Department of Plant Pathology, Montana State University, Bozeman, MT 59717, USA

Background and objectives
Cercospora leaf spot (CLS) is a disease of major importance for sugar beet growers and processors, since it reduces both sucrose yield and quality. Control has become difficult due to resistance to benzimidazole fungicides, tolerance to triphenyltin hydroxide (TPTH) and lack of registration for alternative fungicides. The objectives for this research were to identify control alternatives to TPTH using integrated pest management concepts including host resistance, biocontrol agents and their combination with fungicides.

Materials and methods
Field experiments were conducted in Sidney, MT where CLS had caused significant losses over the past 5 years. In 1996, six alternative treatments were compared to TPTH including propiconazole, the systemic acquired resistance activator benzothiadiazole, the biocontrol agent Bac-J, and their combinations. Treatments were applied to cultivars with high, moderate and low resistance to CLS. Bac-J was applied in a rate of 2.7x1013 vegetative bacterial cells per ha in a formulation of 1% (w/v) beta-glucan (Nurture Inc.) in a 0.1% (w/v) methyl cellulose solution. Treatments were initiated at first sign of disease using four applications applied in 250 l water/ha at a 14-day interval. In 1997, a total of 12 treatments were tested as alternatives for TPTH, including propiconazole, chlorothalonil, fenbuconazole, azoxystrobin, tetraconazole, benomyl, copper, Bac-J alone and in combination with all aforementioned treatments except chlorothalonil. Bac-J was applied as a commercial, freeze-dried spore formulation. In both years, area under the disease progress curve (AUDPC), root and sucrose yield, percentage sugar and impurity values were determined.

Results and conclusions
In 1996, disease pressure was moderate (disease severity for unsprayed resistant and susceptible cultivars was 3 and 10%, respectively) and all treatments provided significant disease control, resulting in increased sucrose yield/ha. The combination of Bac-J and propiconazole provided control equivalent to TPTH. Benzothiadiazole reduced disease severity but did not increase yield. The susceptible cultivar sprayed with fungicides provided optimal yields. In 1997, disease pressure was more severe (9-18% disease severity for resistant and susceptible cultivars, respectively). Greatest economic returns were achieved with the moderately resistant variety sprayed with fungicide. In both years Bac J provided significant disease control as a stand-alone treatment, although fungicide treatments provided superior results when used alone. However, improved disease control was observed when fungicides were applied in combination with Bac-J, with the combination treatments providing optimal control and economic return. Another benefit of these combinations is the potential for fungicide resistance management in CLS control programs. In both years, impurity index and loss to molasses were positively correlated with CLS severity. Also in both years the resistant cultivar, whether sprayed or unsprayed, produced lower economic returns than the moderately resistant or susceptible cultivar despite having significantly lower disease severity.