First Report of Flower Blight Caused by <i>Pantoea stewartii</i> subsp. <i>indologenes</i> on <i>Vanda</i> Orchid in Taiwan

Vanda (Orchidaceae) orchids are predominantly native to tropical and subtropical Asia (Khan et al. 2018). Many of them are commercially cultivated as potted plants and cut flowers (Thammasiri 2016). In July 2020 and June 2024, growers in Huatan and Erhsui townships, Changhua, Taiwan, observed necrosis and blight symptoms on the flowers (petals, sepals, and buds) of Vanda ‘Pachara Delight’, with incidences reaching up to 50%. The lesions were brown, water-soaked, and progressed to complete necrosis of the affected tissues. Symptomatic tissues were collected, cut into 3x3 mm pieces, surface-disinfected with 75% (v/v) ethanol, and homogenized in 1 ml of water. The suspension was streaked onto nutrient agar plates. After incubation at 28 °C for 2 days, yellow, circular colonies were consistently isolated. Strains VB1 and Vanda7 isolated from petals and buds in 2020 and 2024 were selected for DNA analyses. The 16S rDNAs were amplified using primer pair 27f/1492r (Lane 1991) and the Platinum Taq DNA Polymerase High Fidelity kit (Invitrogen). The amplicons were cloned into pGEM-T Easy (Promega) and sequenced. The sequences of VB1 and Vanda7 (GenBank accessions PX363153 and PX363152) shared 100% and 99.9% similarity with that of P. stewartii subsp. indologenes RON1871 (accession no. CP116285; 1,464 bp). Multilocus sequences of atpD, gyrB, infB, and rpoB (accessions PX360306-PX360313) were concatenated into 2,260-bp fragments for maximum-likelihood analysis using different Pantoea species as reference strains (Koirala 2021). Both VB1 and Vanda7 clustered with P. stewartii subsp. indologenes. Biochemical assays showed that the two tested strains produced indole and degraded esculin, which are typical traits of P. stewartii subsp. indologenes (Koirala 2021). Pathogenicity of VB1 and Vanda7 was confirmed by puncturing flower buds of six-year-old Vanda ‘Pachara Delight’ plants using 10-μL pipette tips loaded with 10 μL of bacterial suspension (10⁸ CFU/mL; in sterile distilled water) and leaving the tips on the buds. For each strain, three plants with at least three flower buds were inoculated. Control plants were inoculated with 10 μL of sterile distilled water. All tested plants were bagged and maintained in a greenhouse at 32 °C and 85-90% relative humidity. After 7 days, necrotic lesions resembling field symptoms were observed on all inoculated buds, but not on the controls. The bacteria were re-isolated and confirmed by infB sequencing to be 100% identical with the original strains, thereby fulfilling Koch’s Postulates. Based on the results, we identified P. stewartii subsp. indologenes as the pathogen causing flower blight disease on Vanda orchids in Taiwan. This is the first report of this bacterium infecting Vanda species in Taiwan. Due to its impact on the host plant’s ornamental value, the pathogen could pose a significant threat to the Vanda orchid industry if it spreads. Infected nurseries were quarantined and treated with alternating tetracycline and oxolinic acid sprays as emergency treatments. Flower buds collected at 3- and 6-month post-treatment were confirmed to be pathogen-free. Consequently, Taiwan remains a pest-free area for P. stewartii subsp. indologenes.