A Semiselective Medium for the Isolation ofXanthomonas campestrispv.juglandisfrom Walnut Buds and Catkins

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Abstract
This brilliant cresyl blue-starch (BS) medium recovered 52-100% of the X. campestris pv. juglandis colony-forming units (cfu) compared to King''s medium B (KB) when isolations were from naturally-infested Persian (English) walnut [J. regia L.] tissues, and eliminated 97% of the microorganisms found on KB. The starch hydrolysis zone surrounding X. campestris pv. juglandis colonies made them distinctive and easily distinguishable from the other bacteria that grew on BS medium. Six other X. campestris pathovars grew on BS medium: pv. begoniae, pv. campestris, pv. incanae, pv. malvacearum, pv. phaseoli and pv. vesicatoria. The role of dormant and developing walnut buds and catkins as overwintering sites for X. campestris pv. juglandis was investigated by using the BS medium. Buds and catkins from 2 early blooming cultivars (Payne and Ashley) were infested, with the frequency of bud and catkin infestation ranging 50-80% and 0-90%, respectively. The frequent infestation of walnut buds and catkins by X. campestris pv. juglandins indicated that this pathogen has a resident phase (sensu Leben) on walnuts.

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