Effect of photosensitisers on growth and morphology of Phytophthora citrophthora coupled with leaf bioassays in pear seedlings

Abstract

The phytopathogenic oomycetes of the genus Phytophthora cause devastating economic losses worldwide.<br />Naphthodianthrone compounds, present in plant extracts of buckwheat and Saint John’s wort act as photosensitiser<br />agents and exhibit antimicrobial activity against a number of pathogens. In this study, we investigated the potential<br />inhibitory effects of fagopyrin and hypericin on Phytophthora citrophthora (R.E. Sm. &amp; E.H. Sm.) Leonian 1906, the<br />main causal agent of rot diseases in deciduous trees. Fagopyrin had the highest inhibitory effect in the colony growth<br />at a concentration of 2% of a stock solution (3 mg/mL), inducing clubbed hyphae with round tips. Notably, hypericin<br />also inhibited the radial colony growth and increased the hyphal branching at the subapical region, while also promoting<br />the formation of enlarged cells with irregular shapes growing collectively as biofilm-like structures. In terms of the<br />mycelial dry weight, although both photosensitisers had considerable inhibitory effects, the fagopyrin treatment was<br />most effective. Leaf bioassays showed that under dark conditions the photosensitiser pre-treated zoospores formed a<br />dense, but aberrant, mycelial growth with penetration defects. In contrast, when the zoospore production was performed<br />under light conditions, the zoospores failed to cause necrotic lesions and penetration events implying that their<br />virulence was impaired. These findings shed light on the biological effects of fagopyrin and hypericin in the regulation<br />of the mycelial growth, morphology and pathogenicity of P. citrophthora.