Investigating physiological effects due to artificial infections of grapevine with Verticillium nonalfalfae

Ailanthus altissima is among the most invasive woody species worldwide, outcompeting native trees. The fungus Verticillium nonalfalfae (VN) is promising for A. altissima biocontrol, and its effects on the host have been studied via visual assessment in a range of host species. However, little research was performed to address fungal effects on the physiological processes of non-target woody plants. We investigated the occurrence of visual and non-visually recognisable perturbations of VN infection on potted vines to evaluate the potential risks of the biocontrol pathogen on viticulture. Eighteen four-years-old Vitis vinifera (cultivar Grüner Veltliner grafted on Kober 5BB) potted plants were inoculated with VN conidial suspension of the fungus (F), while nine plants were treated with sterile water (C, control). Disease symptoms and physiological parameters were monitored throughout the experiment (seven evaluation dates), while leaf water potential, leaf mass per area (LMA) and biomass were measured at the end of the study when plant tissue was sampled for re-isolation of the fungus. In our trial, inoculations with VN induced characteristic wilting symptoms only in Ailanthus (used as side control of the inoculum), while vines remained asymptomatic, thus indicating a high degree of host specificity of VN. Limited or no impact was detected on the physiology of the non-target V. vinifera. Furthermore, the LMA and biomass measured in the two experimental groups were not different. Although fungal colonisation induced vascular discolouration in both species, the fungus could only be re-isolated from dying Ailanthus but not from vine tissue. Results suggest that V. vinifera cv Grüner Veltliner is resistant to the applied VN isolate. However, the susceptibility and physiology of additional grapevine cultivars, as well as other native woody species to VN, should be studied before promoting large-scale use of the biocontrol agent.