FUNGICIDES FOR CONTROL OF SUGARBEET POWDERY MILDEW 

ONTARIO, OREGON, 2002

  John J. Gallian, Dixie Vargas, and Paul Foote, Sugarbeet Specialist and Plant Pathologist, and Scientific Aide, University of Idaho, Twin Falls Research and Extension Center, PO Box 1827, Twin Falls, Idaho 83303-1827, and Research Technician, Amalgamated Sugar Company, P.O. Box 700, Paul, ID 83347.

  Powdery mildew of sugarbeets, caused by Erysiphe polygoni, occurs each year in much of the sugarbeet growing area of Idaho, Eastern Oregon, and Washington. Cultural practices have little practical effect on the disease, with the exception that it will be less severe under sprinkler irrigation.  Fungicide treatment should begin when disease first appears or has been detected nearby (0-3% disease) in order to avoid economic loss. Economic loss occurs when average disease for the season exceeds 10% mature leaf area diseased. In most years, two fungicide treatments are necessary to prevent substantial economic loss in the Treasure Valley of Idaho and Eastern Oregon.  One fungicide treatment is usually sufficient in the Magic Valley, and in some years treatment is necessary in the upper Snake River.  Do not rely on any perceived curative action of a fungicide to manage powdery mildew.  When the disease is treated is usually more important than which fungicide is used.

  METHODS

  A study was conducted at the Wettstein Farm in Ontario, Oregon in 2002 to test the efficacy of six fungicides for the control of sugarbeet powdery mildew.  The fungicides are listed in Table 1.

  Table 1.  Fungicides, companies and chemistry class for 2002 powdery mildew test.

  Variety Hilleshog 2984 Rz was planted on March 20 with a 5 3/8 inch seed drop. The previous crop was onions.   Soil type was Greenleaf silt loam with furrow irrigation and the first irrigation was applied on May 1.  One pint of Round Up was applied on March 30 and three Progress, Upbeet, Stinger, and Destiny (MSO) tank mixes were applied for weed control on April 5, 11, and 30.  On May 2 Upbeet, Progress and Destiny were applied for weed control followed on May 16 by Outlook and Trust (trifluralin).  Insecticide applications of Lorsban on July 15 and DeClare (methylparathion) on August 19 were applied for black bean aphid control.

  Experimental design was a randomized complete block with 19 treatments and 5 replications.  Individual plots were 6 rows (11 ft.) wide by 30 ft. long.  Fungicide applications were made on July 17 and August 7 using a CO₂ backpack sprayer.  The first application was applied at 20 psi. using XR 1103 nozzles and the second application was applied at 30 psi using XR 1102 nozzles.  All applications were applied at 23.6 GPA.  Weather conditions were clear, wind 1-5 mph and 83-102 ^°F for the July 17 application, and clear, wind 3-6 mph and 67-79 ^oF for the August 7 application.

  Disease ratings were taken by plot on July 17 and August 7, the same day and immediately prior to each application, with a final disease rating on September 4.  Both sides of 25 recently matured leaves in each plot were rated for percent leaf area infected with powdery mildew using a 0 to 5 rating scale with the following values: 0 = no disease; 1 = 1-10%; 2 = 11-35%; 3 = 36-65%; 4 = 66-90%; 5 = 91-100%.  Percent mature leaf area diseased (%MLAD) was calculated from the average disease rating for each treatment.  The experiment was harvested on October 14,15,and 16.  Roots were dug using a tractor mounted two-row lifter, then hand topped and weighed. Two sugar samples, consisting of 8 beets each, were taken to Amalgamated Sugar Company's tare laboratory at Nyssa, Oregon, for sugar analysis.

  RESULTS

  The average disease ratings and % MLAD are given in Table 2.  Disease was uniform throughout the test, and there were no significant differences in disease among treatments immediately prior to the first fungicide application. Performance of most treatments was good to excellent.  All treatments except the two sulfur treatments had significantly lower disease ratings than the untreated check on August 7.  On September 4, the two sulfur treatments and only one of the 5 Headline treatments did not have lower disease ratings than the untreated control. 

  Most treatments had significantly (p=.0.05) higher root yields than the untreated check, ranging from a 3.25 T/A to 5.73 T/A increase in yield (Table 3).  There were no differences among treatments in percent sugar, gross sugar/A, conductivity, extraction, and recoverable sugar in pounds/ton and pounds/A. The increased gross return due to treatment, without deducting the cost of fungicide and its application, ranged from $2/A to $233/A based on the Amalgamated Sugar Company contract at $23.00 net return/100 lb sugar. Generally, treatments with the lowest disease ratings gave the highest return per acre.

  ACKNOWLEDGMENTS:  We thank Lou Wettstein and Chris Payne for their cooperation. Appreciation is extended to Dennis Searle, Terry Cane and Robert Huffaker of the Amalgamated Sugar Company LLC, for assistance in taking disease ratings.