K. N. Conner, Alabama Cooperative Extension System, Auburn University, AL 36849;
J. Olive, Alabama Agricultural Experiment Station, Auburn University, Mobile, AL 36689;
L. Zhang, Department of Entomology and Plant Pathology, Auburn University, AL 36849;
J. Jacobi, Alabama Cooperative Extension System, Auburn University, Birmingham, AL 35223; and
M. L. Putnam, Botany and Plant Pathology, Oregon State University, Corvallis 97331
Bacterial gall symptoms were observed on Loropetalum chinense (R. Br.) Oliv. in two separate commercial nurseries in South Alabama during the spring of 2012. Limb dieback and plant death was first reported by the growers. Plants with dieback symptoms had galling and irregular dark callus formation on the lower stem and lower branches. Galls were small, 0.2 to 1 cm, inconspicuous, and in some cases girdled the stem causing breakage of the main stem. In both locations, 30 to 40% of the crop was affected. Similar symptoms have been observed on L. chinense in nursery and landscape plantings in central Alabama, North Carolina, and Georgia in previous years. Bacterial colonies were isolated from four plants representing two different locations. Isolates were recovered from surface sterilized symptomatic tissue on nutrient agar and King's medium B (KMB). All isolates were gram-negative and fluoresced blue-green under UV light after 48 h of growth at 28°C on KMB. One representative isolate from each site was identified as Pseudomonas savastanoi based on their fatty acid profiles (similarity index of 0.776; MIS-TSBA, version 4.0, MIDI Inc., Newark, DE) and LOPAT tests (2). The identity was confirmed by sequencing a 900-bp portion of the 16S rDNA gene, which revealed 98% similarity to the P. savastanoi type strain in NCBI (Accession No. AB021402). In greenhouse pathogenicity tests, eight Loropetalum liners were inoculated with a bacterial suspension (107 CFU/ml) of each of the two isolates. Plants were inoculated by injecting the suspension into the lower stem after wounding by puncturing with needles or slicing sections of the bark. Controls were inoculated with water. All plants inoculated with the bacteria developed gall symptoms in 8 weeks under 90% relative humidity at 30°C. The bacteria were reisolated from five inoculated plants. DNA was extracted from each isolate, amplified using primer pair 27F/1492R targeting the 16S rDNA gene (1), and sequenced. Sequences (900 bp) from all isolates shared 98 to 99% similarity to P. savastanoi type strain in GenBank (Accession No. AB021402). Nucleotide sequence data reported are available in GenBank under accessions JX915832 to 37. To our knowledge, this is the first report of bacterial gall of L. chinense caused by P. savastanoi in the United States. Given the increasing prevalence of this disease in South Alabama, its confirmation is a significant step toward management recommendations for growers.
References: (1) D. J. Lane. 16S/23S rRNA sequencing. Page 115-175 in: Nucleic Acid Techniques in Bacterial Systematics. E. Stackebrandt and M. Goodfellow, eds. John Wiley and Sons, New York, 1991. (2) N. W. Schaad et al. Laboratory Guide for Identification of Plant Pathogenic Bacteria. 3rd ed. The American Phytopathological Society, St. Paul, MN, 2001.