Graeme W. Bourdôt

A model of the escape of Sclerotinia sclerotiorum ascospores from pasture

Abstract A multi-layer physical model, sporesim -1 d , based on the gradient transfer theory (K-theory) of turbulent dispersal (analogous with the molecular diffusion of gasses) is described for the transport of Sclerotinia sclerotiorum ascospores within and above a grass canopy following their release from apothecia at ground level. The ‘steady-state’ diffusion equation is solved numerically and the spore escape fraction is estimated. sporesim -1 d s context is the risk analysis of S. sclerotiorum used as a mycoherbicide to control Cirsium arvense in pasture. In validation tests sporesim -1 d was internally consistent and produced a vertical wind speed profile similar to that measured in a grassland. In further validation tests, measured vertical profiles of atmospheric concentrations of Lycopodium clavatum spores in a wheat crop, and Venturia inaequalis spores in an apple orchard and in a grassland, were closely approximated by the model, as was measured data on the concentration of S. sclerotiorum ascospores deposited downwind of a small area source in a grassland. Escape fractions for grassland predicted by sporesim -1 d , were 50% lower than predicted by both a Lagrangian model (Plant Disease 82 (1998) 838) and a one-layer version of sporesim -1 d , sporesim -1 l , indicating that the vertical compartmentalisation in sporesim -1 d , allowing wind speed and pasture leaf area index (LAI) to vary with height, results in a more realistic estimate of the escape fraction. Simulations using sporesim -1 d revealed an increase in the escape fraction with increasing wind speed, and an order-of-magnitude fall with increases in LAI from values typical of a closely grazed sheep pasture (ca. 2) to those of more laxly grazed cattle pastures and intact grassland (ca. 7). This result implies that any additional risk of disease in a susceptible crop growing downwind of a pasture treated with a S. sclerotiorum mycoherbicide may be reduced by grazing management. Reduction in the risk of sclerotinia rot in kiwifruit (Actinidia deliciosa) vines, and in apple scab disease in apple trees, caused by V. inaequalis, appears possible by maintaining a dense grass under-storey. A simple empirical model for spore escape with one parameter and two variables (LAI and wind speed) derived from the mechanistic model provided a good description (r2=0.998) of simulated escape fraction. Combined with information on release rates of S. sclerotiorum spores at a biocontrol site, this model will enable a times-series analysis of spore emission, and coupled with a Gaussian plume model, prediction of minimum isolation distances between a biocontrol site and a susceptible crop.

Pastoral weeds in New Zealand: Status and potential solutions

Abstract Currently there are some 187 plant species, almost all exotic in origin, occurring as “weeds” in pastures in New Zealand. Judging from their occurrence in scientific papers published in the proceedings of the New Zealand Plant Protection Society, 65 of these species are, or have been considered historically, to be significant pastoral weeds. While 34 of these pastoral weeds are currently being managed under Regional Pest Management Strategies, 15 of these regionally managed species are apparently un‐researched in New Zealand, implying that their management does not have a scientific basis. The aggregate cost of pastoral weeds to the New Zealand economy is estimated to be NZ

The potential global distribution of the invasive weed Nassella neesiana under current and future climates

1.2 billion per annum, based on an analysis made in 1984, but this analysis is hampered by a lack of both objective data on the impacts of weeds on pastoral production and an accurate and comprehensive national census of the main problem species. Ongoing naturalisations, from an existing pool of exotic plant species estimated currently at 25 000, and new arrivals through international trade in plant species and germplasm, can be expected to steadily add to the list of pastoral weeds. Prioritisation of these weed species will be necessary if New Zealands pastoral agricultural industries are to make long‐term, economically optimal decisions about their management. Such prioritisation will require robust models of their current and potential distributions, their rates of population increase and spread, a full understanding of the merits and pitfalls of alternative control options, and robust models of their impacts on pastoral productivity.

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

Nassella neesiana (Trin. and Rupr.) (Chilean needle grass), native to South America, has naturalised sporadically in the UK, France, Italy and Spain, and more widely in Australia and New Zealand, where it has become a serious grassland weed. As a first step towards a global risk analysis we project a CLIMEX model of N. neesiana distribution globally under current climate and six future climate scenarios. Under current climate, areas not yet invaded, but climatically suitable, are eastern and south-western Africa, the north-west coast and south-eastern USA in North America, high-elevation areas in Central America, south-eastern China, northern Africa along the Mediterranean Sea, parts of Nepal, India and Pakistan, and Europe. Under the future climate scenarios, a mean global reduction of 32% in the area of suitable climate is projected, with marked reductions in the native range (34%) and also in Africa (67%), Asia (30%), North America (36%), and Australia (42%). These range contractions are primarily attributable to projected increases in temperatures leading to lethal heat stress excluding the plant from areas currently designated as sub-tropical and tropical humid. By contrast, projected expansions eastward in Europe and westward in New Zealand, result in increases in suitable area of 70 and 60%, respectively. Based on these results, which were consistent across the climate-change scenarios, the countries most at risk from N. neesiana are located in western and eastern Europe. A prudent biosecurity strategy would be to prevent the species spreading from the nascent foci already established there. Such a containment strategy would require controls to limit human-assisted dispersal of the species’ fruit and to ban the species from propagation and distribution throughout Europe.

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

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.

Management strategies for an invasive weed: A dynamic programming approach for Californian thistle in New Zealand

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.

Dairy production revenue losses in New Zealand due to giant buttercup (Ranunculus acris)

Abstract Invasive plants can cause significant problems in natural and agricultural ecosystems. Although research has already been conducted on the economics of a single‐control option for some invasive weeds, we extended the analysis by developing a dynamic optimisation model that evaluates the net benefits of a range of possible control options simultaneously in order to identify the optimal strategy (mix of control options). This paper focuses on Californian thistle (Cirsium arvense) in pasture in New Zealand. The net benefit is maximised by considering the costs and efficacy of control options, and the monetary value of animal production. Trajectories of shoot density are developed and the optimal strategies are found. Our results suggest that the introduction of a biological control agent (Apion onopordi), in combination with one or more control options, is the optimal strategy when the initial density of the thistle population exceeds 1.0 shoot m‐2. Results show that in the setting of the model excluding MCPA, MCPB and a Sclerotinia sclerotiorum‐based mycoherbicide reduces the net present value (NPV) by less than 2%.

Does transmission of the rust pathogen, Puccinia punctiformis, require stem mining vectors?

Abstract Giant buttercup (Ranunculus acris L.), a species of European origin, is widespread throughout New Zealand, and is an intractable weed of dairy pastures in parts of the south Auckland, Hawkes Bay, and Taranaki regions and in the districts of Southern Wairarapa, Horowhenua, and Tasman. In this study, the seasonal progression of the weeds ground cover in dairy pastures in the Tasman District was measured during 1989–90 and used, in conjunction with monthly weed‐free pasture dry matter production values, to estimate the annual percentage loss in pasture production due to a typical infestation of the weed subjected to typical control measures. Extrapolation to other infested regions and districts provided a national estimate of milk solids revenue loss due to giant buttercup in New Zealand dairy pastures of

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.

156 million in the 2001–02 season.

A methodology for risk analysis of plurivorous fungi in biological weed control: Sclerotinia sclerotiorum as a model

Abstract Recent research in Europe has suggested that stem mining insects may be important for vectoring the pathogen Puccinia punctiformis, and largely responsible for its systemic infection in the weed, Cirsium arvense. However, here we present comparative survey data showing that the level of systemic disease is the same in Europe and New Zealand, with and without stem miners, respectively, casting doubt on the idea that these insects are necessary for transmission of the fungus.