Effect of sclerotial form and isolate on the pathogenicity and virulence of Sclerotinia sclerotiorum on Ranunculus acris

Abstract

Ranunculus acris (giant buttercup) is a significant weed in dairy pastures in New Zealand, outcompeting grasses and clovers and leading to an increased impacts over time. In recent years, there has been a reduction in efficacy of herbicides registered for the control of R. acris due to evolved resistance. Thus, there is a need for alternative control measures. The fungus, Sclerotinia sclerotiorum, has undergone substantial research to develop it as a mycoherbicide to R. acris using a grain-based formulation. Our study investigated the vegetative survival structure, the sclerote, as a potential source of inoculum for the mycoherbicide. Three bioassays were undertaken to determine: 1) the most effective sclerotial form (whole, ground or powdered) of S. sclerotiorum for the infection of R. acris; 2) the virulence of two S. sclerotiorum isolates (S36 and S37) on R. acris; and 3) if there is a difference in the pathogenicity of S. sclerotiorum on the different chloroplast cytotypes of R. acris. The assays showed that the S. sclerotiorum isolate (S36, S37), inoculum type (whole, ground or powdered), amount of sclerotia applied, and R. acris cytotype all affected necrotic lesion development. By contrast, there was no significant (P>0.05) and consistent difference between the lesions caused by the two isolates of S. sclerotiorum. Sclerotia cut or ground, germinated and infected R. acris tissue quicker than intact sclerotia. There was no difference in the response of the different R. acris cytotypes. Despite the inconsistencies observed between the bioassays, the methods are useful for future screening of potential S. sclerotiorum isolates in a mycoherbicide for R. acris.