Evaluation of resistance to Rhizoctonia solani in soybean and assessment of fungicide sensitivity in isolates from sugar beet and soybean

Abstract

Soybean and sugar beet are commonly grown in rotation in the Red River Valley of MN and ND and southern Minnesota. Both crops are highly susceptible to Rhizoctonia solani. Currently there are no Rhizoctonia resistant soybean cultivars available to the growers. Partially resistant commercial sugar beet cultivars are available, but they are susceptible to R. solani during the seedling stage. Disease management for both crops relies partially on the use of fungicides. The primary objectives of this study were (i) to determine the response of different soybean genotypes to R. solani in the field and growth chamber, and (ii) to determine the sensitivity of R. solani isolates from soybean and sugar beet to the fungicides sedaxane, penthiopyrad, fluxapyroxad, azoxystrobin, and pyraclostrobin. Soybean genotypes were evaluated for their response to R. solani at Waseca and Crookston, MN field locations. All genotypes evaluated at Crookston (n=20) in 2018 and 2019 had high losses in inoculated plots in plant population and yield compared to noninoculated control plots. Losses were also severe for all genotypes (n=36) at Waseca in 2018. Some soybean genotypes had relatively low losses in 2017 and 2019, however, their performance was not consistent across the years. MN 1613CN was the only genotype that performed well in 2017 and 2019, but it had high plant population and yield losses in 2018. Significant genotype by isolate interaction was detected when 16 soybean genotypes were evaluated for their response to four isolates of R. solani in the growth chamber. All genotypes were highly susceptible to isolate Rs 16WC3-2, but they differed in susceptibility to some other isolates. The soybean genotypes used in this study were not resistant to R. solani, but those with relatively low losses in plant population should be investigated further along with additional genotypes. The response of soybean genotypes to R. solani is strongly influenced by the isolate and environment, so future studies should continue to be conducted under different environmental conditions using multiple isolates. Sensitivity of R. solani AG 2-2 isolates (n=35) collected from soybean and sugar beet to the SDHI fungicides sedaxane, penthiopyrad, and fluxapyroxad, and to the QoI fungicides pyraclostrobin and azoxystrobin was determined using a mycelial growth inhibition method. The concentration of fungicide required to inhibit the radial growth of mycelium 50% (EC50) compared to the growth on non-amended media was estimated for each isolate. The mean and range of EC50 values for sedaxane, penthiopyrad, fluxapyroxad and pyraclostrobin were 0.1 (0.03 to 0.3), 0.15 (0.05 to 0.27), 0.16 (0.08 to 0.3), and 0.24 (0.04 to 1.02) µg a.i./mL, respectively. The mean EC50 values of azoxystrobin for 22 isolates ranged from 0.76 to 2.36 µg a.i./mL. EC50 values for azoxystrobin could not be estimated for 13 isolates due to < 50% inhibition in growth, however, a pronounced decrease in mycelial density was observed as fungicide concentration increased. The SDHI fungicides and pyraclostrobin effectively inhibited the growth of the isolates of R. solani in vitro at low concentrations, but new methods are needed to determine in vitro sensitivity to azoxystrobin. In summary, the different levels of partial resistance to R. solani detected in soybean genotypes suggest a need for improving resistance to R. solani, and fungicide sensitivity results indicate that currently labeled fungicides continue to be useful in managing seedling damping-off and root rot.