Nitrogen and ultraviolet radiation modify grapevines’ susceptibility to powdery mildew

Abstract

Potted Cabernet Sauvignon and Chardonnay grapevines were grown in southeastern Australia under either ambient or reduced ultraviolet (UV) radiation to test the effect of UV on powdery mildew (Uncinula necator) susceptibility. Diacetate films were used to screen out UV-B (280-315 nm) and UV-A (315-400 nm). Two nitrogen treatments (0 or 3 g N per plant) were applied at bloom to test interactions of nitrogen status with UV radiation. Cabernet Sauvignon was much less susceptible to U. necator and also responded less to UV and nitrogen. U. necator incidence and severity on leaves were increased dramatically when high nitrogen supply was combined with low UV, particularly in Chardonnay. The differences in infection were not due to variation in canopy microclimate (temperature and humidity) caused by experimental treatments. However, high disease susceptibility in response to high nitrogen status and low UV radiation was related to low concentrations of constitutive phenolic compounds (flavonol glycosides and, to a lesser extent, hydroxy-cinnamic acid derivatives), high leafnitrogen status and photosynthetic rates, high succulence, and reduced cuticular wax deposition. No stilbene phytoalexins could be detected following infection, suggesting that stilbenes are not involved in U. necator resistance in grapevines. These results show that UV radiation affects disease susceptibility of grapevine cultivars and that this susceptibility is modulated by nitrogen supply.