What was it all about?

Beginning in 2015 and ending in 2018, this project aimed to increase the capacity of the vegetable industry to develop integrated disease management programs for bacterial leaf spot of capsicum and chili field crops. As tomato was considered an alternative host for the leaf spot pathogens, investigation of the causal agents of the disease in tomato was also completed.

The research developed a new understanding of the pathogens behind bacterial leaf spot…

• Previously it was assumed that Xanthomonas campestris pv. vesicatoria (now known as X. vesicatoria) was causing bacterial leaf spot in all solanceous crops in Australia. The results of this project, however, indicate this is not the case. Instead there are four different Xanthomonas species associated with the disease.

• The pathogen species are largely host-specific:
  • X. euvesicatoria essentially infects capsicum, chili and tomato
  • X. vesicatoria and X. perforans infect only tomato
  • The fourth species, X. arboricola, was found in association with bacterial leaf spot symptoms and able to weakly infect tomato only.
    
    
• Race-typing of X. euvesicatoria isolates identified races 1 and 7 in Australia. The researchers report that there are currently several commercial capsicum lines in Australia with resistance to race 1 though there are very few, if any at all, readily available with resistance to race 7.

• The research indicates that the diversity of X. euvesicatoria races and other Xanthomonads detected in Australia is very low.

In regard to disease management…

• The researchers looked at the use of copper treatments and copper tolerance in bacterial populations. The testing of Australian isolates revealed all of the pathogens were tolerant to highly tolerant of copper. The researchers report that “copper may still have some role in management of bacterial leaf spot, however, alternative methods are needed to address the ineffectiveness of using copper alone.”
  
  
• Essential oils may be a suitable avenue of alternative treatments, with the researchers noting that preliminary testing indicated oils tested had a strong antibacterial effect against X. vesicatoria and X. euvesicatoria, both as volatile gases and through direct contact. Further work is needed to explore this area.

Other work in the project included a literature review on survival of bacterial leaf spot pathogens between cropping cycles, which highlighted the importance of using disease-free planting material. Survival of the bacterial pathogens in the environment was found to be quite low and disease outbreaks found to be more likely initiated from primary introduction of the pathogen each season, rather than transfer of the bacterium from sources within the environment. The researchers also note that ongoing international trade in seed, together with the high seed-transmissibility of Xanthomonads, increases the risk of co-importation of new races or species into Australia.

One of the project’s major recommendations for the management of bacterial leaf spot – apart from the use of resistant lines where possible – includes heat treatment of seed to prevent primary introduction of pathogens into crops and to mitigate the risk of introduction of new bacterial races, which may circumvent existing plant host resistance genes.

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The project produced the below fact sheets:

• Bacterial leaf spot of tomato and pepper, describing the disease in solanaceous crops, with basic information on how to identify and control
  
  
• Management of bacterial leaf spot of tomato and pepper, providing a more detailed look at available management options for the disease