Direct branch inoculation cannot produce uniform infection, until you cause significant physical damage to plant tissues. Aggressive strains of Xaj can easily penetrate shoots after young leaf inoculation (by clipping leaf margins or injection into main rib). Young plants (one-year old) can be killed in 3 months.  Have you tried to evaluate cut shoots placed into bacterial suspension? It may help to find genotypes with strong resistance, but usually not efficient if the difference between cultivars is small.  

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