The Influence of abiotic factors and host plant physiology on the survival and pathology of plasmodiophora brassicae of vegetable brassicas

This thesis details studies conducted as part of a wider national program to deliver to the Australian brassica industry a multifaceted or `whole of production' approach to the prevention and management of clubroot in vegetable brassica crops. Variation in the pathogenicity of Plasmodiophora brassicae Wor in Australia was studied using the European Clubroot Differential (ECD) series of brassica hosts Forty one collections of P brassicae from important crucifer production regions in Australia generated 24 triplet codes that were more similar to populations of P brassicae reported from the USA than those from Europe. Several triplet codes (16/3/12, 16/3/31 and 16/2/31) were repeatedly assigned and were common to at least three states of Australia. Two types of `resistant' reaction to Australian populations of P brassicae were evident in B oleracea screened m glasshouse and field trials Several lines of broccoli exhibited partial resistance to P brassicae These lines were `resistant' to only one or several Australian populations but this type of `resistance' was incomplete in effect since a very small amount of root galling was observed on `resistant' hosts. In contrast, complete resistance (no root galls) to all of the Australian populations used in this study was observed in three new Syngenta cauliflower breeding lines. The difference m physiological response of the host and growth of P brassicae was studied in resistant and susceptible B oleracea grown in sand solution culture artificially inoculated with spores of P brassicae. Primary (root hair) and secondary (cortical) stages of P. brassicae occurred in both resistant and susceptible hosts The main difference between the resistant and susceptible host reaction was the absence of degradation of the secondary thickening and cells walls of the xylem in the resistant host. An amoeboid form of the pathogen was observed in both the resistant and susceptible hosts suggesting that resistance genes in B oleracea do not act to prevent the development of this amoeboid form Resistance to P brassicae in B oleracea was characterised by an increased deposition of a 1-3 glucan (callose), a decrease in abscisic acid and increase in total glucosinolates. An integrated program to control and prevent the spread of clubroot was developed based on, improved farm and nursery hygiene and strategic application of in-field controls including line (calcium oxide), calcium, boron and the chemical, fluazinam New machines were developed to incorporate these treatments into the root zone of the transplants in bands along the planting rows. Application of fluazinam in this way increased the marketable yield of broccoli by up to 44% compared with the untreated control and was more effective than either of the existing methods of application. This method of application also reduced the cost of lime and calcium cyanamide treatments by 66 %. These soil treatments have been used alone (on low risk sites) or as part of an integrated control strategy (moderate to high-risk sites) to manage clubroot In field trials conducted over a five-year period, manipulation of soil pH using lime (calcium oxide) was generally sufficient to prevent significant yield loss on low risk sites. On moderate to high-risk sites, additional protection in the form of calcium and boron and/or a protectant chemical, such as fluazinam, was required to prevent disease. However, integration of two or more treatments, was cost effective only when the soil pH was low. On a high pH site, incorporation of fluazinam into the transplant row at 1 5 kg a i /ha was the most effective treatment and there was no additional benefit, in terms of profit, of applying additional lime or calcium treatments