Agronomy Talk

Agronomy Talk: Seed Quality in Soybeans

Published on Wednesday, October 17, 2018

Click here for a downloadable version of this Agronomy Talk Update

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. 

Pod Cavity Damage:

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)

 

Maturity Date:

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.
 

Diseases:

Anthracnose

  • Pathogen: Colletotricum spp. 
  • Pod Symptoms: Irregularly shaped brown areas. Small, black fruiting bodies (acervuli) that produce spine-like structures may also form on infected issues
  • Seed Symptoms: Brown to black or small, irregular gray areas with black specks
  • Seed-Borne: Yes
  • Resistance: No
  • Rotation Crops: corn, non-legumes
  • Tillage: Yes
  • Fungicide: Foliar fungicides

Cercospora Blight/Purple Seed Stain

  • Pathogen: Cercospora kikuchii
  • Pod Symptoms: Dark lesions, may not always be present
  • Seed Symptoms: Pink to dark purple discoloration of seed coat 
  • Seed-Borne:  Yes
  • Resistance: Yes, but only leaf blight not seed stain
  • Rotation Crops: Corn; small grain; alfalfa
  • Tillage: Yes
  • Fungicide: Foliar for leaf blight stage

Frogeye Leaf Spot

  • Pathogen: Cercospora sojina 
  • Pod Symptoms: Circular to oval lesions that are red-brown to black
  • Seed Symptoms: Reddish-brown lesions - often on ends of seed
  • Seed-Borne: Yes
  • Resistance: Yes
  • Rotation Crops: Corn, small grains
  • Tillage: Yes
  • Fungicide: Seed treatment and foliar options

Downy Mildew

  • Pathogen: Pernospora manshurica 
  • Pod Symptoms: No external symptoms, internal whitish, fluffy mass
  • Seed Symptoms: Small and lighter seed, crusty fungal growth on seed; dull and white in apperance 
  • Seed-Borne: Yes
  • Resistance: Yes, but many races
  • Rotation Crops: Yes
  • Tillage: Yes
  • Fungicide: N/A

Phomopsis Seed Decay

  • Pathogen: Phomopsis spp/Diaporthe spp. 
  • Pod Symptoms: Black fungal specks (pycnidia) on infected tissue
  • Seed Symptoms: Cracked, shriveled, with chalky, white appearance
  • Seed-Borne: Yes
  • Resistance: Yes, early maturity greater risk
  • Rotation Crops: Corn; wheat
  • Tillage: Yes
  • Fungicide: Seed treatment and foliar options 

Insect Damage:

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. 

 

2018 PFR Fungicide and Insecticide Study:

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.

Disease Complexes:

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. 

Management strategies:

  • Prioritize harvest in most affected fields
  • Slow down combine and properly adjust reel speed 

For next year:

  • Light tillage to bury residue — all of these foliar disease species will overwinter on stubble.
  • Rotate to a non-host crop for 2019.
  • Plant varieties with good disease tolerance.
  • Plan varieties to stagger harvest — minimize the time soybeans sit ready in the field. 

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.
 

 

Click here for a downloadable version of this Agronomy Talk Update

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