June 1, 2017

Managing Soybean Plant Health​

Key Points

  • Well-timed fungicide applications can help reduce foliar diseases and improve overall plant health. 
  • Pod set through seed fill growth stages (R3 to R6) are the most critical period for yield potential.
  • Foliar diseases are more prevalent in wet and humid weather.

Soybean Development and Foliar Diseases

Pod set and pod fill are critical periods of development and yield potential in soybean. Weather is always an uncertainty and many diseases are favored by warm and humid or wet conditions. Just a few of the fungal diseases that may affect soybean include Cercospora leaf blight, frogeye leaf spot, pod and stem blight, and anthracnose. While we cannot control the weather, foliar fungicides provide an option for disease management.

Symptoms of Cercospora leaf blight and frogeye leaf spot develop on leaves (Figure 1).  Leaf defoliation is the big concern with these diseases. Healthy, green plant material is needed for photosynthesis and converting sugars for pod fill; therefore, it is important to protect leaf tissue.

The symptoms of pod and stem blight and anthracnose are similar. Both diseases can be found on stems, petioles, and pods in the early reproductive stages as irregularly-shaped brown blotches. While neither disease usually causes severe yield loss, it is important to scout and assess overall plant health throughout the reproductive stages.

Fungicide Applications

 
Figure 1. Cercospora leaf blight (left) and frogeye leaf spot (right) symptoms.

Deciding whether or not to spray can be a tough decision, which should be based on disease severity and timing. Keeping a close watch on fields and a review of soybean growth stages can help properly time fungicide applications.

Soybean fungicide applications are generally not needed in the early vegetative growth stages (VE through V6). Fungicide applications for late-season diseases are generally made during pod development stages R3 and R5. As previously mentioned, the pod set through seed fill stages (R3 through R6) are the most critical period for seed yield. Leaf loss can significantly reduce yield if diseases attack during early seed filling. Spraying fungicides after R6 is generally not recommended.

Weather conditions can also help determine proper fungicide timing in soybeans. Wet and humid conditions are more conducive for disease development, but benefits from fungicide treatment can also be seen in stressful conditions such as dry weather, high temperatures, and/or aphid infestations. When these conditions are prevalent near the R3 growth stage, treatment with a fungicide may be warranted.

More than one fungicide application may be needed in environments with high disease pressure. The length of time foliar fungicides are active ranges from 14 to 21 days. When deciding to use foliar fungicides, these factors should be kept in mind: 

  • Planting soybean after soybean can increase the risk of foliar disease. 
  • Soybean products rated as more susceptible to diseases such as frogeye leaf spot tend to have greater yield response to fungicides. 
  • Foliar disease is more prevalent in wet and humid conditions.   
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Sources:

Sources:
Brown, C. OMAFRA Publication 811. Diseases in field crops—soybean. http://www.omafra.gov.on.ca/english/crops/pub811. Brown Spot—Septoria leaf blight. 2006. Iowa State University Extension. http://extension.agron.iastate.edu. Mueller, D. 2006. Fungicides: Plant health fungicide applications. Iowa State University Integrated Crop Management. IC-496(17). http://www.ipm.iastate.edu/ipm/icm/2006/6-26/fungicides.html. Soybean Anthracnose. 2006. Iowa State University Extension. http://extension.agron.iastate.edu. Web sources verified 05/30/17. 130611060136