UV-C Irradiation Can Reduce Growth of Phoma lingam in Vitro and in Planta on Oilseed Rape Brassica napus

The short-wave and highly energetic ultraviolet‑C (UV-C) radiation has a disinfectant effect on various microorganisms. It is also known that UV‑C radiation can have an effect on stimulating plant defense. In this study, we used the phytopathogen Phoma lingam as a model organism to examine UV‑C at a wavelength of 254 nm application as an alternative to control this pathogen in Brassica napus. The aim of the study was to determine direct effects on the pathogen and indirect effects of stimulating the plant defense. Mycelia of P. lingam were grown in vitro and treated with different doses of UV‑C (0.015–1.57 kJ/m²). Mycelia diameters were then measured 1, 2, and 7 days after UV‑C treatment. In the in planta tests, cotyledons were injured by a needle and inoculated with 10⁶ spore suspension of P. lingam. To determine the direct effects the oilseed rape seedlings were treated with different UV‑C doses (0.2–1.5 kJ/m²) 1, 3, or 7 days after inoculation with P. lingam. To investigate the indirect effects the UV‑C treatments (0.2–1.5 kJ/m²) were applied 1, 3, or 7 days before inoculation with P. lingam. The in vitro experiments showed a significant reduction in mycelia growth on agar plates one and two days after UV‑C treatment. However, the fungal growth recovered; after 7 days no significant differences were detectable. The in planta results showed an effect on disease severity affected by the application time of UV‑C treatment and the applied UV‑C dose. UV‑C application 1 day before inoculation significantly increased the disease severity. Whereas, UV‑C treatment 7 days before inoculation with a UV‑C dose of 0.8 kJ/m² reduced the disease severity by 44%. If the UV‑C application was done after inoculation, the disease severity could be reduced by approximately 68% at a dose of 0.8 kJ/m² 1 day after inoculation. The results of the study show, that UV‑C treatment can stimulate plant defense and damage the pathogen directly.