Sclerotinia spp., a potential biological control agent of Canada thistle (Cirsium arvense [L.] Scop.)

In a search for a biological control agent of Canada thistle (Cirsium arvense [L.] Scop.), a survey of endemic pathogens of Canada thistle was conducted in Montana during the summer of 1981.Pathogens from six genera, Alternaria, Fusarium, Septoria, Puccinia, Pseudomonas, and Sclerotinia were obtained.Of these pathogens, Sclerotinia was selected for further study due to its lethality on Canada thistle.Sclerotinia inoculum for greenhouse or field studies was prepared by culturing isolates of Sclerotinia spp. on autoclaved oat or wheat kernels.The infested kernels were applied fresh or after being air-dried.In greenhouse studies using a kernel inoculation technique, the isolates of Sclerotinia differed in virulence on Canada thistle and also on eleven crops tested.A S. sclerotiorum (Lib.) de Bary isolate was used in all of the field studies.In field trials conducted on cultivated land infested with Canada thistle, up to 73% of the thistle shoots died within nine weeks of treatment.In addition to attacking the thistle crown and causing wilting and death of the shoots, Sclerotinia infected the roots, extending more than 35 cm into the root system.The damage to the root system reduced the thistle shoot density by 8 to 51% the following year, while the density increased over 40% in the control plots.Soil incorporation of the inoculum reduced the thistle density to lower levels than surface application.In statewide field trials to test S. sclerotiorum under varied environmental and habitat conditions, 19 to 80% kill of the thistle shoots died within 4 to 8 weeks.The trial site with the highest level of thistle control was in a pasture and, contrary to most of the sites, had not been disturbed by cultivation.At this site, the thistle density in the treated plots was reduced 95% the following year.Additional research into the host range and genetics of Sclerotinia would be necessary in the development of this fungus as a biological control agent of Canada thistle.SCLBROTIMIA SPP., h