Many Australian pepper producers have reported poor control of bacterial spot of pepper (caused by Xanthomonas campestris pv. vesicatoria) despite using copper bactericides at recommended rates. This prompted us to investigate whether copper-tolerant strains of the pathogen are present in Australia and whether they are a contributing factor to poor control. We screened strains of X. campestris pv. vesicatoria in the laboratory, in broths containing different concentrations of CuSO4, and established the maximum copper concentration at which each strain could grow. About one in four strains (75 total) collected between 1999 and 2000 tolerated ≥1.0 mM CuSO4 and could be considered copper tolerant. Only one strain (of 12) collected before 1987 could tolerate 1.0 mM, and comparison of the two populations by fitting regression models revealed that the mortality of strains in the two populations differed significantly across all concentrations of copper. It was necessary to apply higher rates of copper to strains collected between 1999 and 2000 to achieve mortality levels equivalent to those in the older population. This is strong evidence that the prevalence of copper-tolerant strains has increased in Australian populations. We also assessed whether copper-tolerant strains may increase substantially in a field population of X. campestris pv. vesicatoria when copper is repeatedly sprayed. In the field, the proportion of copper-tolerant strains increased after 12 weekly sprays of copper, and they were very prevalent in the population after 21 sprays. This is the first report of copper tolerance in Australian populations of X. campestris pv. vesicatoria.

Soilborne fungal diseases threaten the viability of the Australian processing beetroot industry. Globally, Pythium spp., Aphanomyces cochlioides and Rhizoctonia solani are the predominant soilborne fungal pathogens responsible for a root rot complex in beet crops. In Australia, the disease problems have been exacerbated in recent years because crops are now grown virtually year round, and under environmental conditions favourable to infection. This has lead to increased inoculum levels in soils sown to beetroot. Moreover, nowhere in the world does there seem to be a single strategy that is completely efficacious in controlling these pathogens, so an effective management strategy will almost certainly involve a combination of tactics. The most likely combination seems to involve fungicide seed treatments, rotations of gramineous or biofumigant crops and the use of disease-resistant varieties of beet. The first of these, fungicidal seed dressings, are a cost-effective means of reducing pathogen inoculum and protecting young beets from infection. Hymexazol may be particularly useful in the Australian system, since it is active against both Pythium and Aphanomyces. To combat Rhizoctonia, it should be applied in combination with either pencycuron or toclophos methyl. Second, rotations of gramineous crops, such as maize or oats, or biofumigant brassica crops, such as white mustard, brussel sprouts or cabbage should also reduce the build-up of inoculum and offers a tactic to relieve the problem in the longer term. Third, alternative beetroot cultivars with resistance to Rhizoctonia have now been developed in USA breeding programs and should be considered by the Australian industry. Resistant varieties are potentially very useful since they offer a long-term approach to disease management that can be easily incorporated into existing production systems. Fourth, the alteration of sowing dates to avoid periods of high disease risk (e.g. confining sowing to the cooler, drier months) needs to be seriously considered by the Australian industry if it is committed to disease management. There is also evidence to suggest that seed priming may warrant consideration as a tool to use in combination with fungicide dressings to reduce disease in young plants. Biological seed treatments and soil fumigation appear to be tactics of limited value to the Australian beet industry.

No abstract