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

Abstract

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.

Management of bacterial halo blight and other coffee crop diseases

“…Phosphite could also induce plant defense mechanisms by releasing superoxide, causing death of infected cells, and enhancing the accumulation of phenolic compounds (DANIEL & GUEST 2006). Moreover, phosphite can inhibit pathogen growth, as demonstrated for a mutant of P. syringae resistant to the antibiotic rifampicin, Pseudomonas syringae Psp191R1 by BULTREYS et al (2018). The authors reported that phosphite had a strong effect on in vitro growth of the pathogen, as well as on bacterial growth on the leaf surfaces, with a high acidic effect.…”

Phosphonic acid: a long‐standing and versatile crop protectant

“…Studies focusing on mycorrhiza, which can sometimes be suppressive towards plant pathogens, 58 showed that root colonization can either be stimulated or suppressed by phosphonic acid 59–62 . It is likely that the compositions of root and leaf microbiomes are perturbed directly by the antimicrobial action of phosphonic acid, 63 or indirectly by defence and signalling pathways, 64 and perhaps via alterations in plant carbohydrate or amino acid exudates. Any of these mechanisms could influence the plant microbiome and thus indirectly pathogen population and colonization of plant tissues.…”

Sonication-Assisted Production of Fosetyl-Al Nanocrystals: Investigation of Human Toxicity and In Vitro Antibacterial Efficacy against Xylella fastidiosa

“…Fosetyl-Al is a systemic fungicide that has been also applied against bacterial diseases. A recent work proposed the role of fosetyl-Al in the control of the colonization of pear and cherry leaf surfaces by Pseudomonas syringae [11] and for the control of the bacterial canker of kiwifruit caused by P. syringae pv. actinidiae (Brunetti and Pilotti, personal communication).…”