Control of foliar bacterial diseases: alternatives to copper

Current industry standards for phytobacteria control include copper protectants and plant host resistance. The effectiveness of copper to manage bacterial diseases is often poor, despite registered application rates being about ten times greater than required to inhibit bacterial growth in vitro. This discrepancy between in vitro compared to in-field bacterial suppression is related to the low availability of the bactericidal form of copper (i.e Cu2+) in field environments. This is further exacerbated by copper tolerant bacterial populations. Host resistance, although an effective strategy to control bacterial diseases, is limited by low availability of suitable resistance genes which are often also overcome by pathogen evolution. Previous research has indicated plant oils could be useful as an alternative for disease control (1-3). To identify new products for disease control, oil from clove, coriander, fennel, lavender, oregano and thyme were evaluated in vitro for efficacy to suppress Australian isolates of Pseudomonas syringae pv. porri, P. syringae pv. syringae, Xanthomonas euvesicatoria and X. vesicatoria, either as a volatile gas or through direct contact. Selected oils were then tested in pot trials to evaluate disease control in capsicum plants to X. euvesicatoria. An overview of the limitations of copper for disease control and results of potential oils for control of foliar phytobacteria will be presented.