Effect of azoxystrobin applications based on soil temperature on Rhizoctonia root and crown rot of sugarbeet

Rhizoctonia root and crown rot, caused by Rhizoctonia solani AG-2-2, is becoming more severe in sugarbeet (Beta vulgaris L. ssp. vulgaris) fields in North Dakota and Minnesota, USA. Azoxystrobin controls Rhizoctonia solani in artificially inoculated trials when applied just prior to infection. However, it is difficult to know exactly when infection occurs in the field. Field research was conducted during 2003-05 in North Dakota and Minnesota (2004 only) with sugarbeet cv. Beta 3800 to determine the soil temperature at which azoxystrobin should be applied to control Rhizoctonia solani. Azoxystrobin was applied at 0.67 L ha-1 in 187 liter of solution in a 17.8 cm foliar band to each of the four middle rows of six row plots. Fungicide applications were made when average daily soil temperature at the 10 cm depth was between 10 to 13°C, 13 to 16°C, 16 to 19°C, 19 to 23°C, 23 to 26°C, 26 to 29°C and greater than 29°C. Consistently better disease control and higher recoverable sucrose was obtained when azoxystrobin was applied at between 10 and 23°C.