Re-evaluating the management of Rhizoctonia solani on sugarbeet in Michigan through the use of variety tolerance and Quadris applications.

Abstract

Rhizoctonia root and crown rot, caused by the fungal pathogen Rhizoctonia solani, is a major soilborne disease of sugarbeet in Michigan.  This disease has caused substantial losses to growers, both in terms of total root yield and sugar concentration.  In Michigan, severe infections have been observed to cause up to a 15-ton reduction in yield and a 1 percentage point decrease in sugar content.  Throughout the region, this disease is primarily managed by planting tolerant sugarbeet varieties and the use of fungicide applications.  The most common fungicide used is Quadris (Azoxystrobin), which many growers apply twice during the season, with the first application T-band in-furrow at planting and the second in a 7-inch band at the 6-10 leaf stage.  The combination of these management tools has been effective in managing Rhizoctonia in Michigan.  As sugarbeet varieties continue to improve, it has been observed that variety tolerance to Rhizoctonia is stronger in many contemporary varieties than in many varieties available when these recommendations were first made.  With this increased variety tolerance, it is hypothesized that two applications of Quadris may no longer be necessary in the majority of the Michigan beet production region.  Therefore, the current study compared a Rhizoctonia susceptible variety and a tolerant variety, each with four combinations of Quadris applications, including no Quadris applied, one application in-furrow, one application at the 6 to 10 leaf stage, and two applications.  This study was conducted over the course of three years, being done in a different non-inoculated grower field each year.  In 2020 and 2021, the fields had high levels of Rhizoctonia, while the field in 2022 had a moderate level of disease.  To evaluate the impact of the different treatments, the number of dead or dying beets were counted in 1200 feet of row, and yield and sugar data were collected.  Preliminary results from the first two years have shown that for a tolerant variety in a field with high levels of disease, there was no difference between a single application or two applications of Quadris.  For the susceptible variety in this environment, two applications of Quadris provided greater control than either single application, all three of which provided greater control than no applications of Quadris.  If the number of Quadris applications can be reduced in certain situations, this would provide cost savings to growers and reduce the risk of Rhizoctonia solani developing resistance to Quadris.