First Report of Xanthomonas fragariae Causing Angular Leaf Spot on Strawberry Plants in México

The state of Michoacán is the most important strawberry producer in México. During January 2007, field-grown strawberry plants cv. Aromas showing vein necrosis were observed in 3 ha in Zamora County, in fruit production fields. The average disease incidence in the field was 80%. Infected plants presented water-soaked lesions limited by veins on the lower leaf surfaces, which enlarged to form angular spots (1). Additionally, most affected plants presented severe necrosis in the main veins and reddish to necrotic lesions on the upper leaf surfaces. Gram-negative bacteria were consistently isolated from leaves with water-soaked lesions. Isolated bacteria produced mucoid, yellow colonies on YDC, grew on tween and nutrient agar (NA), but not on SX media. Strains produced non-fluorescent colonies on King's B media, were positive starch hydrolysis, negative esculin hydrolysis; and produced acid from fructose but not from arabinose, galactose, celobiose, and trehalose. Growth was inhibited by 2% NaCl (3). Indirect ELISA analysis (NEOGEN, Lansing, MI) was conducted using antibodies specific for Xanthomonas fragariae. Conventional PCR assay using the primer pairs 241A/241B was performed (2). The ELISA test was positive. The expected 300- and 550-bp bands were observed in the PCR analysis. The bacteria was identified as X. fragariae Kennedy and King. Pathogenicity tests were conducted twice in a greenhouse (24 ± 4°C) on a total of five strawberry cv. Aromas plants. The main vein of each of three leaves per plant were punctured using sterile needles. Pathogen inoculum was obtained from 6- to 8-day-old NA cultures. Bacteria were applied onto the wounds with a sterile cotton swab dipped into the bacterial suspension (10⁵ CFU/ml). Inoculated plants were covered with plastic bags for 48 h. Symptoms resembling those seen in the field developed on all inoculated plants after 9 days. X. fragariae was re-isolated from the necrotic lesions and identified by PCR. Control plants were similarly inoculated with water but did not develop symptoms. To our knowledge, this is the first report of X. fragariae causing angular leaf spot in strawberry in Michoacán, México. References: (1) J. L. Maas, ed. Compendium of Strawberry Diseases. The American Phytopathological Society, St. Paul, MN, 1998. (2) M. R. Pooler et al. Appl. Environ. Microbiol. 62:3121, 1996. (3) N. W. Schaad et al. Laboratory Guide for Identification of Plant Pathogenic Bacteria. 3rd ed. The American Phytopathological Society, St. Paul, MN, 2001.