Iminoctadine tris (albesilate) resistance in Geotrichum citri-aurantii and its management

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The bis-guanidine fungicide iminoctadine tris (albesilate) (ITA) is widely used to control citrus sour rot pathogen Geotrichum citri-aurantii but its resistance status in the orchards is still unclear. In the present study, ITA resistance in G. citri-aurantii was assessed and resistance mechanisms and its alternative fungicides were explored. ITA resistance frequency in field isolates of G. citri-aurantii collected between 2018 and 2020 from orchards in China was 6.2%. Consecutively culturing on potato dextrose agar (PDA) for 10 generations showed that ITA resistance for field resistant isolates was relatively stable, whereas resistance for the laboratory induced resistant isolates declined by 53.9%. The competitive ability on citrus fruit for resistant isolates was substantially lower than that of the sensitive isolates. Resistance mechanism studies showed that an ATP-binding cassette (ABC) gene (EVM0003677) and two major facilitator superfamily (MFS) genes (EVM0000087 and EVM0003101) might be involved in ITA resistance only in the induced resistant isolates but not in the field resistant isolates. Bioassay of 26 fungicides found that the demethylation inhibitor fungicides metconazole and propiconazole, and the dinitro-aniline fungicide fluazinam were highly effective to ITA-sensitive isolates of G. citri-aurantii and their EC50 values were 0.33, 0.56, and 0.54 mg/L, respectively. The protective control efficacies of ITA, propiconazole, metconazole, and fluazinam all at 400 mg/L were 100%, 60.4%, 33.9%, and 18.5%, respectively. No positive cross resistance was detected between ITA and metconazole, propiconazole or fluazinam. The mixture of ITA and propiconazole at the mass ratio of 9:8 had a synergistic co-toxicity factor of 26. These results have significant implications for our awareness of ITA resistance in orchards and its management.