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

Abstract

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.

Genetic diversity and aggressiveness of Erwinia psidii on Eucalyptus spp. in Brazil

“…All isolates included in their study were pathogenic to coffee seedlings but were divided into three different aggressiveness classes: highly, moderate and weakly aggressive. Rodrigues et al (2017) not only observed differences in aggressiveness among P. syringae pv. garcae isolates, but also differences in the expression of aggressiveness according to the temperature.…”

“…Rodrigues et al . () not only observed differences in aggressiveness among P. syringae pv. garcae isolates, but also differences in the expression of aggressiveness according to the temperature.…”

Coffee bacterial diseases: a plethora of scientific opportunities

“…Aggressiveness variation among Psgc strains is supported by studies conducted in Kenya and Brazil (Ithiru et al ., ; Rodrigues et al ., ; Maciel et al ., ; Mwangi et al ., ). Interestingly, Kairu () found that cultivar SL 28 was resistant to Brazilian strains but susceptible to Kenyan strains, suggesting a possible virulence variability among bacterial populations.…”

“…One of the major difficulties when trying to establish comparisons among different studies is that several inoculation methods have been used to evaluate plant resistance. In some studies, leaves were previously wounded using different techniques (Ithiru et al, 2013;Rodrigues et al, 2017a;Maciel et al, 2018); in others they were not wounded (Rodrigues et al, 2017a). Some of those studies have shown that the reactions of the coffee genotypes were highly dependent on the inoculation method and that some methods seem to be better in discriminating differences in resistance to the pathogen (Ithiru et al, 2013;Rodrigues et al, 2017a).…”

Can the leaf age influence the susceptibility to bacterial-leaf-spot and bacterial-halo-blight on coffee seedlings?

“…The selection of resistant coffee plants is usually made on field (ITO et al, 2008) and greenhouse conditions (MOHAN et al, 1978;RODRIGUES et al, 2017a). However, the spatial dispersion of bacteria inoculum on field conditions is not uniform, leading to unreliable responses and failure to select the best plants.…”