Integrated management of Cercospora leaf spot on sugar beet

Abstract

Cercospora leaf spot (CLS), caused by Cercospora beticola, is the most important foliar disease of sugar beets in Montana. Losses in research plots ranging from 5-15 metric tons per hectare and 0.5-1.5% lower sugar in the last 7 years. Increased levels of storage rot, sugar impurities, and loss of sugar to molasses have also contributed to losses. Current management strategies are heavily dependent upon the application of a few registered fungicides. Continued use of these fungicides is threatened by registration removal through the Food Quality Protection Act of 1996, and the development of fungicide resistance in Cercospora beticola. This study examined management strategies for CLS integrating host-plant resistance, a CLS prediction model, registered fungicides, and a Bacillus mycoides biocontrol agent, BmJ. The effects of these strategies on disease development and yield were evaluated in field trials from 2001 to 2003. Disease levels were rated 4-5 times per season to generate a disease value for each treatment quantified as the area under the disease progress curve. Yield was determined as metric tons of beets and kilograms of extractable sucrose per hectare, and percent sucrose. Results showed more resistant varieties to give equal disease control with 1-2 fewer fungicide applications than more susceptible varieties without sacrificing yield in moderate to light CLS pressure. BmJ applications (4/year) reduced disease levels below the accepted economic threshold and gave disease control equal to fungicide applications when mixed with a single spray of a half rate of tetraconazole at disease onset. BmJ was highly effective in CLS management when combined with more resistant varieties. The levels of C. beticola fungicide resistance to benomyl, azoxystrobin and tetraconazole isolated from research plots in 2001 and 2002 were measured by spore germination and mycelial growth assays. Total insensitivity to benomyl was observed in more than 70% of isolates and reduced sensitivity to azoxystrobin and tetraconazole up to 10 ppm was also recorded. Preliminary results indicate growing more resistant varieties of sugar beet and spraying BmJ may aide in managing fungicide resistance by reducing the number of fungicide applications necessary for CLS control.