FUNGICIDES FOR CONTROL OF SUGARBEET POWDERY MILDEW AT THE UNIVERSITY OF IDAHO RESEARCH AND EXTENSION CENTER, PARMA, ID, 2001.

John J. Gallian and Tiffany McKay, University of Idaho.  Cooperators:  Dennis Searle, Terry Cane, and George Schroeder, Amalgamated Sugar Co.

        Powdery mildew of sugarbeets 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 area.

        With the loss of Bayleton (triadimefon), only sulfur has been available as a treatment for this disease.  Sulfur can give fair control as a protectant, but timing of application is critical.  If timing is missed slightly, especially for the first application, sulfur treatment will be inadequate and economic loss is likely.  Recently new fungicides in the strobilurin and triazole classes of chemistry are becoming available for control of sugarbeet powdery mildew.  In addition, a benzimidizole fungicide that has been used for Cercospora control on sugarbeets will likely be approved for powdery mildew.

METHODS

A study was conducted at the University of Idaho, Parma Research and Extension Center in 2001 to test the efficacy of six fungicides for the control of sugarbeet powdery mildew caused by Erysiphe polygoni. The fungicides are listed in Table 1.  Microthiol Disperss 80% WP sulfur was included in the test as a check.

  Table 1.  Fungicides, companies and chemistry class for 2001 powdery mildew test.                                   

The trial was planted on April 5, with a 3.5 inch seed drop and later hand thinned to 7-10 inches.  Soil type was a Greenleaf silt loam with 1.0% organic matter.  The field was furrow irrigated, and the first irrigation was applied on April 31.  Fourteen pounds of Temik was applied at planting and again on May 29, and three standard Progress treatments were applied for weed control.  Individual plots were 6 rows wide (11 ft) x 30 ft long and the 4 center rows were treated.  Experimental design was a randomized complete block with 18 treatments and 5 replications.  Fungicide applications were made on July 25 and August 15 using a CO₂ backpack sprayer at 30 psi.  XR11003 nozzles were used for all applications except XR11004 nozzles were used for the two sulfur treatments on August 15 only.  All treatments were applied in 23.5 gal/A of water for the July 25 application.  For the August 15 application, all treatments were applied in 23.5 gal/A of water except for the 2 sulfur treatments, which were applied in 31.5 gal/A.  Weather conditions were clear, wind 1-3 mph, and 67-78 °F for the July 25 application, and clear, wind 1-3 mph and 67-80 °F for the August 15 application.

Disease ratings were taken by plot on July 24 and August 14, one day prior to each fungicide application, and a final rating was taken on September 5.  Both sides of 25 recently matured leaves in each plot were rated for percent of 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 plot.  The experiment was harvested on October 4 & 5.  Roots were dug using a tractor mounted spade lifter.  Roots from each plot were hand weighed, and 2 sugar samples, 8 roots each sample, were taken from each plot and sent to the 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 treatment plots immediately prior to the first fungicide application. Fungicide performance of most treatments was good to excellent.  All fungicide treatments had significantly lower disease ratings than the untreated check on August 14 and September 5.  USF2004, Stratego, Laredo at the 8.0 oz rate, and Eminent appear to have some curative activity.

Sugarbeets had not been previously grown on the field and root yields were high in all treatments.  All treatments except Laredo at the 6.0 oz rate had higher root yield than the untreated check, ranging from a 3.19 T/A to 7.33 T/A increase in yield (Table 3).  All treatments had higher gross sugar per acre, and most had higher recoverable sugar per acre than the untreated check.  There were no differences in disease, root yield, or recoverable sugar per acre between the 5 lb/A and 10 lb/A Microthiol sulfur treatments. There were no differences among treatments in sugar content, conductivity, extraction, and recoverable sugar in pounds per ton.

The increased gross return due to treatment, without deducting the cost of fungicide and its application, ranged from $79/A to $275/A based on the Amalgamated Sugar Company payment per ton at $22.00 nets.  The cost of fungicide and application was not available to calculate net return for most treatments, except for Microthiol sulfur.  The net return for the best Microthiol sulfur treatment was $119/A after deducting two applications at a cost of $15 each.

Disease pressure on the treatments in the experiment was likely more severe than it would have been in a commercial field because a continual source of disease inoculum came from 5 untreated check plots, untreated borders between each plot, and untreated borders surrounding the field.  When the entire field is treated in a commercial situation, the inoculum pressure is likely to be lower and the efficacy of fungicide treatments may be better than in an experiment such as this.