Fungicide resistance in Botrytis cinerea populations in California and its influence on control of gray mold on stored Mandarin fruit

Authors: Saito, Seiya; Xiao, Chang-Lin

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/20/2018
Publication Date: 12/1/2018
Citation: Saito, S., Xiao, C. 2018. Fungicide resistance in Botrytis cinerea populations in California and its influence on control of gray mold on stored Mandarin fruit. Plant Disease. 102(12):2545-2549. https://doi.org/10.1094/PDIS-05-18-0766-RE.
DOI: https://doi.org/10.1094/PDIS-05-18-0766-RE

Interpretive Summary: In recent years, the acreage of mandarin citrus fruit has increased significantly in California in response to consumers' demand for "easy-to-peel" citrus fruit, and storing mandarin fruit in cold facilities before packing has become a common practice to retain fruit quality and extend marketing opportunities. Extended storage, however, can increase the risk for development of postharvest fruit rots. Gray mold caused by the fungus Botrytis cinerea is an emerging fruit rot disease affecting stored mandarin fruit in California. Control of B. cinerea has largely been dependent on the use of fungicides. However, B. cinerea is a high risk fungal pathogen for the development of resistance to certain groups of fungicides. Fungicide resistance in the pathogen often results in the failure of disease control. In this study, we tested resistance of B. cinerea isolates obtained from mandarin fruit to four citrus postharvest fungicides. We found that 83-98% of the isolates were resistant to azoxystrobin, 71 to 93% of the isolates were resistant to pyrimethanil, 63-68% of the isolates were resistant to thiabendazole, and no fludioxonil resistance was detected in a 2-year survey. Of the 200 B. cinerea isolates, 5%, 24%, and 62% were resistant to 1, 2 or 3 classes of fungicides, respectively. Inoculation tests were conducted to evaluate if the fungicides at label rates control various resistant phenotypes on fruit. Most fungicides failed to control gray mold on mandarin fruit inoculated with respective fungicide-resistant phenotypes. Our results suggest that alternative control methods need to be integrated into existing decay control programs to target this emerging disease on mandarin fruit.

Technical Abstract: Gray mold caused by Botrytis cinerea is an emerging postharvest disease affecting stored mandarin fruit in California. To develop effective control programs, fungicide sensitivities to four citrus postharvest fungicides were determined. One hundred B. cinerea isolates each in 2015 and 2016 were obtained from decayed fruit collected from packinghouses and tested for resistance to the fungicides. Sensitivity to azoxystrobin was examined based on the point mutation in the cyt b gene using PCR, while resistance to fludioxonil, pyrimethanil and thiabendazole were examined on fungicide-amended media. For azoxystrobin, 83% and 98% of the isolates were resistant in 2015 and 2016, respectively. Based on the in vitro fungicide tests, 71% and 93% were resistant to pyrimethanil, and 63% and 68% were resistant to thiabendazole in 2015 and 2016, respectively. No fludioxonil resistance was detected in both years. Five fungicide-resistant phenotypes were detected, and the most common phenotype was triple resistance to azoxystrobin, pyrimethanil and thiabendazole, accounting for 59 and 65% in 2015 and 2016, respectively. Of the 200 B. cinerea isolates, 5%, 23.5%, and 62% were resistant to 1, 2 or 3 classes of fungicides, respectively. Inoculation tests were conducted to evaluate if the fungicides at label rates control various resistant phenotypes on fruit. Most fungicides failed to control gray mold on mandarin fruit inoculated with respective fungicide- resistant phenotypes. Our results suggest that alternative control methods need to be integrated into existing decay control programs to target this emerging disease on mandarin fruit.