Resistance to bacterial spot (Xanthomonas gardneri) on foliage and fruit of commercial processing tomato cultivars

Abstract

Bacterial spot of tomato (Xanthomonas gardneri Šutić) is an economically important disease of processing tomatoes in Ontario, Canada, resulting in premature defoliation and fruit damage. Breeding efforts for host resistance focus on assessments of foliar health as opposed to fruit health but anecdotal reports from industry suggest a poor relationship between fruit and foliar resistance. To investigate this, nine commercial cultivars were inoculated at the vegetative (foliar experiment) or reproductive (fruit experiment) stages in replicated field experiments from 2016 to 2018. In the foliar experiment, the standardized area under the disease progress curve (sAUDPC) for defoliation was 51% to 54% higher for ‘TSH18’ than ‘H9706’, ‘Hypeel 696’, and ‘H3406’, but equivalent to ‘CC337’. Fruit disease incidence was 49% and 47% lower for ‘CC337’ than ‘TSH18’ and ‘H9706’, but equivalent to ‘H3406’ and ‘Hypeel 696’. Fruit disease severity was 63% and 60% lower for ‘CC337’ than ‘H9706’and ‘H3406’, respectively, but equivalent to ‘TSH18’ and ‘Hypeel 696’. However, in the fruit experiment, fruit disease incidence was equivalent among cultivars, while the disease severity index for ‘H9706’ (3.4) was higher than ‘Hypeel 696’ (0.7). Furthermore, rank correlation analysis between sAUDPC and fruit disease variables failed to meet the criteria for a significant and strong relationship (r ≥ 0.8 or  ≤ −0.8 and P ≤ 0.05). Additional research is needed to better understand the mechanisms of fruit infection by X. gardneri. In the meantime, scientists should consider the limitations of assessing only foliar damage as an evaluation method for bacterial spot management tools in tomato.