A ProMED-mail post
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June 27, 2003
Source: Amer Phytopathol Soc, DISEASE NOTES [edited]

First report of a Leaf Blight of onion caused by Xanthomonas spp. in Georgia

FH Sanders, DB Langston Jr., JH Brock, and RD Gitaitis, Department of Plant Pathology, University of Georgia, Coastal Plain Experiment Station, Tifton 31793; DE Curry, University of Georgia Cooperative Extension Service, Lyons 30436; and RL Torrance, University of Georgia Cooperative Extension
Service, Reidsville 30453. Plant Dis. 87:749, 2003; published on-line as D-2003-0409-02N, 2003. Accepted for publication 16 March 2003.

In October of 2001 and 2002, a leaf blight was reported affecting Vidalia onion (_Allium cepa_) cvs. Pegasus and Sweet Vidalia, respectively, in one field each. Lesions on onion seedlings began as a water-soaked, tip dieback that gradually blighted the entire leaf. Symptoms on onion transplants appeared as elongated, water-soaked lesions that typically collapsed at the point of initial infection.

In both cases, disease was very severe on seedlings, and disease incidence was 50 percent or more in both fields. Warm temperatures combined with overhead irrigation and above average rainfall likely enhanced the severity and spread of disease. Disease was not detected on more mature onions once cool, dry conditions occurred later in the season, and no significant economic loss occurred.

Seed was tested from seed lots of the aforementioned cultivars and _Xanthomonas_ spp. were not found. Diseased tissue was macerated in sterile, phosphate-buffered saline, and 10 microliters of the resulting
suspension was streaked on nutrient agar plates. Yellow-pigmented, gram-negative, rod-shaped bacteria were isolated routinely from diseased tissue. Bacteria were catalase-positive, cellulolytic, oxidase-negative, amylolytic, proteolytic, and utilized glucose in an oxidative manner.

Analysis of whole cell, fatty acid methyl esters (FAME) using the Microbial Identification System (MIS, Sherlock version 3.1; MIDI, Inc., Newark, DE) identified four representative strains of the bacterium as a pathovar of _Xanthomonas axonopodis_ (similarity indices 0.75 to 0.83). Known _Xanthomonas_ spp. from onion from Colorado and Texas (1,2) had similar FAME profiles when analyzed by the MIDI system.

Onion plants were grown under greenhouse conditions for 2 months and inoculated by injecting the base of a quill with 1.0 ml of bacterial suspensions (10 million CFU /ml) of the _Xanthomonas_ sp. isolated from Georgia, and negative controls were inoculated with 1 ml of sterile water. Disease symptoms developed on plants inoculated with bacterial suspensions in 4 to 7 days and _Xanthomonas_ sp. was isolated from the lesions produced. Disease symptoms occurred when the same suspension was sprayed on onion foliage. No symptoms occurred on plants inoculated with 1 ml of sterile water.

To our knowledge, this is the first report of _Xanthomonas_ spp. affecting Vidalia onions.

References:
(1) T. Isakeit et al. Plant Dis. 84:201, 2000.
(2) H.F. Schwartz and K. Otto. Plant Dis. 84:922, 2000.

[The identity of the pathovar associated with the current outbreak of onion leaf blight remains to be determined. Do any of our readers have information on this point? Please let me know.
According to plant pathologists at Colorado State University, Xanthomonas leaf blight of onion caused by _Xanthomonas campestris_ was first reported in Hawaii in 1975 and was subsequently reported as a yield limiting disease in Colorado in 2000. Annual yield losses due to the pathogen exceeded 17 percent in southern Colorado, and entire crop failures have occurred in other production areas. Disease management involves use of bactericides. - Mod.DH]