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Soybean Seed Treatments and Mycorrhizal Fungi

The role of aboveground plant parts is to synthesize–via photosynthesis–organic compounds from CO2 and water using light energy absorbed by chlorophyll in the leaves. The primary role of the below-ground portion [roots] of plants is to gather raw materials (nutrients and water) that are required for the plant to function and develop. The ability of plant roots to obtain these raw materials and water from the soil is dependent on their ability to proliferate into as much of the surrounding soil as possible. Thus, uptake of nutrients and water from the soil by plants is dependent on expansive root proliferation within the particular soil in which they are growing.

Mycorrhizae are a group of fungi that colonize plant roots and increase their absorptive capacity, thus establishing a beneficial relationship with the host plant by acting as auxiliary roots. This association in effect extends the effective rooting zone of the plant, which enhances the uptake of water and nutrients. These fungi are referred to collectively as arbuscular mycorrrhizal or AM fungi because they develop arbuscules that act as exchange sites for nutrients between the fungus and the host plant. AM fungi have been shown to increase water uptake by plants and decrease recovery time from wilt following relief from drought stress.

Here are a few points about mycorrhizal fungi.

   They are very common soil microorganisms that colonize the roots of most crop species.

   Together, the host plant and AM fungi form a symbiotic relationship called a mycorrhiza. This association is always essential for the AM fungi because the host plant is their only source of energy.

   AM fungi produce hyphae or mycelia that allow them to forage for some nutrients more effectively than roots alone.

   AM fungi can increase the uptake of phosphorus and trace elements such as copper and zinc which are relatively immobile in soil.

   Practices such as tillage [damage to hyphae], high rates of phosphate fertilizer, fallow periods [no host plant for their nutrition], and rotating with crops that are incapable of forming mycorrhizas are detrimental to AM fungi. Conversely, cover crops are supportive of optimum AM fungal populations.

In the last decade, fungicides with multiple modes of action have been routinely applied as a coating to planted soybean seed to control both seed- and soil-borne fungal pathogens. Most commercial soybean seed products are now sold with pre-applied anti-fungal seed treatments that contain a mix of contact and systemic fungicides that target multiple metabolic processes in fungi, and that have varying mobility in the soil. It is therefore logical to assume that these seed-applied fungicides might adversely affect AM fungi that normally would colonize soybean roots in an optimum soil environment.

Cameron et al. at South Dakota State Univ. conducted a study to determine if various seed-applied fungicides inhibit colonization of soybean roots by AM fungi, and determine if there were interactions between fungicide formulations and soybean genotype. They report their results in an article titled Fungicidal Seed Coatings Exert Minor Effects on Arbuscular Mycorrhizal Fungi and Plant Nutrient Content that appears in Agronomy Journal, Vol. 109, Issue 3 [2017]. A summary of the conduct of and findings from their research follow.

   Three fungicide seed treatments were used: 1) CruiserMaxx Advanced [Mefenoxam (4), Fludioxonil (12), and Thiamethoxam (IRAC 4)]; 2) Evergol Energy SB [Metalaxyl (4), Penflufen (7), and Prothioconazole (3)]; and 3) Vibrance [Sedaxane (7)]. [FRAC/IRAC codes for each active ingredient are shown in brackets.] There was also a control treatment with untreated seed.

   Soil was inoculated with four AM fungal species that had been isolated from agricultural soils.

   Fungicide seed treatment had no effect on AM fungal root colonization, nutrient concentration, or plant biomass of the three soybean genotypes used in the study. The authors attribute this to relatively low fungicide doses because of the direct application to seed.

   The lack of fungicide seed treatment effect indicates that the different fungicide modes of action [4 in the three treatments] did not differentially affect AM fungal colonization on soybean roots.

   There was no significant interaction between fungicide seed treatment and soybean genotype for AM fungal colonization, plant nutrient content, and plant biomass; however, soybean genotype significantly affected all three variables.

   These results, plus those from other studies, suggest that a diverse AM fungal community in soil may be an important component in resisting any potential negative effects of seed-applied fungicides.

Results from the above study may not be indicative of what will happen in all soils in which fungicide-treated soybean seed are planted, but they certainly do indicate that fungicides that are applied to soybean seed do not automatically have a negative impact on native AM fungi. This news, coupled with the above points about the value of AM fungi and ways to ensure their viability, should further encourage soybean producers to continue to use effective fungicide seed treatments on soybean.

Composed by Larry G. Heatherly, July 2017,