Finding a suitable alternative to the small pool of existing antifungal agents is a vital task in contemporary agriculture. Therefore, intensive research has been conducted globally to uncover environmentally friendly and efficient agents that can suppress pathogens resistant to the currently used antimycotics. Here, we tested the activity of boric acid (BA) and its derivative phenylboronic acid (PBA) in controlling the early blight symptoms in tomato plants infected with pathogenic fungus Alternaria alternata. By following the appearance and intensity of the lesions on leaves of the tested plants, as well as by measuring four selected physiological factors that reflect plant health, we have shown that both BA and PBA act prophylactically on fungal infection. They did it by reducing the amount and severity of early blight symptoms, as well as by preventing deterioration of the physiological traits, occurring upon fungal inoculation. Phenylboronic acid was more efficient in suppressing the impact of A. alternata infection. Therefore, we conclude that BA, and even more so PBA, may be used as agents for controlling early blight on tomato plants, as they are both quite effective and environmentally friendly.

BACKGROUND: Phenylboronic acid (PBA) is an environmentally non-toxic substance with antimicrobial activity. Due to increasing ecological limitations in phytopharmacy and considering the development of resistance of phytopathogenic bacteria to available antibacterial agents, here we explore a possible role of PBA as an antibacterial agent of choice.RESULTS: We determined a minimal inhibitory concentration (MIC) of PBA in vitro on the Pseudomonas syringae pv. tomato (Pst) (0.5 mg/mL) and Erwinia amylovora (0.8 mg/mL), two of the most damaging plant pathogenic bacteria. In comparison, boric acid MIC was 2.5-6-fold higher than that of PBA, indicating enhanced antibacterial efficacy of the latter. Moreover, we determined the effect of PBA on cell growth and viability of both bacteria and have shown that PBA has bactericidal effect in concentrations > 1.0 mg/mL, whereas in lower concentration it is bacteriostatic. In addition, we have shown that PBA impairs Pst ability to cause symptoms on tomato plants in a dose-dependent manner, whereas solely applied PBA did not affect plant morphology at bactericidal concentrations.CONCLUSION: We report, for the first time, that PBA is a suitable agent for controlling phytopathogenic bacteria. PBA has bacteriostatic activity in lower, and bactericidal activity in higher (> 1.0 mg/mL) concentrations. When applied on tomato plants, PBA managed to suppress symptoms caused by Pst, while having no adverse effect on plants at the bactericidal concentrations. As an additional benefit, PBA is environmentally friendly.

The ascomycete fungus Alternaria alternata causes early blight, one of economically the most important tomato diseases. Due to frequent use of fungicides, A. alternata has developed resistance with negative economic and environmental consequences. Research of new ways to control fungal pathogens has turned its eye to environmentally friendly chemicals with low toxicity such as boronic acids. The aim of our study was therefore to test the antifungal effects of phenylboronic and boric acid in vitro on A. alternata. We isolated the pathogen from a symptomatic tomato plant and determined the minimum inhibitory concentration of phenylboronic and boric acid on A. alternata mycelial growth using the poisoned food technique. The antifungal effect was tested on a wide range of phenylboronic and boric acid concentrations (from 0.04 % to 0.3 %) applied separately to agar with mycelial disc of the pathogen. After five days of incubation, phenylboronic acid at low concentration (0.05 %) completely inhibited mycelial growth. Boric acid, in turn, did not significantly slow down mycelial growth but did reduce sporulation and confirmed its fungistatic effect. Our findings point to the potential use of phenylboronic acid to control phytopathogenic fungi. This is, to our knowledge, the first report on its antifungal effect on an agriculturally important pathogen in vitro. Moreover, since A. alternata is also a human pathogen, these results may have clinical ramifications.