First Report of Xanthomonas dyei Causing Bacterial Blight on Glossy Abelia

Glossy abelia (Abelia × grandiflora) is an evergreen ornamental shrub used in landscaping globally. From Jun. 2023 to Feb. 2024, bacterial blight symptoms were observed in > 50% of ~20,000 glossy abelia plants (cv. Kaleidoscope, Radiance, and Rose Creek) in an ornamental nursery in North Carolina, USA. Symptoms consisted of small, irregular, water-soaked spots that became brown and coalesced, with defoliation from 20 to 80%. Symptomatic samples were sent to the Purdue University Plant and Pest Diagnostic Laboratory for preliminary identification. A symptomatic leaf was rinsed 3× in sterile deionized (DI) water, cut into 1 mm wide strips, placed in 5 mL of sterile DI water for 5 min, from which a 10 µL loop was streaked onto KB (King's medium B). Individual yellow predominant colonies were transferred to KB after 48 h at 25°C, and the 16s rRNA was sequenced with the 27F/1492R primers (Lane 1991; GenBank accession PP734871). To confirm, three symptomatic plants (cv. Radiance) were sent for isolation to Clemson University in Feb. 2024. One symptomatic leaf per plant was surface sterilized with 70% ethanol. The toothpick method (Noh et al. 2024) was used to extract bacteria into sterile distilled water; suspensions were streaked on nutrient agar plates. Predominant colonies after 48 h incubation at 28°C were convex, mucoid, and yellow. Three isolates (Abs1, Abs2, and Abs3), one per plant, were purified from single colonies; all produced positive results using the Xanthomonas ImmunoStrip® (Agdia) and hypersensitive response 24 h after being infiltrated (~108 CFU/ml) into Nicotiana benthamiana leaves. PCR amplification and sequencing of the gyrB, rpoD, dnaK, and fyuA genes was done for all three isolates using the bacterial suspension as DNA template with the primers and conditions described in Young et al. (2008). The sequences of each gene were 100% identical among isolates and those from Abs1 were deposited in GenBank (accession # PP739553-PP739556). The gene sequences most closely matched Xanthomonas dyei, with 98.60% (848/860 nt), 99.65% (866/869 nt), 99.44% (884/889 nt), and 99.42% (681/685 bp) identity with the gyrB (GQ183098), rpoD (GQ183084), dnaK (GQ183125), and fyuA (GQ183111) sequences, respectively, of the X. dyei type strain ICMP 12167. A neighbor joining tree using the concatenated sequences of these four genes showed Abs1 clustered with X. dyei. To fulfill Koch's postulates, three branches of three healthy ~1-year-old abelia plants were spray inoculated with Abs1 suspension in sterile tap water (~5×108 CFU/ml). An abelia plant sprayed with sterile tap water was the control. The experiment was conducted three times, one cultivar per time (cv. Kaleidoscope, Radiance, and Rose Creek). All plants were maintained in a growth chamber at 21°C with 12 h photoperiod. Leaf chlorosis, significant defoliation, and stem blight symptoms were observed on inoculated branches within 7 days after inoculation with similar disease severity among cultivars. Symptoms were not observed on control plants. Due to excessive defoliation, bacteria were reisolated from blighted stem tissue of inoculated branches and confirmed to be the same isolate via PCR and sequencing as described above. To the best of our knowledge, this is the first report of X. dyei causing disease on Abelia x grandiflora worldwide. Given the severe disease symptoms on this high value crop, more research is needed to determine the inoculum source, disease distribution, and to develop management strategies.