Crape myrtle (Lagerstroemia sp.) is a popular ornamental tree in the United States and the industry produced 2,781,089 trees in 2010 with a value exceeding US $42.8 million (1,4). A new disorder of crape myrtle has been observed since 2011 in numerous nurseries in Florida, which was characterized by dark brown, angular to irregularly shaped, oily-looking spots surrounded by yellow halos. The disease primarily affects lower leaves that eventually turn yellow and can lead to rapid defoliation of susceptible cultivars. Plants examined in field surveys at the University of Florida, North Florida Research and Education Center, Quincy, FL in 2012 and 2013 also had similar symptoms on cvs. Arapaho, Carolina Beauty, Tuscarora, Whit IV Red Rocket, Whit VIII Rhapsody in Pink, and White Chocolate. The disease severity ranged from 20 to 70% and all the plants were infected. A yellow-pigmented, gram-negative, oxidase negative bacterium was consistently isolated from symptomatic leaves (two leaves from each of five plants). Pathogenicity tests were performed using five isolated bacterial strains on potted crape myrtle cv. Arapaho. Three plants were inoculated with a 108 CFU/ml suspension of bacterial strains in sterile deionized water, and covered with transparent plastic bag for 48 h. Two control plants were inoculated with sterile distilled water. The inoculated plants were then incubated in a greenhouse at 30 to 34°C for 14 days. Symptoms of dark brown, angular to irregularly shaped lesions were observed only on the inoculated plants after 7 days. The bacterium was re-isolated from the inoculated symptomatic plants as described above, thus fulfilling Koch's postulates. Fatty acid methyl ester profiling of the five isolated bacteria using GC-MIDI (Microbial IDentification Inc, Newark, DE) revealed the identity of the bacterium as Xanthomonas axonopodis with an identity index of ~0.80, but matched multiple pathovars. Total genomic DNA was extracted from the pure bacterial culture using UltraClean Microbial DNA Isolation Kit (MO BIO Laboratories, Carlsbad, CA). The genomic DNA was subjected to PCR assay using universal primers 27f/1492R (3) targeting the complete 16S rRNA gene and primers 16F945/23R458 (2), which target the partial 16S-23S internal transcribed spacer region. PCR amplification using primer pairs 27f/1492R and 16F945/23R458 resulted in amplicons of 1,450 and 1,500 bp, respectively. The amplicons were gel purified and sequenced directly at Florida State University. BLAST analysis of the sequences (Accession Nos. KF926678, KF926679, KF926680, KF926681, and KF926682) revealed the identity of the bacterium as X. axonopodis, ranging from 98 to 99%, with several strains in the NCBI database. Phylogenetic analysis using the neighbor-joining method showed that our strains were distantly clustered with X. axonopodis pv. dieffenbachiae when compared to other available strains in the database. To our knowledge, this is the first worldwide report of a bacterial leaf spot on crape myrtle caused by X. axonopodis. This information should aid in the development of breeding lines with resistance to bacterial leaf spot and effective disease management practices.
References: (1) C. S. Furtado. Garden Bull. 24:185, 1969. (2) C. Guasp. Int. J. Syst. Evol. Microbiol. 50:1629, 2000. (3) D. J. Lane. Page 115 in: Nucleic Acid Techniques in Bacterial Systematics, 1991. (4) USDA. 2007 Census of Agriculture, Washington, DC. 3:25, 2010.
