Optimizing fungicide timing for the control of Rhizoctonia crown
Editor’s note: This article is from the archives of the MSU Crop Advisory Team
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Recent weather conditions may have been suitable for the development of Rhizoctonia crown and root rot and possibly damping-off. Quadris can be applied early in the sugar beet growing season for control of Rhizoctonia damping-off and Rhizoctonia crown and root rot caused by Rhizoctonia solani anastomoses groups (AG) 4 and 2-2, respectively. Work at MSU/MSC has evaluated fungicide application timings based on crop growth stage and soil temperature thresholds in inoculated small-scale trials and in commercial fields with a history of Rhizoctonia crown and root rot. Soil temperature thresholds 50°F, 59°F and 68°F were selected for fungicide application timings and used to test whether soil temperature could be used to better time applications of azoxystrobin. In both small and large plot trials, timing applications after attainment of specific soil temperature thresholds did not improve efficacy of Quadris in controlling damping-off or Rhizoctonia crown and root rot, compared to application timings based on either planting date, seedling development, or leaf stage in a susceptible (E-17) and a resistant variety (RH-5). Application rate and split application timings of azoxystrobin had no significant effect on severity of crown and root rot. Other environmental factors such as soil moisture may interact with soil temperature to influence disease development. The variety RH-5 had higher sugar yield attributes than the susceptible variety (E-17) in seasons conducive and non-conducive to crown and root rot development. The rate of 0.4 or 0.6 fl.oz./1000 row feet were equally effective but the best timing over the three years of trials was at leaf stage 4 to 6 and 6 to 8.
Quadris is only labeled for banded applications for Rhizoctonia crown and root rot control (up to leaf stage 6 to 8) and at a lower broadcast rate for foliar diseases such as leaf spot. The broadcast rate (max 15.5 fl.oz./A) is equivalent to about 20 percent of the in-furrow application rates and an equivalent rate for control of crown and root rot as a broadcast application would be about 40-60 fl.oz./A. There are no known data for crown and root control using 15.5 fl.oz./A of Quadris and it is likely that control of crown and root rot would be compromised as such a low rate. Mixing Quadris and Roundup is not an illegal tank mix, but there is no information available on the efficacy of such a mixture, given maximum permitted rates, for crown and root rot control. In addition, no phytotoxic studies have been completed using this combination, and the risk of crop damage to the beets has never been assessed.