Detached Twig Assay to Evaluate Bacterial Canker on Peaches

Area of Science:

• Plant Pathology
• Horticultural Science
• Agricultural Botany

Background:

• Bacterial canker, caused by *Pseudomonas syringae* pv. *syringae* (*Pss*), leads to significant yield losses and tree mortality in global stone fruit crops, particularly impacting peach production in the United States.
• Current methods for assessing peach scion resistance to *Pss* are limited, relying on field observations or small-scale laboratory studies, hindering systematic germplasm evaluation.
• The lack of a standardized, scalable laboratory assay impedes reproducible and large-scale phenotyping of bacterial canker tolerance in peach.

Purpose of the Study:

• To adapt and optimize a detached dormant twig assay for peach to enable systematic and scalable phenotyping of bacterial canker tolerance.
• To establish a reliable laboratory method for evaluating peach germplasm response to *Pseudomonas syringae* pv. *syringae* infection.
• To support future breeding programs and germplasm screening efforts for bacterial canker resistance in peach.

Main Methods:

• Dormant peach shoots from nine accessions were segmented, surface-sterilized, and inoculated with a *Pss* suspension or buffer control.
• Following six weeks of incubation, inner bark lesion sizes were assessed using a newly developed visual rating scale and quantified with ImageJ.
• Spearman correlation analysis was employed to compare visual scores, quantitative measurements, and assess the influence of shoot segment diameter on disease severity.

Main Results:

• The adapted detached twig assay demonstrated strong positive correlations between visual disease scores and ImageJ-based lesion measurements (ρ = 0.80-1.00, *p* < 0.01), confirming its reliability.
• Shoot segment diameter showed weak-to-moderate negative correlations with disease severity, indicating it is not a primary driver of susceptibility in this assay.
• The assay proved practical, reproducible, and scalable for phenotyping bacterial canker tolerance across different peach accessions.

Conclusions:

• The optimized detached dormant twig assay provides a valuable and standardized tool for evaluating bacterial canker tolerance in peach germplasm.
• This method facilitates reproducible, large-scale screening, crucial for advancing breeding efforts aimed at developing resistant peach varieties.
• The assay addresses a critical gap in current research, enabling more efficient identification of peach genotypes with enhanced tolerance to *Pss*.