Giant buttercup (Ranunculus acris L.) is a serious weed of dairy pastures in many regions of New Zealand and at its peak can occupy over 50% of the pasture area. It is unpalatable to cattle and is avoided to the extent whereby pasture plants in the immediate vicinity are also avoided. The national revenue loss incurred by New Zealand dairy farmers from current infestations of giant buttercup is estimated to be $118 million annually. The emergence of resistance to the herbicides historically used to control this weed has led to the need for alternative options for control. Previous work investigated the potential of the fungus Sclerotinia sclerotiorum as a mycoherbicide against giant buttercup. This poster reports on a continuation of this developmental work in which we compare the pathogenicity of three different formulations of S. sclerotiorum. The standard formulation (WH1) and two novel biopolymer formulations (T425 and T425N) were tested in bioassays on excised giant buttercup leaves and the resulting lesions measured after 24, 48 and 72 hours. After 72 hours both T425 and T425N had produced significantly (P<0.05) larger lesions than WH1, indicating that the two novel biopolymer formulations were more pathogenic towards giant buttercup than their rudimentary counterpart.

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The ability of a simple "two parameter" model based on the relative leaf area of weeds to describe grain yield losses in wheat (Triticum aestivumL.) was assessed. Wheat was sown at 100 and 300 plants/m 2 and oversown with six densities of mustard (Brassica nigra L.) to generate different levels of "weed" leaf area. Results showed that the model could be simplified to a one parameter model in this particular experiment. The single parameter q, defining the rate at which crop yield declines with increasing relative weed leaf area, did not vary between times of leaf area determination or between wheat sowing densities. However, other workers have shown that q varies substantially between experiments, limiting the predictive value of the model.

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Phoma macrostoma is a cosmopolitan fungus pathogenic to a wide range of herbaceous and woody plant species but nonpathogenic to most grasses Recently a strain of P macrostoma pathogenic to Californian thistle (Cirsium arvense) and other broadleaf weeds was registered as a bioherbicide in Canada for use in turf grass and arable cropping systemsThis strain causes a condition known as white tip disease that is characterised by symptoms of yellow to white chlorosis commonly referred to as photobleaching Symptoms characteristic of white tip disease have been observed on Californian thistle in New Zealand but it is uncertain if the causal agent is P macrostoma The aim was to determine if strains of P macrostoma that cause white tip disease and are genetically similar to the Canadian bioherbicide strain are present in New Zealand The presence of such strains would facilitate the registration of this pathogen as a bioherbicide since most safety requirements will have been satisfied by overseas research People in the agricultural industry are encouraged to look for Californian thistle with white tip symptoms and send samples to AgResearch for identification