This project aimed to increase the capacity of the vegetable industry to develop integrated disease management programs for bacterial leaf spot (BLS) of capsicum and chili field crops. As tomato was considered an alternative host for the BLS pathogens, investigation of the causal agents of the disease in tomato was also completed.
Previously it was assumed X. campestris pv. vesicatoria (now known as X. vesicatoria), was causing BLS in all solanceous crops in Australia. The results of this project clearly indicate this is not the case, instead there are four different Xanthomonas species associated with the disease. Furthermore, the species are largely host specific, where X. euvesicatoria essentially infects capsicum, chili and tomato, and X. vesicatoria and X. perforans only tomato. The fourth species, X. arboricola, was found in association with BLS symptoms and able to weakly infect tomato only. Given this weak pathogenicity, its ability to cause BLS disease is questionable and as such the bacterium is unlikely to need control. Furthermore, race‐typing of X. euvesicatoria isolates identified races 1 and 7 in Australia. There are currently several commercial capsicum lines in Australia with resistance to race 1. There are very few, readily available with resistance to race 7. Importantly, the diversity of X. euvesicatoria races and other Xanthomonads detected in Australia is very low, however, as there is no regulation for these bacterial pathogens it is likely new races and species could be introduced with seed.
Copper tolerance testing of Australian isolates revealed all were tolerant to highly tolerant. Although the minimum amount of copper used in the tests is well below the amount of copper routinely applied in the field, disease control is ineffective. This highlights that copper tolerance in bacterial populations is not the complete answer as to why copper is ineffective for disease control. Copper may still have some role in management of BLS, however, alternative methods are needed to address the ineffectiveness of using copper alone. To this end, essential oils showed promise as preliminary testing indicated they have a strong antibacterial effect against X. vesicatoria and X. euvesicatoria, both as a volatile gas and through direct contact.
The literature review on survival of BLS pathogens between cropping cycles highlighted the importance of using disease free planting material. Survival of the bacterial pathogens in the environment is quite low and disease outbreaks are more likely initiated from primary introduction of the pathogen each season rather than transfer of the bacterium from sources within the environment.
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. This means existing resistance genes managing disease in Australia may become unreliable in the future. Additionally, overuse of a single management strategy such as a resistance gene or a single chemical can provide strong evolutionary pressure on the bacterial pathogen, leading to mutation and the local emergence of resistance‐breaking races or chemically tolerant populations.
The major recommendations for the management of BLS are:
• Consult with your seed supplier about assurances on the health status of the seed or alternatively,
• 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. This treatment will also mitigate risk of other potential exotic and endemic pathogen threats
• Use of resistant capsicum lines where possible
• Combining copper with manganese‐zinc ethylene bisdithiocarbamate (EBDC) (e.g mancozeb) early in the crop cycle to improve availability of bactericidal forms of copper and thus early infections, later applications are not practical due to with‐holding periods of the EBDC
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