fields during pod fill and maturation.
• Feeding may cause delayed maturity, green stem, and abnormal pods. Seeds fed upon may be shriveled, deformed, undersized, or aborted.
• Late planted and late maturing soybeans are at a particular risk.
• Fields with broadleaf weed growth, especially shepherd’s purse, may be more susceptible, field margins can contain higher numbers.

Figure 5. Brown stink bug showing piercing-sucking mouthparts below head and between legs.

Soybean Podworm – Corn Earworm

• Corn earworm can also feed on soybean foliage and pods.
• Open canopies of late planted crops can serve as egg laying sites.
• If defoliation reaches 20% or more during pod fill, or 5-10% of pods are damaged, then treatment is justified (Bailey, 2014).

Figure 6. Soybean podworm feeding on soybean.

Rhizoctonia

• Rhizoctonia solani is a soilborne fungus that infects the roots and stems of soybeans.
  • Overwinters as survival structures called sclerotia.
• Symptoms of this disease are rusty brown, dry, sunken lesions on stems and roots near the soil line.
• Soybeans can also appear stunted, chlorotic, and wilted as a result of root decay.
• This pathogen is favored with high soil moisture and warm soil temperatures, 81ºF (27ºC).
  • Because of this, it is common in late planted soybean fields.

Figure 7. Red discoloration at soil line due to Rhizoctonia solani.

Figure 8. Cankers in roots due to rhizoctonia root rot.

Figure 9. Close up of red discoloration due to Rhizoctonia solani.

Phytophthora Root and Stem Rot

• Caused by the soil-borne fungus Phytophthora sojae (also known as Phytophthora megasperma f.sp. glycinea).
• Associated with wet soil conditions.
  • Commonly occurs on heavy, poorly drained or compacted soils.
  • May occur on any soil saturated for an extended period of time.
• Displays seed rot, seedling blight and root/stem rot phases.

Figure 10. Phytophthora infected soybean on right, compared to a healthy soybean on the left. Note the dark brown lesion.

Figure 11. Soybean plants wilted due to Phytophthora rot.

Cercospora Leaf Blight and Seed Stain

• Caused by the fungal pathogen Cercospora kikuchii, which attacks both the leaves and the seeds of soybeans.
• Favored by warm and wet conditions.
• The disease is spread as spores are blown or splashed onto soybean plants from infected residue, weeds or other soybean plants.
• Leaves will have a general bronzing to purpling discoloration.
• Seeds are infected through their attachment to the pod. Infected seeds may show a pink to pale or dark purple discoloration.

Figure 12. Bronzing on leaves due to Cercospora.

Figure 13. Cercospora seed stain on soybean.

Frogeye Leaf Spot

• Frogeye leaf spot, Cercospora sojina, is most common in the mid-South, Mississippi Delta, and Southeastern soybean growing areas.
• Disease development is favored by warm, humid conditions and frequent rains following disease onset can lead to serious epidemics.
• The center of lesions become light brown to light gray, and the border remains dark.

Figure 14. Frogeye leaf spot on soybean.

Viruses

• An increase in vector populations can increase the chance of viral infections in soybean fields.
  • Soybean mottle virus is vectored by bean leaf beetle.
  • Soybean mosaic virus is vectored by aphids.

Figure 15. Soybean leaf with symptoms of bean mottle virus.

Figure 16. Aphid feeding on soybean leaf.

Bailey, W. C. (2014, September 3). Soybean Podworm in Soybean. Retrieved from Integrated Pest Management University of Missouri.