First Report of Damping‐off Caused by Phytopythium helicoides on Cabbage

In July 2025, damping-off symptoms were observed in 2-week-old cabbage (Brassica oleracea var. capitata) seedlings that had been directly sown in a vegetable field in Osaka, Japan (Figure 1a). In affected plants, stems were thin, brownish and softened. Thirteen out of 180 cabbage seedlings exhibited these symptoms. Oogonium-like structures were observed in diseased tissues (Figure 1b). Four oomycete isolates were obtained from diseased tissues by incubation on oomycete-selective NARM agar (nystatin, ampicillin, rifampicin and miconazole) (Morita and Tojo 2007) at 31°C for 1 day. The isolates were subsequently cultured using the grass blade method (van der Plaats-Niterink 1981). Hyphal growth responses to temperature were examined for one-two days on potato-carrot agar following the method of van der Plaats-Niterink (1981). Because all four isolates exhibited identical morphological characteristics and temperature-dependent hyphal growth responses, a representative isolate, OMU041, was selected for species identification and pathogenicity testing. Morphological and cultural characteristics of strain OMU041 were consistent with those of Phytopythium helicoides strain CBS286.31 (Figures 2 and 3, Table 1; van der Plaats-Niterink 1981). The mitochondrial cytochrome c oxidase subunit I (COX1) and subunit II (COX2) regions were amplified and sequenced using the primer pairs OomCoxI-Levup / CoxI-Fm85mod (Robideau et al. 2011) and Cox2-F (Hudspeth et al. 2000) / Cox2-RC4 (Choi et al. 2015), respectively. The COX1 and COX2 sequences of strain OMU041 shared 99.55% and 99.29% sequence identity, respectively, with those of P. helicoides strain CBS286.31 (Robideau et al. 2011). The COX1 and COX2 sequences of strain OMU041 were deposited in GenBank under Accession Nos. LC903162 and LC903163, respectively. The living culture was deposited in the Genebank Project (NARO, Tsukuba, Japan) under Accession No. MAFF 248271. For the pathogenicity test, cabbage seeds were sown in commercial nursery soil (Tanemaki Baido; Takii Co. Ltd., Japan) in 9-cm pots and maintained in growth chambers at 20°C or 30°C for 5 days. Each treatment consisted of two pots, with five seeds per pot. The inoculum was prepared by culturing strain OMU 041 on bentgrass seed agar at 31°C for 7 days (Kawasumi et al. 2022). For inoculation, 10 mL of inoculum was applied to the basal portion of each plant. Plants were then maintained under two temperature regimes (constant 20°C or 30°C) with a 12-h photoperiod (115–120 µmol m−2 s−1). The experiment was repeated three times. Two weeks after inoculation, damping-off symptoms were observed in 11% and 80% of the seedlings at 20°C and 30°C, respectively (Figure 4a,b). Oospores with and without antheridia were observed in infected tissues (Figure 4c,d). Phytopythium helicoides was re-isolated from symptomatic stems, thereby fulfilling Koch's postulates. Uninoculated control plants remained asymptomatic, and P. helicoides was not isolated. Because P. helicoides has not previously been reported on cabbage worldwide, this study represents the first report of damping-off of cabbage caused by this species. Disease severity was greater at 30°C than at 20°C. In the study field, the mean air temperature during the week preceding disease occurrence was 28.7°C (minimum 23.4°C and maximum 34.8°C). This high temperature, which falls within the optimal range for disease development by P. helicoides, may have promoted the occurrence of the disease.