Cross-resistance between Pseudomonas chlororaphis strain AFS009 metabolites (Howler EVO) and fludioxonil in Botrytis cinerea.

Howler EVO is a biological fungicide based on metabolites of the bacterium Pseudomonas chlororaphis strain AFS009. One of the metabolites, pyrrolnitrin (PRN), is a chemical analogue of the phenylpyrrole fludioxonil used to manage gray mold of fruit crops caused by Botrytis cinerea. Resistance to fludioxonil in B. cinerea is well documented and linked to mutations in the transcription factor mrr1, leading to overexpression of the ATP-Binding Cassette (ABC) transporter gene BcatrB. Moderately resistant isolates are designated MDR1 and MRD1h based on the specific variation of mutations in mrr1 and the level of BcatrB expression. This study investigated EC50 values of 54 B. cinerea isolates sensitive and with moderate resistance to fludioxonil for sensitivity to fludioxonil and Howler EVO. The Pearson correlation coefficient indicated a strong correlation between EC50 values of fludioxonil and Howler EVO. Isolates that were moderately resistant to fludioxonil and classified as MDR strains were also moderately resistant to Howler EVO. The effect of Howler EVO and fludioxonil on BcatrB gene expression was studied by qPCR. Both fungicides induced the BcatrB gene expression significantly up to 100-fold in sensitive B. cinerea isolates. Howler EVO significantly induced the BcatrB gene expression in all MDR1 isolate but not in the MDR1h isolate. In detached fruit assays on cherry, sensitive B. cinerea isolates were completely inhibited by formulated fludioxonil (Scholar) and significantly suppressed in growth by Howler EVO. However, MDR1 and MDR1h isolates produced disease in Scholar and Howler EVO treatments. Our results indicate cross-resistance between the synthetic fungicide fludioxonil and the biofungicide Howler EVO, indicating that, at least for some biofungicides, resistance management is necessary.