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A field experiment was conducted from October 1992 to March 1997 in a sheep-grazed pasture in Canterbury, New Zealand, to determine the effects of the fungus Sclerotinia sclerotiorum on the long-term dynamics of a population of Cirsium arvense. The pathogen was applied in mid-spring either once or in three consecutive years when the C. arvense shoots were vegetative rosettes, using a granular, mycelium-on-wheat preparation that lodged on the C. arvense leaves, stems and in the leaf axils. The single application caused disease in the C. arvense that was confined to the application year. The disease resulted in a temporary (17 months) reduction in population size through initial mortalities among treated shoots and resultant reductions in root growth, adventitious root bud, subterranean shoot and, subsequently, aerial shoot population sizes. The soil seedbank was 80% lower in the treated plots than in the control plots in the first year. Seedlings were never found. The annually repeated application of S. sclerotiorum did not result in the expected continuing decline in the C. arvense population relative to the control population.

The hypothesis that late autumn root biomass in Californian thistle (Cirsium arvense) is a linear function of aerial shoot biomass duration over the previous growing season was tested in experiments conducted in 1995-96 and 1996-97 in sheep-grazed pasture in Canterbury. A range of biomass durations was generated by single and multiple mowings early, midway and late in the growing season. Linear regressions of autumnal root weight on shoot biomass duration explained 26% and 91% of the variation in root mass in 1995-96 and 1996-97 respectively. Mowing was more effective in limiting autumnal root biomass the later in the growing season it was conducted, as a result of the greater impact of later-season defoliation on shoot biomass duration.

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