Copper pesticides are the only products available to growers to combat outbreaks of bacterial diseases in vegetable crops in Australia. Copper products need to be applied to the plant prior to infection for them to be effective. Their efficacy is also compromised if coverage of the target plant if poor. In recent years there have been increasing reports by Queensland growers of poor control of bacterial spot of capsicum, bacterial fruit blotch of melons and black rot of brassicas despite frequent application of copper pesticides. At the same time, strains of bacterial plant pathogens tolerant of copper applied at recommended rates have caused disease control failures in a range of crops worldwide. This report details a three year investigation into poor field control of bacterial spot, fruit blotch and black rot epidemics in Queensland. We sought to understand whether copper-tolerant bacterial strains of these pathogens were contributing to the poor field control. Also, we investigated ways to improve the efficacy of copper pesticides and improve disease control by optimising spray application techniques.

This work showed that copper-tolerance is present in Queensland populations of bacterial spot of capsicum and we demonstrated that copper-tolerant strains of bacterial spot increase in a population after 12 copper sprays and dominate a field population after 21 sprays. Copper-tolerant strains are contributing to poor control of this disease with copper sprays in the field, particularly in north Queensland production districts. Strains of the fruit blotch bacterium with tolerance to copper are also present in low frequency in north Queensland production districts. There was no evidence that copper tolerance exists in populations of black rot from the Granite Belt.

Copper hydroxide/mancozeb tank mix combinations should now replace existing industry standard treatments, because they gave better control of all three pathogens compared to copper hydroxide (Kocide DF) applied alone (the current industry standard). Air assisted boom sprayers fitted with twin jet nozzles were the best means of achieving overall spray coverage of capsicum, rockmelon, and watermelon plants. Young capsicum plants should be sprayed with 300L/ha and this should be increased to 800L/ha when the plant canopy has fully developed. Volumes of 300L/ha for young watermelons, 500L/ha for young rockmelons, and 1000L/ha for mature vines of both melon types are required for optimal spray coverage of melon crops.