Rhizoctonia crown and root rot caused by the fungus, Rhizoctonia solani AG 2-2 is one of the most damaging sugarbeet diseases worldwide. Losses are highest in warm, irrigated, production areas where sugarbeets are planted intensively. Maintaining rotations with non-host crops such as corn or small grains and avoiding beans or alfalfa before beets will help keep soil populations of this strain of Rhizoctonia low. Once soil populations of this fungus are built up, the value of cultural controls such as rotation, avoiding cultivating soil into the row, maintaining adequate, balanced fertility for good crop growth and maintaining adequate soil drainage is greatly reduced. Where disease pressure is high, growers can plant specialty varieties with resistance, but the resistance is incomplete. These varieties typically have yield potentials 10-15% less than the best approved varieties and insufficient curly top virus resistance for growing areas where this disease is major threat. Some newer varieties such as Beta 4546 used in this research are only 5-10% lower yielding than the best approved varieties. Because predicting disease development and loss is difficult, growers have long wanted a control where yield potential is not compromised. Since 1995, we have explored the potential for chemical control by preventing crown infections of young plants. Our research and that of others clearly shows that most infections occur through the crown from sclerotia deposited there during cultivation and that application of effective fungicides to the crown prior to cultivation will provide good control. Research from 1995-1998 served as the basis for a Section 18 label for Quadris in Montana, Wyoming, Nebraska and Colorado. Data summarizing Quadris data from 1997-1999 are shown in Table 1. 1997 was a year of high disease severity while 1998 was a year of moderate disease severity and 1999 was a year of low disease severity. Our objective in 1999 trials was to evaluate proven fungicides and experimental fungicides on both a high yielding susceptible variety and a high yielding resistant variety.

Research was done at the Eastern Agricultural Research Center at Sidney, MT on a Savage silty-clay loam soil. The plot design was a split plot (varieties) randomized complete block with six replications. Plots were single rows 30 ft long and all plots except the uninoculated controls were inoculated at the 4 leaf stage with 14 grams / plot of ground barley infected with R. solani AG 2-2. Fungicide applications were made just prior to application of inoculum using a 6 inch band applied over the row with a single Spraying Systems 8002 VS nozzle @30psi= 18 gallons/acre. Following application, plots at the 4 leaf stage (6/2/99) were cultivated and 1 inch of rain occurred preventing the need for irrigation. Applications made at the 8 leaf stage were done on 6/15/99. Cool temperatures in May and June reduced disease development to low levels of severity-mostly superficial, arrested dry lesions at the point of inoculation or shallow, dry rot cankers in the center of the crown. Plots were harvested 9/28/99 and samples sent to

Holly Sugar for determination of tare, % sugar and sugar loss to molasses.

RESULTS

Results are presented in Table 2. There were no significant differences in yield, % sugar or recoverable sugar/acre from any treatment owing most likely to low disease development during the relatively cool 1999 growing season. While not significant @ P=0.10 the resistant variety Beta 4546 had approximately 50% less damage from inoculation but still showed yield increases from fungicide application (P=0.25). Both varieties showed yield increases from Quadris 0.15 oz ai/1000 row ft, Stratego 8 oz product/1000 row ft, Flint 0.1 and 0.2 oz ai/1000 row ft, Flint 0.1 oz ai/1000 row ft plus 127, BAS 500 1.54 0z ai/1000 row ft, KZ 165, and Moncut at 0.25 and 0.38 oz ai/1000 row ft. Quadris applied at 0.075 oz ai/1000 row ft at the 4 plus 8 leaf stage increased yields on the susceptible variety KW 2262. It is important to note that timing of application maybe different if cultivation is later than the 4 leaf stage used here. Application should be made just before the first cultivation.

While low disease development precludes definitive conclusions, it appears that fungicide application will be beneficial on even high yielding resistant varieties but not as profitable as applications to susceptible varieties.

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*Presented at the Snake River Sugarbeet Conference, Twin Falls, Idaho, January 13, 2000.

Barry Jacobsen, Douglas Collins, Nina Zidack¹, Joyce Eckhoff and Jerry Bergman²