Evaluating spray programs for managing copper resistant Pseudomonas syringae pv. tomato populations on tomato in the Limpopo region of South Africa

Abstract

Bacterial speck of tomato caused by Pseudomonas syringae pv. tomato is an economically important disease in the Limpopo production region of South Africa. Two field trials were conducted to evaluate different spray programs for management of the disease. In the first trial, applications were only made in the field, and the main product evaluated was acibenzolar-S-methyl (ASM), with and without copper + mancozeb. In the second trial, spray programs were already started in the nursery and then continued in the field. The main products evaluated in the second trial were ASM and biological products. In the first trial, ASM field programs included a South African registration (3.75 g/100 L every 21 days), a USA registration (1.16 g/100 L to 2.63 g a.i./100 L every 14 days) and an ASM low rate (1.16 g/100 L weekly) application. These spray programs were all effective, and comparable in suppressing foliar infections and severely infected fruit (fruit with more than five lesions). The addition of copper + mancozeb to ASM spray programs did not improve control. In most of the first and second set of trials, the commercial standard of weekly applied copper + mancozeb was ineffective in suppressing foliar and fruit damage. It furthermore resulted in a significant increase in fruit infections in most trials. The ineffectiveness of copper + mancozeb spray programs, and programs that contained these compounds, were due to copper resistant P. syringae pv. tomato strains. The strains were identified in all trials through in vitro copper sensitivity testing. This was further confirmed through a tomato seedling bioassay using a copper resistant strain. In the second trial, starting spray programs in the nursery was beneficial. ASM applications in the nursery, followed by ASM + copper + mancozeb field applications were more effective in suppressing fruit infections, compared to only field applications of the latter products. The application of B. subtilis in the nursery, followed by field applied biologicals (B. subtilis and Microbacterium maritypicum) combined with ASM sprays, also improved the reduction of infected fruit, when compared to field only applications of ASM + copper + mancozeb.