First Report of Stem Rot Caused by Fusarium oxysporum f. sp. opuntiarum on Sulcorebutia heliosa in Italy

Sulcorebutia heliosa, Cactaceae family, is a succulent plant used as ornamental. During June 2018, several 2-year-old plants of S. heliosa, grown in a nursery located in Ventimiglia (43°47′48.648″N, 7°35′34.616″E, Imperia province, northern Italy), showed symptoms of a stem rot. Stems wilted and collapsed. Internal tissues rotted, and eventually affected plants dried and died. About 10% of 1,000 plants were affected. Small fragments (3 mm) were taken from the margins of affected collar and stem tissues and plated onto potato dextrose agar. Colonies of a fungus were isolated that produced whitish to pale purple colonies and pale purple pigments in the medium. On carnation leaf agar medium, colonies produced unicellular, oval to elliptical microconidia supported by short monophialides. Conidia measured 4.3 to 8.2 × 1.7 to 3.4 µm (average, 5.9 × 2.6 µm) (n = 50). On the same medium, colonies produced slightly falcate macroconidia, not formed in sporodochia. Macroconidia were slightly falcate, with a foot-shaped basal cell, a short apical cell, and three (sometimes four) septa. They measured 22.6 to 41.6 × 3.4 to 4.5 µm (average, 31.0 × 3.8 µm) (n = 50). Chlamydospores were smooth walled, terminal or intercalary, in singles or in pairs, and measured 4.8 to 8.6 µm (average, 6.9 µm) in diameter (n = 50). These morphological characteristics are typical of Fusarium oxysporum (Leslie and Summerell 2006). A pure culture of the isolate DB18GIU36 was used to extract DNA, using the E.Z.N.A. Fungal DNA Mini Kit (Omega Bio-Tek, Darmstadt, Germany). The elongation factor 1α gene (EF1α) was amplified using primers EF1/EF2 (O’Donnell et al. 1998) and sequenced (GenBank accession no. MK050503). BLASTn analysis of the 677-bp segment showed a 99% similarity with the sequence KY379852 of F. oxysporum. Successively, the intergenic spacer (IGS) was amplified using primers CNS1/CNL12 (Appel and Gordon 1995) and sequenced (GenBank accession no. MK050504). BLASTn analysis of the 1,202-bp segment showed 100% similarity with the sequence FJ985530 of F. oxysporum f. sp. opuntiarum. Pathogenicity of one isolate was tested on three 12-month-old healthy plants of S. heliosa by dipping roots in a 4.3 × 107 CFU/ml conidial suspension, grown on potato dextrose broth. The roots of three control plants were dipped in sterile water. Successively, all plants were transplanted into pots filled with steamed substrate and maintained at 25 to 35°C. After about 30 days, the first wilt symptoms appeared on inoculated plants, from which F. oxysporum f. sp. opuntiarum was reisolated (78% reisolation) and identified by the amplification of the EF1α and IGS gene portions (GenBank accession nos. MK910767 and MK910768, respectively). As the disease progressed, stems rotted and plants died. Controls remained symptomless, and attempts to reisolate the pathogen from these failed. This is the first report of F. oxysporum f. sp. opuntiarum on S. heliosa in Italy, as well as worldwide. Although in Italy the production of S. heliosa is still limited, many other succulent plants belonging to Cactaceae, hosting F. oxysporum f. sp. opuntiarum, such as Schlumbergera truncata (Lops et al. 2013), are extensively grown, with a risk of severe losses.