5.5.11
A SURVEY OF SENSITIVITY TO DMI FUNGICIDES IN PEANUT LEAFSPOT PATHOGENS FROM GEORGIA, USA

KL STEVENSON1, GB PADGETT2, and AK CULBREATH3

1University of Georgia, Athens, GA 30602-7274, USA; 2Louisiana State University, Winnsboro, LA 71295-5179, USA; 3University of Georgia, Tifton, GA 31793-0748, USA

Background and objectives
Georgia is the largest producer of peanuts in the US, but the crop suffers from severe foliar disease problems that require multiple fungicide applications each year. Cercospora arachidicola and Cercosporidium personatum cause early and late leafspot of peanut respectively, and together are the target for most foliar fungicide applications. Because of their activity on soilborne peanut pathogens in addition to superior leafspot control, the demethylation-inhibiting (DMI) fungicides propiconazole and tebuconazole are currently the fungicides of choice in peanut disease management programs. The major objective of this study was to survey populations of the peanut leafspot pathogens in Georgia by sampling in peanut fields with known histories of sterol inhibitor use, in order to establish the current level of sensitivity to propiconazole and tebuconazole in these pathogen populations.

Materials and methods
Monoconidial isolates of C. arachidicola and C. personatum were obtained from leaves collected from peanut fields in Dodge, Irwin, Sumter, and Tift Counties in Georgia, USA, in 1996. Some fields received applications of DMIs during the growing season and others had no current or previous exposure to DMIs. Sensitivity assays were conducted in vitro, using a 96-well microtiter plate assay in potato dextrose broth amended with 12 different concentrations of propiconazole or tebuconazole ranging from 0 to 3 ppm[1]. Aliquots of homogenized broth cultures of each isolate were added to plates containing the fungicide amended medium and incubated at 25C for 2 weeks. Optical density of each well was recorded with an automated plate reader. ED50 values were estimated based on linear regressions of probit-transformed optical density values on log-transformed fungicide concentrations.

Results and conclusions
ED50 values for tebuconazole were obtained for 526 and 95 isolates of C. arachidicola and C. personatum, respectively. ED50 values for propiconazole were obtained for 548 and 99 isolates of C. arachidicola and C. personatum, respectively. For both fungi, both fungicides, and all locations, ED50 values followed a lognormal distribution. ED50 values for C. arachidicola ranged from 0.0002 ppm to 1.08 ppm for tebuconazole and 0.0006 ppm to 0.77 ppm for propiconazole. Significant differences in sensitivities of C. arachidicola isolates to both DMIs were found among locations, but the differences did not appear to be associated with fungicide exposure history. Mean ED50 values for isolates of C. arachidicola from fields unexposed to DMIs was 0.028 ppm and 0.039 ppm for propiconazole and tebuconazole, respectively. The ED50 values did not differ significantly from the ED50 values of isolates from fields that had been treated with DMIs (0.036 ppm for both propiconazole and tebuconazole). The sensitivities to propiconazole and tebuconazole were positively correlated, although there was considerable variation in the response (r=0.20). Mean ED50 values for C. personatum ranged from 0.016 ppm to 0.027 ppm for propiconazole and 0.029 ppm to 0.111 ppm for tebuconazole. These mean ED50 values for tebuconazole are very similar to mean ED50 values based on germ tube assays reported for other non-DMI exposed isolates of this fungus[2]. Due to widespread use of DMI fungicides in commercial peanut fields in Georgia, establishment of a true baseline for these pathogens to DMIs was not possible. However, results of this survey will serve as a relative baseline for detection of shifts in sensitivity and enable assessment of current resistance management strategies in peanut leafspot control programs.

References
1. Pijls CN, Shaw MW, Parker A, 1994. Plant Pathology 43,726-32.
2. Schwarz MR, Marine DV, Taylor S, Rogers WD, 1990. Proceedings American Peanut Research and Education Society 22,79.