Arbuscular mycorrhizal colonization response to three seed-applied fungicides

In soybean [Glycine max (L.) Merr.], arbuscular mycorrhizae (AM) enhance nutrient and water status and may increase root resistance to soilborne pathogens. However, the fungicides that are routinely applied to the seed may reduce AM colonization, reducing these benefits. Thus, the objective of this research was to assess the effect of three commonly used seed-applied fungicides on AM colonization of soybean in Iowa. Soybean seeds were treated with the fungicides mefenoxam, fludioxonil, mefenoxam + fludioxonil, and a nontreated control. Soil fumigation with a mixture of 1,3-dichloropropene and chloropicrin was used as a tool to measure any direct effect of the fungicide on plant growth or yield parameters. There was a significant fumigation by seed treatment interaction in 2005. Seed-applied fungicides that contained fludioxonil favored AM colonization in nonfumigated Soil, where fludioxonil-treated plants had double the root colonization of the control (6 vs. 2.8%, respectively) and five times more root colonization than plants treated with mefenoxam (6 vs. 1.1%, respectively). In the fumigated soil, plants treated with mefenoxam alone or in combination with fludioxonil had lower colonization than the control and fludioxonil-treated plants. Fumigation did not significantly reduce or increase mycorrhizal colonization across locations. No differences in grain yield, final stand, or grain composition were found among seed-applied fungicides or between nonfumigated and fumigated soil. With the exception of mefenoxam in fumigated soil in 2005, there was no evidence of a reduction in mycorrhizal colonization of soybean roots with seed-applied fungicides under field conditions.