Antibacterial activity of fosetyl‐Al, ethyl‐phosphite and phosphite against Pseudomonas syringae on plant surfaces and in vitro

Fosetyl‐Al (the aluminium salt of ethyl‐phosphite) is an acid product used to fight oomycete diseases and sometimes used against bacterial diseases, but its antibacterial mode of action is largely unknown. Therefore, the direct effects of fosetyl‐Al and neutralized fosetyl‐Al on the colonization of leaf surfaces by Pseudomonas syringae were compared. Control of colonization was highest and almost complete in acidic conditions (99.7%), showing the importance of acidity for maximal antibacterial efficacy on plant surfaces. However, the high inhibition obtained with neutralized fosetyl‐Al (88.9%) suggested another direct effect. Therefore, it was investigated in vitro whether ethyl‐phosphite has antibacterial activity, possibly related to its phosphite content. Inhibition of P. syringae growth by ethyl‐phosphite was observed and influenced by the carbon source and by phosphate. Growth inhibition reached 83.1% in unshaken conditions in a phosphate‐depleted glucose medium. Phosphite drastically increased the effects and induced up to 99.7% growth inhibition. Mass spectrometric analysis indicated that P. syringae hydrolysed ethyl‐phosphite to phosphite using a cell‐linked phosphatase activity. Specific phosphite toxicity was observed in phosphate‐sufficient succinate media and phosphite‐induced phosphorus starvation was observed in phosphate‐depleted glucose media, indicating that ethyl‐phosphite and phosphite have direct environmental impacts on bacteria. Phosphite toxicity for P. syringae probably occurred in the phyllosphere. The study indicates the potential and limitations of fosetyl‐Al as a direct antibacterial product and helps in understanding the fosetyl‐Al control of flower bud blast in pear. It raises the prospect of using phosphite in plant bacterial disease control.