Mammillaria humboldtii found in Mexico is a short-globose ornamental cactus species of the Cactaceae family, which has gained increasing popularity in China. It is characterized by tuberculate stems, dimorphic areoles, small pink flowers and pitted seed cell walls. The populations of wild M. humboldtii are critically endangered and are now of international conservation concern. In July 2021, stem rot symptoms were observed on M. humboldtii in a commercial greenhouse located in Zhangzhou (117°39'44.0064″E, 24°28'3.7236″N), Fujian Province (southern China). The typical symptoms were water-soaking, rotting and wilting on the stem, eventually leading to necrosis of the plants within 20 to 30 days. The vascular system of infected stems and roots showed a reddish-brown discolouration. The disease affected approximately 10% of 1000 plants. Fungi were isolated from the diseased stems of 26 samples, which were chopped into small pieces (5 × 5 mm), surface-sterilized with 75% ethanol for 40 s, and placed onto potato dextrose agar (PDA). After seven days of dark culture at 28°C, morphologically similar fungal isolates with whitish aerial mycelium and purple pigment were observed. On carnation leaf agar (CLA), isolates produced sickle and slightly curved macroconidia with three to four septa, measuring 12.8 to 27.9 × 1.9 to 3.8 μm (n = 15), and unicellular, ovoid to elliptical microconidia measuring 3.8 to 7.7 × 1.4 to 2.5 μm (n = 30). Smooth walled chlamydospores were terminal or intercalary, single or in pairs, measuring 9.2 to 13.1 μm (n = 15) in diameter. For molecular identification, the internal transcribed spacer (ITS) region of rDNA (Schoch et al. 2012), translation elongation factor-1α (EF1-α) (Maryani et al. 2019) and gene coding endopolygalacturonase 1 (PG1) (Hirano et al. 2006) of the representative isolate FJMH7 were amplified, purified and sequenced. BLASTn analysis of the ITS, EF1-α and pg1 sequences (GenBank accession numbers: ON832660, ON843495 and ON843496) showed 100%, 99.70% and 98.96% identity with F. oxysporum (GenBank accession numbers: KX611626, OM801797 and KF437345), respectively. Phylogenetic analysis based on the the concatenated ITS and EF1-α sequences and pg1 genes placed isolate FJMH7 with F. oxysporum reference strains in the phylogenetic trees. Based on morphological identification and sequence analysis, this isolate was identified as F. oxysporum. For the pathogenicity assay, six 6-month-old healthy plants of Mammillaria humboldtii were inoculated by dipping roots in a conidial suspension (106 conidia/mL) of isolate FJMH7 cultured in Bilai's medium for three days. Six noninoculated plants treated with Bilai's medium served as a control. Plants were transplanted into pots filled with sterilized soils and placed in a glasshouse at 25°C. After 15 days, all the inoculated plants exhibited rot symptoms on stems, which were similar to those observed in the commercial greenhouses. All inoculated plants were dead 30 days after inoculation. Control plants did not show any symptoms. F. oxysporum was reisolated and confirmed based on morphology and sequencing. No fungi were reisolated from control plants. To fulfil Koch's postulates, the pathogenicity assay was repeated twice with the same results. To date, F. oxysporum isolates have been reported on golden barrel cactus (Echinocactus grusonii) (Polizzi et al. 2004), night-blooming cereus (Hylocereus undatus) (Wright et al. 2007), apple cactus (Cereus peruvianus monstruosus) (Garibaldi et al. 2011), Schlumbergera truncate (Lops et al. 2013), Astrophytum ornatum (Quezada-Salinas et al. 2017) and Nopalea cochenillifera (Santiago et al. 2018). To our knowledge, this is the first report of F. oxysporum on M. humboldtii in China, indicating that this pathogen could cause wilt and rot disease on different cactus hosts.
Keywords: Causal Agent; Crop Type; Fungi; Fusarium oxysporum; Mammillaria humboldtii; Ornamentals; cactus; herbaceous/flowering plants; stem rot.