July 10, 2003
Source: American Phytopathological Society, DISEASE NOTES [edited]

An outbreak of a Leaf Spot Disease of cabbage in Southern Florida caused by Xanthomonas campestris pv. armoraciae

K. Pernezny, University of Florida, Everglades Research and Education Center, 3200 E. Palm Beach Rd, Belle Glade 33430; J. B. Jones, University of Florida, Department of Plant Pathology, 1453 Fifield Hall, Gainesville 32611; P. D. Roberts, University of Florida, SW Florida Research and Education Center, 2686 SR 29 N, Immokalee 34142; and E. Dickstein, University of Florida, Department of Plant Pathology, 1453 Fifield Hall, Gainesville 32611. Florida Agric. Exp. Sta. Journal Series R-09462; Plant Dis. 87:873, 2003; published on-line as D-2003-0501-02N, 2003. Accepted for publication 16 Apr 2003.

From October to December 2001, a leaf spot disease was observed in numerous commercial fields of red and green cabbage (_Brassica oleracea_ var. _capitata_ L.) in the Everglades Agricultural Area, south and east of Lake Okeechobee and in the environs of Immokalee in southwestern Florida.

Discrete water-soaked to greasy-appearing spots were observed in the leaf blades with no evidence of marginal V-shaped lesions characteristic of black rot caused by _Xanthomonas campestris_ pv. _campestris_.

Profuse bacterial streaming was observed when cut leaf sections were examined microscopically. A bacterium that formed yellow colonies on nutrient agar was consistently isolated from these lesions. 10 bacteria were isolated, purified, and characterized. All strains were aerobic, Gram-negative rods. Strains were positive for esculin hydrolysis, proteolysis in litmus milk, and gelatin liquefaction. Strains were negative for urease production, nitrate reduction, oxidase, and utilization of asparagine as a sole source of carbon and nitrogen.

Fatty acid methyl ester analysis indicated a match with Florida library strains of _X. campestris_ pv. _raphani_ (similarity indices 0.605-0.738). Suspensions (20 million CFU/ml in phosphate-buffered saline) of 2 Oklahoma strains identified as _X. campestris_ pv. _armoraciae_ provided by J. P. Damicone (3) and 4 representative Florida strains were applied to plants using a hand-held sprayer.

Pathogenicity of the strains was tested on 3 replicate greenhouse-grown plants of the following: green cabbage cv. Market Early; red cabbage cv. Salad Delight; radish cv. Red Silk; tomato cv. Sunny; sweet bell pepper cv. Jupiter; and fresh horseradish roots purchased from a retail grocery chain. A strain of _X. campestris_ pv. _campestris_ originally isolated from Homestead, FL was also included in pathogenicity tests. All Florida and Oklahoma strains produced leaf spots, but no V-shaped lesions, on leaves of green cabbage, red cabbage, radish, tomato, and horseradish.

Typical black rot symptoms were observed only in radish and green and red cabbage inoculated with the _X. campestris_ pv. _campestris_ strain. On the basis of these results, we identify the Florida strains as _X. campestris_ pv. _armoraciae_ (1,2,3), recognizing the precedent of _X. campestris_ pv. _armoraciae_ over _X. campestris_ pv. _raphani_ based on extensive genetic and serological data (1).

Our strains appear to be more similar to those causing outbreaks on crucifers in Oklahoma (3) than those in Ohio (2), because Florida strains were pathogenic on tomato.

References:
(1) A. M. Alvarez et al. Phytopathology 84:1449, 1994.
(2) F. Sahin and S. A. Miller. Plant Dis. 81:1334, 1997.
(3) Y. Zhao et al. Plant Dis. 84:1008, 2000.

[_Xanthomonas campestris_ pv. _armoraciae_ [Xpa] is also present in Australia, Japan, Brazil, China, Turkey, and India. The disease is favored by cool temperatures in fall and winter, although it infects susceptible hosts over a wide temperature range. Infected plant debris is a source of inoculum and Xpa is known to be soil- and seed-borne. Disease management involves use of bacteria-free seed, planting in well drained soils, and rotation of non-cruciferous crops on a 3-year cycle. Apparently there are no tolerant or resistant cultivars.
Another reference: <www.biogemuese.de/kohlkrank/navigato.htm> - Mod.DH]