Aggressiveness of Strains and Inoculation Methods for Resistance Assessment to Bacterial Halo Blight on Coffee Seedlings

Characterization of resistance response to pathogens is a fundamental strategy in plant breeding programmes. Thus, in this study, we developed an effective inoculation method for resistance tests of bacterial halo blight (BHB), caused by Pseudomonas syringae pv. garcae on coffee. Firstly, aggressiveness of eight selected bacterial strains as well as seven mixed strains was assessed on coffee seedlings of Mundo Novo IAC 376‐4 cultivar. Two experiments were conducted comparing three inoculation techniques: initially, we compare the methods of sprinkling and multiple needles on four cultivars of Coffea arabica; later, four cultivars and seven genotypes were compared for the multiple needles method and the abrasion technique. Severity was evaluated according a disease rating scale (DRS), considering either leaf surface area affected for sprinkling, or affected inoculated area for inoculation by multiple needles and abrasion. The area under the disease progress curve of disease (AUDPC) was calculated considering weekly evaluations of disease from seven until 42 days postinoculation (DPI). The standard deviation (SD), coefficient of variation (CV) and confidence interval (CI) variables were used to denote the effectiveness of the methods. According to the results, strain IBSBF 1197 was the most aggressive, and three other strains showed high aggressiveness. All inoculation methods were able to discriminate the resistance response to BHB, wherein the sprinkling method was less efficient than multiple needles and abrasion technique was more efficient than multiple needles. In addition, disease evaluations at 14 DPI showed a high correlation coefficient with area under AUDPC at 42 DPI, validating the early selection to BHB. Results of inoculation methods indicated the abrasion on first pair of leaves, together with evaluations on fourteenth DPI, as the most promising technique for early selection on coffee breeding to BHB.