USE OF A DETACHED LEAF BIOASSAY FOR SCREENING SWEET CHERRY CULTIVARS FOR BACTERIAL CANKER RESISTANCE

Detached leaves harvested from trees of sweet cherry (Prunus avium L.) in an orchard setting were used for an in vitro assay of resistance to bacterial canker of sweet cherry.
Two methods of inoculation were tested, a leaf disc infiltration technique and a midrib leaf injection technique.
Variables tested included age of leaves, sample date, and inoculation method.
Harvested detached leaves were surface sterilized, inoculated with known pathogenic strains of Ps. syringae pv. syringae, a saprophytic strain of Pseudomonas fluorescens or sterile distilled water.
Inoculated leaves were incubated for 7–14 days at 25°C with a 12 or 16-hour photo period.
Symptom development was rated using a 0–4 (leaf infiltration) or 0–3 ( wound injection) scale with zero indicating no symptom development and the highest numbers indicating expanding chlorotic and necrotic leaf tissue.
Cultivars tested were of known susceptibility and included ‘Merpet’ (resistant), ‘Merchant’ (resistant), ‘Royal Ann’ (susceptible), ‘Sweetheart’, ‘Lapins’ and ‘Bing’. The leaf infiltration technique was successful in confirming the pathogenicity of the Ps. syringae pv. syringae strains used.
It did not allow discrimination of levels of resistance of the cultivars tested.
The midrib wound injection technique offered more useful results.
Mature leaves were better subjects for inoculation than younger succulent leaves primarily because of sensitivity to the surface sterilization procedures.
An inoculum concentration of 10⁸ colony forming units/ml was necessary for good symptom development.