ISSUE #364

364 - Managing Cercospora Leafspot Tolerance or Resistance to Fungicide

6-24-1999

Economic losses to Cercospora Leafspot were in excess of $45,000,000 to American Crystal growers in 1998. Expenditures for Cercospora control were about $20,000,000. Tin and benzimidazole (Topsin, Benlate) fungicides have been used for Cercospora control for almost two decades in Minnesota and North Dakota.

Serious yield losses occurred in the early 1980’s where the Cercospora fungus developed resistance to Topsin and Benlate. Use of these products was almost discontinued until recent years. The USDA lab at Fargo, ND first revealed tolerance to tin fungicides in 1994 in Southern, MN. The Sugarbeet Research and Education Board of Minnesota and North Dakota has funded a project to determine incidence and severity of tolerance or resistance to fungicides each year since 1995. Agriculturists collect samples each August with analysis done by USDA scientists.

Attached township maps in figures 1-5 summarize the 1998 survey results for American Crystal. Tolerance means normal past use rates of a fungicide presently have reduced effectiveness for Cercospora control. Resistance means normal or higher fungicide use rates now give no Cercospora control. The table below defines the resistance and tolerance catagories and relates them to the shaded townships on figures 1-5.

Resistance/Tolerance CategoryPercent of Leafspots in Category
None 0
Low 1-25
Moderate 26-50
High 51-75
Very High 76-100

Data in figure 1 shows resistance to Topsin and Benlate is moderate to high or very high in about half the Red River Valley townships. Use of these products would be questionable in many areas in 1999. Their use should be only once per season, and only in a tank mix. Figures 2&3 indicate that Tin tolerance is particularly high in the southern Red River Valley and lower in the northern Red River Valley. Use tin fungicides carefully this year. Tolerance to mancozeb fungicides is much higher in the Drayton factory district compared to all other districts, figures 4&5. Its use should be limited in the Drayton district.

Knowledge of local levels of tolerance or resistance to fungicides are key to successful Cercospora control in 1999. These 5 maps provide that information. Your agriculturist has similar information by individual section to further help you better manage Cercospora in 1999. Visit with your agriculturist to review this information and plan a Cercospora management strategy for 1999.

More Cercospora management information is available in the 1998 Sugarbeet Research and Extension reports pg. 243-289. Specific management strategies will be discussed in the next Ag Notes in late June.

REMEMBER THE "120" OR LESS