Fungicide Seed Treatments in Soybeans: Factors to Consider

• Facebook
• Twitter
• LinkedIn
• Email

• Fungicide seed treatments help to establish soybean stands by reducing the impact of important seedling diseases 
• Fungicide seed treatments deliver approximately three weeks of protection to the seed/seedling
• Not all fungicide seed treatments are effective against all seedling pathogens
• Early planted fields of continuous, no-till soybeans with drainage issues and a history of seedling pathogens are at the highest risk for seedling disease issues

Fungicide seed treatments are one tool to assist producers manage problematic seedling diseases in soybeans. However, seed treatments are not always needed and are not created equal. Understanding the factors and conditions where these technologies may provide the greatest benefit will help with achieving a positive ROI. This article will discuss seed treatments that may be used to manage fungal pathogens in soybeans and situations where their use may be most beneficial.

Basics of fungicide seed treatments
Fungicide seed treatments help protect the developing soybean seedling during the highly vulnerable period from planting through early vegetative growth stages (≈V1-V2). Seed treatments do not fumigate the soil and kill all pathogens within a field. Rather, the pathogen must come out of a protected, dormant state and encounter the active ingredient within the soil or seedling for them to be effective. If pathogens are not present in the field or if conditions do not favor growth and activity of soilborne pathogens during this initial period after planting, the benefits seed treatments will not be realized. Therefore, it is essential that you know the field history and that any seedling disease issues encountered are properly diagnosed if encountered to allow the strategic development of in integrated pest management plan the following seasons. 

After planting, seed treatment fungicides protect the seed coat and may move into a limited area of soil around the seed. Many active ingredients can be taken up by the seedling roots and moved to developing stem/foliar tissues through the conductive tissues of the plant as the seedling develops. Work has shown that seed treatments can provide approximately three-weeks of protection of the seedling. After this point, fungicides taken into plant tissues are degraded or diluted, and roots have grown outside of the initial zone of protection in the soil. 

When are seed treatments best used?
Research has shown that producers will experience a higher probability of achieving a return on investment when seed treatments are used in fields that are at elevated levels of risk for important seedling diseases. Table 1. has a generalized risk calculator for helping you determine the relative risk for seedling disease in a field, provided that the field has a history of seedling disease issues. In general, risk for seedling diseases are greatest in early planted, no till, continuous soybean fields with drainage issues. 

What soil pathogens may be encountered?  
Table 2. lists the potential seedling diseases that could impact your soybeans, conditions that favor development, and other factors that may affect their impact on the current crop. Diseases caused by oomycetes (Pythium and Phytophthora root rot) are favored by poorly drained soils saturated with water, whereas other diseases (e.g. Rhizoctonia root rot) are favored by moist, but not saturated soils. All of these pathogens are soilborne, meaning that they infect from the soil at/near planting, and do not move long distances on air currents like some other diseases. These pathogens all overwinter in soils as tough overwintering structures or in/on crop residue.

Table 1. Use this table to help you in determining the relative risk for a seedling disease to be an issue, and help you decide if a fungicide seed treatment may provide a potential benefit. For this table to work the seedling pathogen MUST be present in the field. This is just a guide and does not guarantee a particular disease will or will not be a problem in a given field/season. High risk = 12-15; Moderate risk 8-11; Low risk 7 ≥
 

Table 2. Pathogens causing seedling diseases that are frequently encountered in soybeans. 

Seed treatments differ in efficacy
Remember, not all fungicide active ingredients are effective against all pathogens. Some are specific for certain groups of fungi (e.g. oomycetes such as Pythium and Phytophthora) or even specific species of fungi (e.g. Fusarium virguliforme). The table below provides the relative performance of fungicide active ingredients currently available in soybean. Ratings are taken from the NCERA 137, Extension soybean pathology group ratings. Efficacy categories: E = Excellent; VG = Very Good; G = Good; F = Fair; P = Poor; NR = Not Recommended; NS = Not Specified on product label; U = Unknown efficacy or insufficient data to rank product. Ratings of NR may mean that the fungal group listed is not a target of the specific fungicide active ingredient. More information on these ratings can be found at:  https://crop-protection-network.s3.amazonaws.com/publications/fungicide-efficacy-for-control-of-soybean-seedling-diseases-filename-2020-03-18-150238.pdf 

1. Products may vary in efficacy against different Fusarium and Pythium species. 
2. Areas with mefenoxam or metalaxyl insensitive populations may see less efficacy with these products.
3. Listed seed treatments do not have efficacy against Fusarium virguliforme, causal agent of sudden death syndrome.