Determination of fungicide resistance in Botrytis cinerea from strawberry in the Central Coast Region of California

Authors: POKORNY, ANDREW - University Of California; Smilanick, Joseph; Xiao, Chang-Lin; FARRAR, JAMES - University Of California Agriculture And Natural Resources (UCANR); SHRESTHA, ANIL - University Of California

Submitted to: Plant Health Progress
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/19/2016
Publication Date: 3/2/2016
Citation: Pokorny, A., Smilanick, J.L., Xiao, C., Farrar, J.J., Shrestha, A. 2016. Determination of fungicide resistance in Botrytis cinerea from strawberry in the Central Coast Region of California. Plant Health Progress. 17:30-34. doi: 10.1094/PHP-RS-15-0053.

Interpretive Summary: Gray mold caused by the fungus Botrytis cinerea is a major disease of strawberries, and the control of gray mold is dependent on the use of fungicides. However, B. cinerea can develop resistance to fungicides depending on the chemical class and its usage patterns. In 2013, 59 isolates of the fungus were obtained from California’s northern strawberry growing region and tested for sensitivities to four different commercially available fungicides, pyraclostrobin, boscalid, fenhexamid, and iprodione. Results indicated that 37%, 29%, and 31% of the isolates were resistant to pyraclostrobin, boscalid, and fenhexamid, respectively; and all isolates were highly sensitive to iprodione. The findings suggest that appropriate measures need to be taken to rotate fungicidal modes of action to manage this problem in strawberry growing areas of the central coast region of California.

Technical Abstract: A study was conducted in 2013 to investigate the occurrence of fungicide resistance in Botrytis cinerea populations in California’s northern strawberry growing region; specifically in Watsonville and Salinas. In mid-May, 59 samples consisting of a single diseased fruit or plant part with gray mold symptoms were collected from six different strawberry fields. A single-hyphal tip isolate of B. cinerea from each sample was then utilized for in vitro mycelial growth assays comparing sensitivities to four different commercially available fungicides, pyraclostrobin, boscalid, fenhexamid, and iprodione. The fungicide active ingredients were suspended in cooling agar at the following range of concentrations: 0.01, 0.1, 1.0, 10.0, and 100.0 mg/l which were then inoculated with an individual isolate. Colony diameters were measured and compared to control plates to determine the percentage of relative growth of an isolate and the concentration resulting in 50% growth reduction relative to the control (EC50 value). Results indicated that all of the 59 isolates were highly sensitive to iprodione. However, other isolates exhibited some degree of resistance to the remaining fungicides, with 37% resistant to pyraclostrobin; 29% resistant to boscalid; and 31% resistant to fenhexamid. In several instances, isolates were found to be dual- and triple-resistant to these fungicides. Therefore, appropriate measures need to be taken to rotate fungicidal modes of action to manage this problem in strawberry growing areas of the central coast region of California.