internationally collected data concerned with variation for pathogenicity in Plasmodiophora cKssicae and for clubroot resistance in three Brassica species, together with additional experimental evidence, were used to elucidate the major genetic features of this host-parasite relationship. The major components of resistance in B. campestris and B. napus were differential and xobahly oligogenic. Good agreement was found between the data and simple gene-for-gene schemes. In contrast, the major component of resistance in most B. oleracea genotypes appeared -. 0 be non-differential and there was evidence of pathogen variability for non-differential pathogenicity. A strong association between pathogenicity on any resistant B. campestris genotype and or. all resistant B. napus genotypes was apparent, suggesting the possibility of genes in common. There was evidence that P. brassicae collections carrying certain characters for differential pathogenicity on these two species were less pathogenic on B. oleracea. However, there was no ether evidence of pathogenic specialization to a particular host species. Several analytical pro-Mdures were employed which may prove useful in clarifying the important features of other host-parasite combinations.

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Grapevine powdery mildew, caused by the fungus Erysiphe necator, is a major disease affecting grape yield and quality world‐wide. In conventionally‐managed vineyards, this disease is controlled mainly by regular application of sulfur and synthetic fungicides; and in organic agriculture, by sulfur and canola‐based oils. Health and environmental impacts associated with the use of sulfur, development of resistance to certain synthetic fungicides, and demand for residue‐free grapes, all point to a need for effective alternatives to sulfur and synthetic chemicals. Accordingly, seven greenhouse experiments were conducted to evaluate the potential of novel (abiotic) materials, as well as certain biological agents, for controlling powdery mildew. Application of Bacillus subtilis, milk (200 mL/L), whey (30 g/L), canola‐based oils (2–4 mL/L) and Ecocarb (potassium bicarbonate, 3 g/L) all reduced the severity of powdery mildew, and in some cases by 96% compared with untreated controls. Such treatments thus offer potential as alternatives to sulfur in organic viticulture if their efficacy can be duplicated under field conditions – the subject of paper 2 in this series (Crisp et al. 2006, Australian Journal of Grape and Wine Research12, pp. 203–211).

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