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Bacterial Leaf Spot of Radicchio (Cichorium intybus) is caused by Xanthomonas hortorum

Radicchio (Cichorium intybus) is ranked 22 among crops in Monterey County, California, with a farm gate value of $19,531,000 (3). Beginning in 2002, a leaf spot disease of radicchio was observed in Monterey County. The disease began as small lesions and in some cases coalesced into larger, irregular spots. Lesions were maroon to dark brown; in some cases, the margins of brown lesions became dark maroon with aging. Each leaf spot was observable from both adaxial and abaxial leaf surfaces. Symptoms primarily occurred on the outer foliage of the heads, though on occasion the head cap leaf could develop lesions. Disease incidence in the first year resulted in up to 10% unharvested radicchio because of cap leaf infections or reduced head size if outer wrapper leaves were all removed; outbreaks in subsequent seasons were more limited. Bacteria forming yellow mucoid colonies were isolated from surface disinfested symptomatic tissue that was macerated and streaked onto sucrose peptone agar medium. Bacteria were gram negative, did not fluoresce on King's Medium B, and used esculin as a carbon source but used none of the other 48 carbon sources tested using the API 50 CH test strip. Nine isolates from symptomatic radicchio had the same DNA fragment banding pattern generated by repetitive extragenic palindromic sequence polymerase chain reactions (rep-PCR) using the BOXA1R primer. Amplicons of rpoD, dnaK, fyuA, and gyrB for multilocus sequence typing (MLST) were generated using a modification of the scheme developed by Young et al. (4) and sequenced by a commercial laboratory. Concatenated sequences of the four genes from the radicchio isolates were compared to the sequences available in the Plant Associated and Environmental Microbes Database (1). The genetic distance between the nine isolates from radicchio and pathotypes of Xanthomonas hortorum were 0.03 or less and MLST analysis indicated that radicchio isolates were members of the species X. hortorum (2). To complete Koch's postulates, freshly grown cultures were suspended in phosphate buffer and adjusted to approximately 5 × 10⁸ CFU/ml. The inoculum was sprayed onto the undersides of leaves of 40-day-old radicchio plants (C. intybus cv. Leonardo). Plants were incubated at 100% humidity for 48 h and then moved to a greenhouse. Plants sprayed with buffer served as negative controls. For each of the two experiments conducted, there were three and six single-plant replicates per treatment. The buffer treated plants did not develop symptoms but the plants treated with isolates from radicchio developed leaf spots similar to those observed in the field with symptoms beginning to be visible after 5 days. The bacteria isolated from symptomatic tissue on inoculated plants were identical to the original strains when compared with rep-PCR, thus completing Koch's postulates. Results from the two experiments were similar. To our knowledge, this is the first report of X. hortorum causing a leaf spot disease on radicchio. The disease continues to occur sporadically on radicchio grown in coastal California.

References: (1) Almeida et al. Phytopathology 100:208, 2010. (2) Bull et al. Phytopathology 101:847, 2011. (3) Lauritzen, Monterey County Crop Report, 2010; (4) Young et al. Syst. Appl. Microbiol. 31:366, 2008.