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Poster: Epidemiology: Pathogen Dispersal


Sensitivity survey of Alternaria species to SDHI fungicides reveals regional and temporal resistance changes in pistachio orchards of California
P. LICHTEMBERG (1), R. Puckett (1), C. Cunningham (2), V. Leschenne (3), T. Michailides (1) (1) University of California - Davis, U.S.A.; (2) Fresno State University, U.S.A.; (3) Swiss Federal Institute of Technology, Switzerland

In California, resistance of Alternaria spp. to succinate dehydrogenase inhibitor (SDHI) fungicides is common in pistachio orchards and results in difficulties in managing Alternaria late blight (ALB). The objective of this study was to monitor the development and spread of SDHI resistant Alternaria phenotypes and evaluate the cross-resistance among fluopyram (Fp), penthiopyrad (Pe), fluxapyroxad (Fd) and boscalid (Bo). In total, 798 isolates of Alternaria spp. were collected from 35 pistachio orchards where SDHI were used. In vitro assays were used to identify the frequency of SDHI resistant phenotypes and to compare the sensitivity distribution of isolates from previous years. The results revealed 85.8, 83, and 80% phenotypes in the current population resistant to Bo, Fd, and Pe, respectively. The Fp showed the smallest EC50 value 16.74 µg/ml and the greatest frequency of sensitive phenotypes, at 45.8%. For the first time, highly resistant phenotypes for Fp were observed in California pistachio orchards, showing a moderate cross-resistance with Bo, Fd and Pe. Finally, the current population of Alternaria isolates (2015) showed sensitivity shifts toward resistance for Fd and Fp in comparison to those of the 2010 survey. Regional and temporal changes in the resistance of Alternaria spp. population will be discussed as well as the impact of future studies in order to delay SDHI resistance in California.