Azoxystrobin is Needed Before Infection for Control of Rhizoctonia solani in Sugarbeet

Rhizoctonia solani is pathogenic to sugar beet, causing seedling damping-off, and crown and root rot in mature sugar beet plants. Rhizoctonia root rot was reported as the most important problem faced by sugarbeet producers in Minnesota and North Dakota. Azoxystrobin, a strobilurin fungicide, was recommended for controlling R. solani, but growers needed to know when to apply the fungicide relative to when infection takes place for effective disease control. The objective of this greenhouse study was to determine the best time to apply azoxystrobin relative to the time of inoculation for controlling root rot caused by R. solani AG 2-2 IIIB. Four-leaf stage sugar beet plants received fungicide application at 0, 3, 10, 14 and 21 days after inoculation and at 0, 7, 14, 21 and 28 days before inoculation. Treatments included a non-inoculated control and an inoculated control. Azoxystrobin was applied as a hypocotyl drench, and inoculation was done by placing R. solani grown on barley grain 2 cm below the soil and from 2 cm from the roots. Fourteen days after treatments were completed, plants were evaluated for root rot disease severity. Azoxystrobin applied before inoculation resulted in significantly lower disease severity compared to when it was applied after inoculation, except when inoculation was followed two hours later by the fungicide treatment. This research demonstrated that azoxystrobin needs to be applied before infection takes place to provide effective control and the fungicide provided protection for up to 28 days.