Effect of Resistance and Ethaboxam Seed Treatment on the Management of Phytophthora Root Rot in Illinois and Iowa

Abstract

Phytophthora root and stem rot (PRR) is a limiting factor for soybean production. Seed treatments are used for early-season management, but efficacy can depend on seed selection and the local environment. Ethaboxam is a new fungicide commercially available as a seed treatment to control oomycetes. Field experiments were established in Illinois and Iowa in 2017 and 2018 to evaluate the effect of ethaboxam + metalaxyl on PRR. Experiments included soybean lines with no resistance gene, Rps1c or Rps1k, and different levels of partial resistance. Seed treatments increased soybean stands in all locations and years. Significant yield effects were observed only in two locations that were inoculated with Phytophthora spp. Groups of soybean lines with the same Rps gene responded differently in each location, showing how Rps gene usefulness depends on the field. A comparison of the effect of seed treatment on lines with different levels of partial resistance showed that partial resistance alone cannot always protect against stand losses. Soybean lines with high levels of partial resistance had consistently higher yields than those with low levels of partial resistance across Illinois locations. These results show that ethaboxam seed treatment can protect early-season stands and that selection of cultivars with high levels of partial resistance is important for PRR management.

Virulence, Aggressiveness, and Fungicide Sensitivity ofPhytophthora spp. Associated with Soybean in Illinois

“…As shown in other studies (e.g. Cerritos-Garcia et al 2021) combining varieties with Rps genes, partial resistance, and seed treatments provide the most effective management for PRR.…”

“…In addition to Rps genes, PRR can be managed with quantitative host resistance and fungicide seed treatments (Dorrance et al 2009; Dorrance 2018; Cerritos-Garcia et al 2021). Quantitative resistance (also known as partial resistance) is not race-specific, and therefore broadly effective against all pathotypes of P. sojae (Dorrance et al 2003).…”

“…In addition to Rps genes, PRR can be managed with quantitative host resistance and fungicide seed treatments (Dorrance et al 2009;Dorrance 2018;Cerritos-Garcia et al 2021).…”

A global-temporal analysis on Phytophthora sojae resistance-gene efficacy

“…Quantitative resistance (also called “partial resistance”, “horizontal resistance”, or “field tolerance”) to P. sojae is available in commercial soybean varieties 6 , 83 – 85 . Unlike qualitative resistance, quantitative resistance is governed by multiple genes acting in concert to provide partial resistance to the pathogen by allowing infection but limiting the growth and spread of the pathogen within the plant and limiting disease development.…”