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Natural Outbreak of a Bacterial Fruit Rot of Cantaloupe in Georgia Caused by Acidovorax avenae subsp. citrulli


R. R. Walcott, D. B. Langston, Jr., F. H. Sanders, Jr.,
and R. D. Gitaitis, Department of Plant Pathology, University of Georgia, Coastal Plain Experiment Station, Tifton, GA 31793; and J. T. Flanders, University of Georgia Cooperative Extension Service, Cairo, GA 31728


Posted 2 June 2000. Plant Health Progress doi:10.1094/PHP-2000-0602-01-HN.

Reproduced, with permission, from Plant Disease, March 2000.



In April and July 1999, cantaloupe plants (Cucumis melo) from commercial greenhouses and fields in Grady, Colquitt, Mitchell, and Tift counties, GA, exhibited severe foliar necrosis and a fruit rot. Foliar symptoms were V-shaped, necrotic lesions occurring at the margin of the leaf and extending inward toward the midrib. Symptoms on the fruit surface were observed after net development and occurred randomly as round, necrotic, sunken spots or cracks a few millimeters in diameter. A soft rot originating from lesions on the surface of the fruit expanded into the flesh. Approximately 5% of the fruits were affected. Bacteria recovered from cantaloupe fruit and leaf tissues produced nonfluorescent, smooth, off-white colonies on King's medium B. Characteristic of Acidovorax avenae subsp. citrulli, the bacteria produced pits in carboxymethyl cellulose media (WFB 44), and reduced Tween 80 to give a visible precipitate on WFB 68 media (1). Based on fatty acid analysis, all strains were identified as A. avenae subsp. citrulli by Microbial Identification System software, version 3.6 (MIDI, Newark, DE), and similarity indices of 0.06, 0.79, 0.21, and 0.43 were recorded for strains recovered form Grady, Tift, Colquitt, and Mitchell counties, respectively. Using specific oligonucleotide primers (WFB 1/2) (2), PCR conducted on DNA from each strain yielded a 390-bp DNA fragment, confirming similarity to A. avenae subsp. citrulli. Indirect enzyme-linked immunosorbent assay with genus-specific antibodies also verified that the bacteria were Acidovorax spp. Pathogenicity of the A. avenae subsp. citrulli strains was confirmed by inoculating and observing symptom development on 2-week-old watermelon seedlings. Although all strains were identified and confirmed as A. avenae subsp. citrulli, restriction fragment length polymorphism data indicated that the Tift County strain was distinguishable from the others, suggesting that inoculum for these outbreaks may have originated from at least two different sources.


References

1.  R. D. Gitaitis. 1993. Development of a seedborne assay for watermelon fruit botch. Pages 9-18 in: Proc. 1st Int. Seed Testing Assoc. Plant Dis. Commit., Ottawa, Canada.

2.  R. R. Walcott and R. D. Gitaitis. (Abstr.) Phytopathology 88(suppl.):S92, 1998.