Isolate differences in Sclerotinia sclerotiorum applied as a bioherbicide to Cirsium arvense

Abstract

Sclerotinia sclerotiorum (Lib.) de Bary has been proposed for use in a bioherbicide formulation to control Cirsium arvense (L.) Scop. populations in pastures in New Zealand. Our research has investigated whether there is a difference between isolate S36 (the currently favored isolate) and another potential candidate, S37. Mycelial growth and morphology of the two isolates were compared on potato dextrose agar. A detached leaf bioassay was conducted on two populations of the thistle in Canterbury, New Zealand: one at Irwell and another at Lincoln. The fungus was applied as an oat formulation or as sclerotia. Lesion development on detached leaves was compared between the isolates, and molecular analysis was undertaken to determine if any genetic differences existed. S36 exhibited greater colony growth than S37 but no morphological differences were observed. Using the oat formulation on the Lincoln population of the thistle, S37 was more virulent than S36, forming substantially larger lesions. This difference was not observed on the Irwell population. In the detached leaf assays with the Irwell population, negligible (0.04%) germination of the S37 sclerotia occurred compared to 30% of the S36 sclerotia. The latter were more virulent, as assessed by lesion development. Random amplified polymorphic DNA indicated genetic differences between isolates S36 and S37 but whether these polymorphisms are responsible for virulence differences is unknown. This work indicates that the two isolates could be used together in a bioherbicide to overcome differences in virulence on different genotypes of C. arvense.