First Report of a New Leaf Spot Disease Caused by a Xanthomonas sp. on Hamelia patens

Abstract

HomePlant DiseaseVol. 103, No. 2First Report of a New Leaf Spot Disease Caused by a Xanthomonas sp. on Hamelia patens PreviousNext DISEASE NOTES OPENOpen Access licenseFirst Report of a New Leaf Spot Disease Caused by a Xanthomonas sp. on Hamelia patensC. Conner, C. Marble, A. Khan, and D. J. NormanC. ConnerSearch for more papers by this author, C. MarbleSearch for more papers by this author, A. KhanSearch for more papers by this author, and D. J. Norman†Corresponding author: D. J. Norman; E-mail: E-mail Address: djn@ufl.eduhttp://orcid.org/0000-0002-1373-9416Search for more papers by this authorAffiliationsAuthors and Affiliations C. Conner C. Marble A. Khan D. J. Norman † , UF-IFAS, Mid-Florida Research and Education Center, Apopka, 32703. Published Online:13 Dec 2018https://doi.org/10.1094/PDIS-01-18-0122-PDNAboutSections ToolsAdd to favoritesDownload CitationsTrack Citations ShareShare onFacebookTwitterLinked InRedditEmailWechat Lime Sizzler firebush (Hamelia patens Grelmsiz) is an ornamental landscape shrub that is cultivated in USDA hardiness zones 8 to 11 (Polanco 2015). It is popular because of its lime-colored foliage and its red-orange flowers. In the summer of 2016, dark necrotic leaf spots were observed on Lime Sizzler plants being produced in Florida. Isolations on nutrient agar from symptomatic tissue consistently yielded smooth, yellow, butyrous colonies of gram negative bacteria. Two strains of the bacterium, X1963 and X1964, were selected for further identification and characterization using fatty acid methyl esters analyses with the MIDI identification system (Sherlock version 6.2B, Microbial ID, Newark, DE), partial 16S rDNA, and gyrB gene sequencing. Using MIDI, the two strains had similarity indexes of 0.870 and 0.812 for Xanthomonas axonopodis pv. citrumelo and X. axonopodis pv. manihotis, respectively. For partial 16S rRNA and gyrB sequencing, we used primers and amplification protocols recommended by Ah-You et al. (2009). For both strains, sequences of 16S rRNA were identical. Similarly, sequences of gyrB gene were also identical for both strains. When using 16S rRNA and gyrB sequences in the National Center of Biotechnology Information (NCBI) BLAST of the nucleotide collection/nonredundant (nt/nr) database, 99% identity with a number of Xanthomonas spp. was found. The 16S rRNA and gyrB sequences were deposited in the NCBI nucleotide database under accession numbers MH591441 and MH681992, respectively. To satisfy Koch’s postulates, the two strains were inoculated on 10 plants each of three Hamelia species: H. patens Grelmsiz (Lime Sizzler firebush), nonnative H. cuprea, dwarf firebush, and native H. patens var. patens (Ahmad et al. 2012). Plants were approximately 15 cm in height with 15 to 25 leaves per plant. Inoculations were done in a greenhouse with temperatures between 21 and 32°C and light levels between 9,688 and 12,917 lm/m2. Suspensions of each strain were sprayed on plants to run-off at a concentration of 1 × 108 CFU/ml. Plants were bagged for 24 h to raise humidity to aid infection. Within 2 weeks, leaf spots developed on all three Hamelia varieties. The same characteristic leaf spots yielded the same type of colony as the one originally inoculated and the identical 16S rRNA sequence. Inoculation experiments were repeated with the same results. To our knowledge, this is the first report of bacterial leaf spot on H. patens caused by Xanthomonas sp.