Optimizing protocols to evaluate brown rot (Monilinia laxa) susceptibility in peach and nectarine fruits

This study assessed and optimized an in vivo method to evaluate the levels of susceptibility/resistance in fruit from the EEAD-CSIC peach germplasm to an isolate of Monilinia laxa (Aderhold & Ruhland) Honey from peach. A total of four commercial cultivars and six genotypes, descendants of three families, of peach [Prunus persica (L.) Batsch] were superficially inoculated in fruits as uninjured inoculation fruit test. Inoculum was obtained from artificially infected peach fruit after five days of incubation under a photoperiod of 12 h supplied by fluorescent lighting system. Spores were harvested from the fruit, incubated at 20-26 degrees C, of 40-60% RH, being careful to avoid contamination. Production of inoculum (conidia) was rapid and adequate using these inoculation and incubation conditions (five days at 23 degrees C) indicating that the M. laxa used was highly pathogenic and inoculation protocol suitable to screen the peach material and commercial cultivars. Lesion diameter and colonization extent were measured on inoculated fruits to estimate disease severity (colonization severity and lesion severity) to establish levels of susceptibility/resistance to brown rot. Results from the screening test showed that although all peach genotypes and commercial cultivars screened were susceptible to the M. laxa isolate, the lack of sporulation on lesions of 'Calante' cultivar and the significant differences in colonization and lesion severities among the genotypes indicated the existence of genetic tolerance to M. laxa infection. The reasons for the differences in tolerance to infection and implications for breeding peach fruit in the Ebro valley are discussed.