First Report of <i>Plectosphaerella cucumerina</i> Causing Stem Rot on Broccoli ( <i>Brassica oleracea</i> var. <i>italica</i> ) in China

Broccoli (Brassica oleracea var. italica) is a nutritious cruciferous vegetable, often celebrated as a superfood due to its unique bioactive compounds (Mohammed & Mohammed 2023). In November 2023, a stem rot disease was observed on broccoli in Wenzhou City, Zhejiang Province (28.05°N, 120.31°E), China. The pathogen primarily targeted the stems near the soil line and aboveground stems. The disease affected stems at the soil line and above, causing brown-black, sunken lesions that softened the tissue. Disease incidence was about 15% across five surveyed fields. To identify the pathogen, five broccoli plants exhibiting stem rot were randomly collected. To isolate the pathogen, symptomatic tissues from five plants were surface-sterilized and plated on potato dextrose agar (PDA). After 5 days at 28°C under dark conditions, eight fungal isolates with identical morphology were obtained using the single-spore method. Colonies were off-white to light yellow, with radiating wrinkles. The fungus produced thin, branched, septate mycelium and ellipsoid to oblong-oval conidia (1.8–3.3 × 5–10 μm; n=30), characteristic of Plectosphaerella sp. (Palm et al. 1995). For molecular identification, the representative isolate WZU-YT1 was used to amplify the ITS (White et al. 1990) and LSU (Vilgalys & Hester 1990) regions. The nucleotide sequences of the ITS and LSU regions from strain WZU-YT1 have been deposited in GenBank under accession numbers PV696910 and PV696909, respectively. The sequences (GenBank PV696910, PV696909) showed 100% identity to Plectosphaerella cucumerina strains NR171712 (ITS) and KY662258 (LSU). A phylogenetic tree confirmed its placement within the P. cucumerina complex. Pathogenicity assays were conducted on ten mature broccoli stems (cv. 'Youxiu'). Five stems were inoculated with the isolate WZU-YT1 and five controls with sterile PDA. After disinfecting the surface with 75% ethanol and wiping it with sterile water, sterile puncture wounds (3 mm deep) were made at the stem-floret junction using an inoculation needle. Five mm diameter mycelial plugs from the 7-day-old fungal cultures, were placed on the wounds, while sterile PDA plugs were used as the control. The stems were then sealed in moist, airtight bags to retain moisture at a temperature of 28°C, without exposure to light (Chen et al. 2023). After 5 days, all stems inoculated with the WZU-YT1 isolate exhibited symptoms identical to those observed in the field, while no symptoms were present in the control group. The pathogenicity test was repeated three times. The fungus reisolated from lesions was confirmed as P. cucumerina, thereby fulfilling Koch's postulates. This is the first report of P.cucumerina causing stem rot on broccoli in China. This disease poses a threat to production, and our findings provide a basis for developing management strategies.