Towards faster phenotyping methods for brown rot susceptibility by artificial inoculation in the orchard

Brown rot (BR) of stone fruits caused by Monilinia spp. is one of the first causes of yield loss and fungicide application worldwide. Efforts aimed to BR tolerance breeding are often hindered by the high dependence of the trait on seasonal conditions and fruit factors. Current phenotyping methods consist in scoring rot incidence after artificial inoculation of conidial suspension on ripe fruit. Although these approaches use standard conditions for pathogen growth, they are not suited for its use in large breeding programs, which require the analysis of a high number of seedlings in a short time. The aim of this work was to develop a fast protocol of “on-tree” artificial inoculation for BR tolerance by inoculating fruits providing high humidity conditions to enhance fungal colonization. On three peach accessions (BO02037003, ‘Bordò’ and ‘Rosella’), six infection treatments were tested, using 20-40 fruit replicates: not protected and not infected (C.o); infected and not protected (I.o); protected by paper bag, not infected (C.Pa); protected by paper bag, infected (I.Pa); protected by plastic bag, not infected (C.Pl); and protected by plastic, infected (I.Pl). Up to 10 seedlings per person per hour could be assessed. Multivariate logistic regression analysis using GLM models, showed significant contributions of the following variables to the infection occurrence: cultivar (p=1.8E-11), treatment (p = 1.3E-15), IAD fruit maturity index (p = 1.7E-6), and cultivar × treatment interaction (p = 7.4E-3). Although I.Pl presented much higher infection rates, I.Pa presented advantages such as the highest infection rate compared to its respective control (C.Pa), allowed to observe significant differences among cultivars (p = 0.003), and did not give place to contaminant infections. For these reasons, I.Pa could be considered a promising method for fast scoring of BR tolerance on fruit, directly in the orchard.