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

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