First Report of Dickeya chrysanthemi Causing Soft Rot on Euphorbia milii in Texas

Euphorbia milii, is a popular ornamental succulent valued for its attractive flowers and use as a landscape plant. Samples of E. milii ‘Maxi Pink Cadillac’ showing wilting in leaves and water-soaked symptoms on stems and leaves were collected in September 2025 from a nursery in Cameron County, Texas, with approximately 10% plant loss. Samples were submitted to the Plant Disease and Insect Diagnostic Laboratory, Oklahoma State University for diagnosis. Symptomatic stems and petioles were washed, cut into small pieces (~ 5 × 5 mm), surface-disinfested with 10% Clorox (0.6% sodium hypochlorite active ingredient) for 1 min, and rinsed three times with sterile water. The stem or petiole tissue was placed in a 2 ml screw cap tube with 2 (3.5 mm) and 5 (2.7 mm) glass beads and macerated using a mini-bead beater (Biospec Products, Bartlesville, OK) for 20 s at 4200 rpm, followed by addition of 250 µl of sterile water and brief vertexing. The suspension was streaked onto nutrient agar (NA) and casamino acid–peptone–glucose (CPG) medium. Bacterial colonies recovered were purified on NA, and strains PL516 and PL517, were selected for further characterization. The strains were streaked onto crystal violet pectate (CVP) agar, where they produced pits within 24 h (Dobhal et al., 2020; Helias et al., 2012). Genomic DNA from PL516 and PL517 were extracted using the DNeasy Blood and Tissue Kit (Qiagen, Germantown, MA), following the manufacturer’s instructions. The housekeeping genes dnaA, gapA, and gyrB were amplified and sequenced for identification and phylogenetic analysis using the primer sets and PCR conditions (Boluk and Arif, 2019; Boluk et al., 2020). Each PCR reaction consisted of 10 µl of GoTaq Green Master Mix (Promega), 1 µl of each forward and reverse primer (5 µM), 1 µl of template DNA, and 7 µl of distilled water. PCR product cleanup, and sequencing were performed as described by Dobhal et al. (2020). Error-free consensus sequences were generated and used for BLASTn analysis. The concatenated sequences of the three housekeeping genes showed 100% query coverage and 99.4% identity with Dickeya chrysanthemi. The sequences were submitted to the NCBI GenBank database under the accession numbers PZ481109-PZ481110 (dnaA), PZ481105-PZ481106 (gapA) and PZ481107-PZ481108 (gyrB) for strain PL516 and PL517, respectively. The pathogenicity assay was conducted on 4-weeks-old E. milii ‘Maxi Pink Cadillac’ plants grown in plastic pots under controlled growth chamber conditions. Plants were inoculated at nodes with 10 µl of bacterial suspension (OD600 = 1; ∼ 10⁹ CFU/ml) of strains PL516 and PL517; control plants were inoculated with 10 µl of sterile water. Each treatment included three replicates. Plants were maintained in a growth chamber under controlled temperature and humidity. Typical symptoms, including wilting and soft rot of the stem, were observed within 24-48 h post-inoculation on plants, while no symptoms developed on the control plants. To fulfill Koch’s postulates, bacteria were re-isolated from symptomatic tissues, streaked onto CVP plates. Colonies forming pits after 24 h were selected and streaked onto NA for isolation and confirmed as D. chrysanthemi by sequence analysis, showing 100% identity to the original isolates. Symptom development, molecular identification, and pathogenicity assay, confirmed D. chrysanthemi as the causal agent. To our knowledge, this is the first report of bacterial soft rot caused by D. chrysanthemi on E. milii in Texas.