Evaluation of copper-alternatives for managing olive knot in California

Abstract: Olive knot, caused by the bacterium Pseudomonas savastanoi pv. savastanoi (Psv), is one of the most economically important diseases of olives worldwide. Currently used high- and super-high-density planting systems with automated hedging and mechanical harvesting have increased the risk of disease spread by providing more wounds for the pathogen. Copper is the best foliar treatment currently available for the control of olive knot. Reported low performance of copper is likely due to delays in application in relation to infection periods. Studies indicated that bactericide treatments need to be applied within 24 h of an injury event for optimal performance. The dependence on copper for controlling bacterial diseases often leads to the development of copper resistance and a reduction in efficacy. High levels of copper resistance were detected in some Psv strains in California. These strains were highly virulent and more difficult to control in inoculation studies but were found only at low frequency. In field studies, the aminoglycoside kasugamycin performed equally to the standard copper hydroxide in reducing knot development on lateral olive twig wounds after inoculation with a copper-sensitive strain and was often better than copper using a highly copper-resistant strain. Oxytetracycline was less effective, and several ‘biological’ treatments (natural products and biocontrols) were either not effective or inconsistent. Kasugamycin has been registered in the United States for the management of bacterial diseases of other crops because it is not used in human or animal medicine, and registration on olive is being pursued.