FUNGICIDES FOR CONTROL OF SUGARBEET POWDERY MILDEW PARMA, IDAHO, 2003

John J. Gallian and Dixie Vargas, 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.

Powdery mildew of sugarbeets, caused by Erysiphe polygoni, occurred earlier this year in much of the sugarbeet growing area of Idaho, Eastern Oregon, and Washington.  Persistent high temperatures, accounted for the early onset of the disease. Fungicide treatment is necessary to prevent economic loss that will occur if disease exceeds an average 10% mature leaf area diseased for the season.  The Treasure Valley and Eastern Oregon usually require two fungicide treatments to prevent economic loss; one fungicide treatment is usually sufficient for the Magic Valley.

METHODS   

A study was conducted at the Parma Research and Extension Center in Parma, Idaho in 2003 to test the efficacy of six fungicides for the control of sugarbeet powdery mildew.  The fungicides are listed below in Table 1.  One experimental product (XN-105-174 A) for Northwest Agricultural Products was tested at two rates.

Table 1. Fungicides, companies and chemistry class for 2003 powdery mildew test.

The field was fertilized in late October, 2002 and bedded into 22-inch rows.  Variety Beta 4773 R was planted on April 4, 2003 with a 3.1-inch seed drop. Nitrogen was applied again on May 15, 2003.  The field was furrow irrigated and the first irrigation was applied on May 22.  Insecticide applications were Temik 20 lbs/A on April 4 for root maggot and Lorsban 1.5 Pints/A on July 29 for black bean aphid.  For weed control, Progress 10 oz/A and MSO 40 oz/A were tank mixed and applied on April 16.  MSO 40 oz/A, Nortron 4 oz/A, and Progress 12 oz/A were applied on April 28 and again on May 14.

Experimental design was a randomized complete block with 22 treatments and 6 replications.  Individual plots were 6 rows (11 ft.) wide by 25 feet long.  Fungicide applications were made to the four center rows of each plot on July 16 and August 6 using a CO₂ backpack sprayer.  Applications were applied at 30 psi using XR 1102 nozzles.  All applications were applied at 25 GPA.  Weather conditions were clear, wind 0-5 mph and 93^oF for the July application and clear, wind 3-5 mph and 88^oF for the August application.

Disease ratings were taken by plot on July 15 and August 5, the day prior to each application, with a final disease rating on September 4.  Both sides of recently matured leaves in each plot were rated for percent leaf area infected with powdery mildew using a 0-5 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 7 – 8. Roots were topped and the two center rows of each plot were dug and weighed using a tractor mounted two-row lifter. Two sugar samples of approximately 8 roots were taken for each plot and analyzed for sugar content and conductivity at Amalgamated Sugar Company’s tare laboratory at Nyssa, Oregon.

RESULTS

There was variability observed in the experiment that became apparent in May and appeared to be fertility related.  Additional N was applied which improved the situation, but the variability was visible for the remainder of the season.  Disease onset was early and rapid, and the first fungicide applications were made later than was preferred.  The average disease was 22% of the mature leaf area at the first fungicide application compared with about 2% in our tests in previous years.  These two situations contributed to the smaller differences among treatments than were measured in previous years.

The average disease ratings and % MLAD are given in Table 2.  Disease was uniform throughout the test, and there were no significant differences (LSD 0.05) in disease among treatments immediately prior to the first application.  All treatments except the experimental product had significantly lower disease ratings than the untreated check on August 6.  However, on September 4, only six of the treatments had significantly lower disease ratings than the untreated control.  Because of the rapid disease development, it is clear that disease control was not maintained in treatments where sulfur alone was applied on the second application date (August 6).

Most treatments had significantly higher (LSD 0.05) percent sugar and Recoverable Sugar in lb/T than the untreated check Table 3.  There were no differences, however, among treatments in root yield, gross sugar, conductivity, extraction percent, and recoverable sugar in pounds per acre.  The increased gross return due to treatment, without deducting the cost of fungicide and its application, ranged from $39/A to $192/A based on the Amalgamated Sugar Company contract at $23.00 net return/100 lb. Sugar.  Three of the treatments did not show an increase.

ACKNOWLEGMENT:  Appreciation is extended to Dennis Searle, Terry Cane, Robert Huffaker, George Schroeder, Greg Dean, and Howard Binford of the Amalgamated Sugar Company LLC, for assistance in taking disease ratings.