The selection of sweet cherry cultivars that are less susceptible to bacterial canker is one of the most promising strategies for reducing reliance on plant protection treatments and increasing crop sustainability.
However, assessing resistance is particularly complex in a perennial tree species characterized by long breeding cycles and considerable variability in the pathogen Pseudomonas syringae pv. syringae (Pss).
A recent study developed and validated a rapid leaf disc assay (Leaf Disc Assay, LDA) capable of reproducing field-observed susceptibility levels with high accuracy.
Validation was based on a three-year survey conducted between 2017 and 2019 in 222 orchards in Chile’s O’Higgins Region, which identified ‘Santina’, ‘Lapins’, and ‘Bing’ as cultivars with low, intermediate, and high susceptibility, respectively.
The role of light proved to be critical: under continuous darkness, the assay actually reversed the cultivar response pattern, making ‘Bing’ appear less susceptible than ‘Santina’.
In contrast, under a 16-hour photoperiod, the assay accurately reproduced the gradient observed in commercial orchards, confirming ‘Santina’ as the least susceptible, ‘Lapins’ as intermediate, and ‘Bing’ as the most susceptible.
The choice of light conditions therefore proved to be a fundamental methodological factor for obtaining biologically representative results.
The assay was subsequently applied to 27 Chilean Pss strains, revealing marked variation in virulence and allowing four aggressiveness groups to be distinguished.
Strain A1M200 caused more than 64% damage even on ‘Santina’, indicating that the resistance observed cannot be considered absolute immunity, but rather a quantitative and potentially more durable form of reduced susceptibility.
The analysis also revealed important cultivar–strain interactions: some isolates showed a greater ability to specifically damage ‘Bing’, whereas ‘Santina’ generally maintained a more stable response to the pathogen population.
The predictive value of the method emerged from comparison with field data: for strains A1M3 and A1M250, Pearson’s correlation between damage detected on leaf discs and mean orchard disease severity was 0.79 and 0.81, respectively, while Spearman’s correlations reached 0.83 and 0.85.
In contrast, artificial inoculation of young shoots in the field showed no significant relationship with susceptibility observed in orchards (r = 0.11).
Fourteen days after inoculation, ‘Santina’ had a bacterial load approximately five times lower than ‘Bing’, with ‘Lapins’ showing an intermediate level.
Reduced susceptibility therefore appears to be associated primarily with the plant’s ability to restrict Pss multiplication rather than to completely eliminate the pathogen.
From an applied perspective, the rapid leaf disc assay (LDA) therefore represents a fast, reproducible tool suitable for the early screening of large segregating populations.
The assay provides results in approximately 14 days and can be performed throughout the year using greenhouse-grown material, allowing highly susceptible genotypes to be discarded at an early stage before they are transferred to costly multi-year field trials.
The integration of rapid phenotyping, characterization of pathogen virulence diversity, and quantification of bacterial growth could therefore represent an important step forward for sweet cherry breeding programs aimed at reducing susceptibility to bacterial canker.
Source: Figueroa-Grenett, F. et al. (2026). A leaf disc assay under controlled photoperiod accurately predicts field susceptibility to Pseudomonas syringae pv. syringae in sweet cherry cultivar. BMC Plant Biology. https://doi.org/10.1186/s12870-026-09603-6
Image source: Janna Beckerman
Andrea Giovannini
PhD in Agricultural, Environmental and Food Science and Technology - Arboriculture and Fruitculture, University of Bologna, IT
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