First Report of Powdery mildew caused by Golovinomyces ambrosiae on Bidens pilosa in Guangdong Province, China

Bidens pilosa L., a widely distributed annual herbaceous plant, is known for its medicinal properties. In January 2021, an outbreak of powdery mildew was observed on B. pilosa growing along roadsides in Shenzhen, Guangdong Province, China, with disease incidence ranging from 60% to 80%. Initial symptoms included small, irregular white powdery patches, primarily on the upper leaf surfaces. These colonies expanded, coalescing to cover leaves, petioles, and stems, ultimately causing leaf distortion and senescence.
Microscopic examination revealed hyaline, septate hyphae with thin walls, measuring 2–8 μm in width. Hyphal appressoria were nipple-shaped. Conidophores were erect or slightly flexuous, 80–150 μm long and 12–18 μm wide (n = 30), typically bearing chains of 2–5 immature conidia with a sinuate outline. Foot cells were cylindrical, erect, and measured 33–100 μm in length and 6–10 μm in width (n = 30). Conidia were hyaline, ellipsoid-ovoid to barrel-shaped, and lacked fibrosin bodies. Primary conidia were ellipsoid-ovoid with a rounded apex and subtruncate base (25–40 × 15–22 μm), while secondary conidia were barrel-shaped with truncate or subtruncate ends (27–35 × 15–20 μm). Germ tubes exhibited a longitubus pattern, originating prominently from the perihilar or apical region. No chasmothecia were observed.
To identify the pathogen at the molecular level, mycelium and conidia were collected from infected leaves for genomic DNA extraction. The internal transcribed spacer (ITS), intergenic spacer (IGS), and beta-tubulin (tub2) regions were amplified using primer pairs ITS1/ITS4, IGS-12a/NS1R, and tub2, respectively. The resulting sequences—568-bp ITS, 366-bp IGS, and 354-bp tub2—were deposited in GenBank (accession numbers OR647592, OR978282, OR978283). BLAST analysis revealed over 99.6% ITS sequence A-366 similarity to Golovinomyces ambrosiae (MT929773) and G. cichoracearum (MH590731). The IGS and tub2 sequences showed 100% similarity to G. ambrosiae (MK383490, OK349420, MW981257, MW981256). Phylogenetic analysis grouped the sequences within the G. ambrosiae complex.
Based on morphological and molecular analyses, the pathogen was identified as G. ambrosiae. To fulfill Koch’s postulates, infected leaves were pressed onto healthy B. pilosa plants (n = 6), with a control group of non-inoculated plants (n = 6). All plants were maintained in a greenhouse at 25°C, with a 14/10-h light/dark cycle and 50% relative humidity. Powdery mildew symptoms appeared on inoculated plants within 10 days, progressing to discoloration and premature leaf drop by day 16. The control group remained symptom-free. Pathogen identity was confirmed via microscopic observation and sequencing of the reisolated fungus.
Although G. ambrosiae has been reported on B. pilosa in Fuzhou, Fujian Province, this study is the first to document powdery mildew caused by G. ambrosiae on B. pilosa in Shenzhen, Guangdong Province, China.