Strobilurin Fungicide Timing for Rhizoctonia Root and Crown Rot Suppression in Sugarbeet

Field experiments were conducted to determine the opti­ mal timing of strobilurin fungicide application to sugar­ beet (Beta vulgaris L.) for suppression of Rhizoctonia root and crown rot (RRCR) caused by the fungus Rhizoctonia solani AG 2-2. Fungicide application timings were made relative to cultivation operations that introduce soil-borne inoculum onto the crown, thus, initiating disease develop­ ment. Sugarbeet crowns were inoculated with R. solani AG 2-2 at times coincident with cultivation in an effort to sim­ ulate the natural infection process. Field plots were estab­ lished near Torrington, Wyoming under sprinkler irriga­ tion in 1999 through 2002 and in Scottsbluff, Nebraska under furrow irrigation in 1999 and 2000. Fungicide appli­ cation timings most effective for RRCR suppression were those made at the time of inoculation or, alternatively, when a half-rate split application was made at the time of inoculation plus 2 weeks later. Disease incidence decreased 64% to 96% and total root yields increased 67% to 1853% compared to the nontreated control (P ~ 0.05). Fungicide applications made at planting were too early and had little effect on disease that developed following inoculation and cultivation. When the application timing was optimal, the different strobilurin fungicides had similar efficacy. However, when the timing was not optima], azoxystrobin had greater efficacy than did trifloxystrobin and pyra­ clostrobin applied at similar rates. Thiophanate-methyl