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A Semiselective Medium for the Isolation of Xanthomonas campestris pv. juglandis from Walnut Buds and Catkins. E. N. Mulrean, Graduate student, Department of Plant Pathology, University of California, Berkeley 94720; M. N. Schroth, professor, Department of Plant Pathology, University of California, Berkeley 94720. Phytopathology 71:336-339. Accepted for publication 30 July 1980. Copyright 1981 The American Phytopathological Society. DOI: 10.1094/Phyto-71-336.

A medium was developed for the semiselective isolation of Xanthomonas campestris pv. juglandis from Persian (English) walnuts, Juglans regia. 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 walnut 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. 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 two early blooming cultivars (Payne and Ashley) were infested, with the frequency of bud and catkin infestation ranging from 50–80% and 0–90%, respectively. The frequent infestation of walnut buds and catkins by X. campestris pv. juglandis indicated that this pathogen has a resident phase (sensu Leben) on walnuts.