Copper alternatives for management of bacterial spot (Xanthomonas gardneri) and bacterial speck (Pseudomonas syringae pv. tomato) in processing tomatoes

Bacterial spot (Xanthomonas gardneri) and bacterial speck (Pseudomonas syringae pv. tomato) limit tomato production in Ontario, Canada. Repeated applications of copper pesticides may lead to copper tolerance in pathogen populations. Field trials were conducted in Ridgetown, Ontario, Canada, to evaluate copper alternatives that demonstrated efficacy on P. syringae pv. tomato or one of the bacterial-spot causing xanthomonads in controlled studies or field studies in other regions. Alternatives included kasugamycin in 2010, acibenzolar-S-methyl (ASM) in 2011 and 2012, Bacillus subtilis QST 713, and extract of Reynoutria sachalinensis in all three years, and formulations of chelated micronutrients, alpha-keto acids, and humic acid in 2012. Alternatives were applied alone, or in a tank mix or alternating with copper hydroxide, except for the formulations of chelated micronutrients, alpha-keto acids, and humic acid, which were only applied alone. Means comparisons indicated that none of the alternatives applied alone reduced disease severity below levels in the non-treated control. Copper hydroxide + ASM had lower disease severity than the non-treated control in 2012 (P=0.0417), while copper hydroxide alone did not. Contrast analysis revealed that copper alternatives reduced disease severity by 29% compared to the non-treated control in 2010 (P=0.0046), but not in 2011 or 2012. Disease severity was 28 and 47% lower for programs including copper hydroxide than programs without copper hydroxide in 2010 (P=0.0009) and 2011 (P=0.0012), but there was no difference in 2012. Alternatives applied in a tank mix had 17 and 18% higher yield in 2011 (P=0.0190) and 2012 (P=0.0013), respectively, than alternatives applied alone, but these yields were equivalent to copper hydroxide alone. Further research is required to better understand the ecology and epidemiology of bacterial plant pathogens on tomato and improve the efficacy of copper alternatives under field conditions.