Published on Wednesday, October 17, 2018
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High rainfall and warm temperatures after maturity physically cause the soybean pod to swell and shrink. Any structural weakness in the pod from diseases or insect feeding will allow moisture into the pod where it affects the soybean itself. The pods then split open from the physical stress of swelling with moisture. Soybeans exposed to warm temperature and high moisture are also subject to germinating in the pod. All soybean varieties are susceptible to damage in exceptionally wet years.
As soybeans physically mature, the pod develops around the soybean. High rainfall just before physiological maturity allows the soybean to expand rapidly, stretching the pod and leaving it vulnerable to splitting open.
Left: Soybean pods stretch when exposed to moisture, leaving them vulnerable to disease. (Cred: Luke Schulte)
Right: Soybean pods split due to high rainfall in Southern Indiana. Visible black lesions are due to a complex of foliar diseases like frogeye leaf spot and Cercospora. (Cred: Steve Gauck)
Soybeans that were physiologically mature during the window of high temperatures and wet weather will have the most damage. Earlier soybeans may have already had hardened pods at the vulnerable stage and later varieties may have had enough flexibility in the pods to avoid splitting. As harvest is further delayed, additional maturity groups are vulnerable to pre-harvest damage.
Cercospora Blight/Purple Seed Stain
Frogeye Leaf Spot
Phomopsis Seed Decay
Insect feeding on the pod compromises the structural integrity of the pod itself. Some aggressive insect feeding may directly damage the seed. Weaknesses in the pod expose the soybean to fungal and bacterial infections.
Stinkbug Feeding — Yellow Lesion (piercing/sucking):
The piercing mouthpart can literally ‘tap into’ the soybean and suck out the sap. This can lead to reduced seed size and flattened pods. Stinkbug feeding can also cause “stay green” as the plant delays senescence in a last-ditch effort to make up for the aborted seeds.
Left: Stink bug feeding damage on a soybean. Photo by: Luke Schulte.
Right: Stink bug damage to a developing seed. Photo by: Mississippi State Extension
Grasshopper Feeding — Jagged Holes in Leaves and Pods (chewing):
Grasshoppers are generally associated with dry weather. Some areas had dry weather (that benefited grasshoppers) that then switched off wet, allowing plant diseases to damage the seeds.
Adult and immature grasshoppers will consume most leaf tissue expect the tough veins, and can feed through pods later in the season.
Photo by: Marlin E. Rice.
Bean Leaf Beetle — Yellow Lesion with Black Edges (chewing):
Direct injury by bean leaf beetle to soybean pods. Photo by: Marlin E. Rice.
This study was conducted at our PFR locations in IN, KY, Central IL, Southern IL, and OH. Fungicide-Insecticide applications can pay off – especially if applied at R3. However, these treatments may only be effective for a few weeks following application. Pre-harvest losses can be devastating if the diseases move in after the protective chemistry has worn off.
This year, there are many pods affected by more than one issue. Faithfully identifying each challenge will help you to manage risk for next year’s crop.
For next year:
The top soybean was aborted from excessive stress. The middle soybean has been damaged by stinkbug (shriveled soybean with physical damage), and the bottom soybean is infested with Cercospora leaf spot. Photo credit: Luke Schulte.
Author: Samantha Miller
Categories: Agronomy, Agronomy Talk
Tags: Agronomy, Beck's Agronomy, Soybean Diseases, Agronomy Talk, discolored soybeans, soybean damage