G. A. Hurrell

Effect of Application Time on the Efficacy of Sclerotinia sclerotiorum as a Mycoherbicide for Cirsium arvense Control in Pasture

An experiment was conducted in sheep-grazed pasture in three regions of New Zealand over three years to evaluate the effect of application time on the efficacy of a dry granule myceliumon-wheat formulation of Sclerotinia sclerotiorum for the biological control of Cirsium arvense . At each site, the experimental mycoherbicide was applied to a previously untreated plot in each month of the year for three years at a dose of 50 g m -2 . Applications made during the spring and early summer months of October, November and December significantly reduced the ground cover of C. arvense for 67, 67 and 44%, respectively of these applications. Reduced ground covers ranged from 38 to 81% of the cover on untreated plots. Applications in late summer and autumn were less effective. Correlations of ground cover by C. arvense with climate parameters suggested that free moisture promoted treatment efficacy, but that intense rainfall after treatment reduced efficacy through wash off. The importance of leaf wetness for the efficacy of the mycoherbicide was confirmed by comparing disease development and mortality in C. arvense shoots with and without enhanced moisture levels under field conditions. A water miscible formulation applied as a slurry was less dependent upon leaf wetness than the dry granule formulation.

Safety zones for a Sclerotinia sclerotiorum-based mycoherbicide: Accounting for regional and yearly variation in climate

Abstract Variation in the width of ‘safety zones’ for sheep and dairy pasture treated with a Sclerotinia sclerotiorum (Lib.) de Bary-based mycoherbicide was quantified using 10 years of climate data from each of five regions in New Zealand. Acceptable and risk-averse zones were defined as the maximum distances in any direction from a treated pasture where ‘added:natural’ ratios of air-borne S. sclerotiorum spores are 1:1 and 1:10, respectively. The 10-year mean 1:1 safety zone had zero width for sheep pasture at all locations, and was at most, 50 m wide for dairy pasture. The width of the 10-year mean 1:10 zone varied regionally from 314 to 443 m for sheep and from 175 to 280 m for dairy pasture, and the 90th percentile 1:10 zones were up to 41% wider. Linear relationships between safety zone width and mean wind speed were evident and these could be used to derive region-specific safety zones.

Risk analysis for biological weed control - simulating dispersal of Sclerotinia sclerotiorum (Lib.) de Bary ascospores from a pasture after biological control of Cirsium arvense (L.) Scop.

Biological control of Cirsium arvense(L.) Scop. in pasture by the plurivorous plantpathogenic fungus Sclerotiniasclerotiorum (Lib.) de Bary mayresult in the formation, escape and aerialdispersal of ascospores, creating an additionaldisease risk in down-wind market garden crops. To determine the width of a safety zone for apasture subjected to this form of weed control,we simulated the spatial pattern in the ratioof added (due to biocontrol) to naturallyoccurring airborne ascospores (due to marketgarden crops) around a 1ha virtual biocontrolpasture under either sheep or dairy cattlegazing over a 91-day emission period in 1996 inCanterbury, New Zealand. This was achievedusing a unique combination of two computermodels; SPORESIM-1D (for spore escape from avegetation source) and PC-STACKS (a modernGaussian plume model for dispersal beyond asource). Plumes of dispersing ascospores weremodelled for each hour of the emission periodfor both the virtual market garden andbiocontrol sites, and the aerial density of theascospores was averaged over the period. Assuming that a 1:1 ratio of added to naturallypresent spores is acceptable, no safety zonewas necessary for either of the modeledpastures. A ten-fold ratio (1:10 added tonatural) necessitated safety zones of 300 and150 m for the sheep and dairy pasturerespectively. Uncertainties associated withextrapolation of this conclusion to individualpasture management scenarios, and to otheryears and climatically different regions arediscussed.

Chondrostereum purpureum and Fusarium tumidum independently reduce regrowth in gorse (Ulex europaeus)

Abstract An experiment was conducted in two gorse populations (Ulex europaeus) in which Chondrostereum purpureum was applied each month as mycelial-agar cultures to the wounds of decapitated stems of previously untreated gorse plants to determine its potential as a mycoherbicide. Summer–autumn (Feb–May) or late winter–early spring (Aug–Sept) applications were effective in both populations, halving stem stump survival (from an average of 56 to 29%). Another experiment in the same gorse populations evaluated the combined effects of C. purpureum applied in May to decapitated stems, and Fusarium tumidum applied as spores in an invert emulsion to regenerative shoots 5–6 months later. There was no evidence of synergism between the two fungi; each pathogen independently reduced the density of regenerative shoots on the decapitated stems by 39–63% averaged over the 12 months following their respective applications. It is concluded that both pathogens have potential as mycoherbicides for gorse regenerating after stem decapitation.

Eradication of nassella tussock (Nassella trichotoma), an unlikely outcome of grubbing

Abstract Records of the numbers of nassella tussock (Nassella trichotoma) plants removed by annual grubbing on pastoral farms in Marlborough and North Canterbury from 1966 until 1988, were used to reveal changes in population density over this period. For North Canterbury, one index of population density, numbers of tussocks grubbed/ha, indicated that populations declined from ca. 12 tussocks/ha in 1966 to relatively stable densities from 1975 until the present of ca. 5 and 2 tussocks/ ha on undeveloped and developed land respectively. Other indices of density derived from data on tussocks grubbed and person-hours worked, support the general conclusion that population decline has occurred. Eradication, however, has not been achieved and the analysis indicates that it is unlikely to occur in the future under the historically practised annual grubbing regimes.

Verticillium dahliae and other pathogenic fungi in Cirsium arvense from New Zealand pastures: occurrence, pathogenicity and biological control potential

Abstract Cirsium arvense (Californian thistle) populations in pastures throughout New Zealand were surveyed in November–December 2005 (30 sites) and January–March 2006 (94 sites) to identify potential biological control agents for this weed. Fungi were isolated from healthy shoots and shoots showing leaf yellowing/browning, stunting or localized lesions. Verticillium dahliae was isolated most frequently, being detected at 30% of the sites in 2005 and at 51% in 2006. Other pathogenic/saprophytic fungi isolated included Sclerotinia sclerotiorum, Plectosphaerella cucumerina and species of Cylindrocarpon, Rhizoctonia and Phoma. Inoculating cut shoots of C. arvense with conidia of V. dahliae, or cutting shoots with a wetted blade previously used to cut infected shoots resulted in yellowing of leaves and shoot death. Spread of V. dahliae infection among plants facilitated by cutting when wet may explain the reported demise of C. arvense populations mown during rainfall and the fungus may have potential as a bioherbicide.

Herbicide cross‐resistance in Ranunculus acris L. subsp. acris

Abstract The responses of seed‐derived progenies of an MCPA‐resistant (R) and an MCPA‐susceptible (S) population of Ranunculus acris to a range of doses of MCPA, 2,4‐D, MCPB, chlorsulfuron, and thifensulfuron were compared under glasshouse conditions. In a second experiment, also under glasshouse conditions, the responses of R and S to the usual field doses of tribenuronmethyl, bentazone, glyphosate, clopyralid, and asulam were compared. R was cross‐resistant to both 2,4‐D and MCPB. There was temporary cross‐resistance to chlorsulfuron and thifensulfuron manifest in slower development of apical mortality in R plants but all R and S plants ultimately died at the usual field dose rates of these two herbicides. There was weak evidence of cross‐resistance to bentazone but no evidence of resistance to either tribenuron‐methyl or glyphosate since both resulted in 100% mortality of R and S at usual field dose rates. Clopyralid and asulam had no effect on either R or S. The results indicate that neither 2,4‐D nor...

Frequency of occurrence and ground cover of Cirsium arvense on pastoral farms in New Zealand: a farmer opinion survey

To determine the peak cover of Cirsium arvense (Californian thistle) on pastoral farms throughout New Zealand (NZ), a postal and telephone-based questionnaire survey of 600 pastoral farms using a stratified-random sampling procedure was conducted. Three key results emerged from the analysis of data received from 502 responding farms: (1) overall, 89% of NZ farms support C. arvense (79%, 87%, 85%, 97% and 94% for beef, dairy, deer, sheep and sheep/beef farms, respectively); (2) the weed is spread across the entire farm in 45% of cases (35%, 40%, 56%, 50% and 53% for beef, dairy, deer, sheep and sheep/beef farms, respectively); (3) the peak percentage grazed area covered by C. arvense is overall 6% (4%, 6%, 6%, 12% and 6% for beef, dairy, deer, sheep and sheep/beef farms, respectively). This analysis provides the first quantitative information on the occurrence and cover of C. arvense on pastoral farms throughout New Zealand.

Mowing during rainfall enhances the control of Cirsium arvense

Pastoral farmers in New Zealand have described dramatic demises in populations of the weed Cirsium arvense (L.) Scop., a perennial herb indigenous to Eurasia, following its mowing during rainfall. To test the hypotheses (1) that the mowing of C. arvense during rainfall increases the control of this weed and (2) that the causal organism in this ‘mowing-in-the-rain’ effect is the vascular wilt fungus Verticillium dahliae, two series of field experiments were carried out in C. arvense-infested pastures in New Zealand, one in autumn 2008 (Experiment series 1, 9 farms), and another in spring/summer 2008–2009 (Experiment series 2, 12 farms). The effect of mowing in the rain as compared to mowing in the dry was to reduce the % ground cover of the thistle in the spring following treatment by 21 and 32% in Experiment series 1 and 2, respectively. Correlations of this ‘rain versus dry’ effect with the incidence of V. dahliae in the subterranean parts of C. arvense shoots sampled in each of the two field experiment series provided no statistical evidence that the effect increased with V. dahliae incidence. Thus these experiments provide no support for the hypothesis that V. dahliae is the biological mechanism for the ‘mowing-in-the-rain’ effect. Nevertheless, they do support mowing during rainfall as a simple and effective management tactic for C. arvense.

Intraspecific plant density effects in Cirsium arvense

Abstract Cirsium arvense (Californian thistle) has been widely studied because of its importance as a weed of agricultural and natural ecosystems throughout much of the temperate world. However, its population dynamics has been largely neglected. As a step toward developing an understanding of the population dynamics of this weed in pasture, an experiment was conducted during the 1998–99 growing season in Canterbury, New Zealand, in which C. arvense monocultures were established at densities of 16, 36, 64, 100, and 144 plants m‐2 in sterilised soil in boxes outside. Five destructive harvests were made from December 1998 to April 1999 and dry weights and numbers of aerial and subterranean plant parts were measured. The growth in size of individual C. arvense plants was retarded as plant density was increased, primarily by a reduction in the formation of new root resulting in reduced recruitment of new aerial shoots. By contrast, the density of the buds on these roots, and their germination rates, were independent of plant density.