Evaluation of pathogenicity, systemic colonisation, and host range of

Abstract

Verticillium wilt of alfalfa (Medicago sativa) is a widespread and destructive disease caused by the soil-borne fungal pathogen Verticillium alfalfae (formerly V. albo-atrum before 2011). Owing to an inadequate understanding of the pathogenicity, systemic colonisation, and host range of V. alfalfae, it has been challenging to develop an effective control measure against this disease. In the present study, seven inoculation methods, including seed inoculation, leaf spraying (LS), fungal plugs placed on leaves (FP), stem injection (SI), root dipping (RD), root injuring and dipping (RI), and watering conidia suspension into soil (WI) were used to analyse the pathogenicity and systemic colonisation of V. alfalfae on alfalfa. The typical verticillium wilt symptoms including V-shaped necrosis of leaves and leaf wilting were observed in alfalfa plants with all inoculation methods. The shortest incubation period (9 days) was observed with stem injection and the longest (40 days) with root dipping. Alfalfa plants inoculated by stem injection showed the highest disease incidence (91%), disease index (65%), and percentage of infected plants (80%), which resulted in the highest shoot biomass loss rate (34%). However, the plants inoculated by fungal plugs showed the lowest disease index (4%), percentage of infected plants (10%), and root biomass loss rate (2%). Further, the stem injection method was used to assess the host range of V. alfalfae on seven non-alfalfa plant species including erect milkvetch, sainfoin, common vetch, sunflower, potato, cotton, and bluish dogbane. The artificial inoculation of V. alfalfae by stem injection successfully infected these plants, with disease incidence ranged from 13 to 100% and disease index ranged from 10 to 69. This indicates that host range of V. alfalfae is not only alfalfa, but also other plant species.