During the winter of 2013, 50% of 20,000 plants of Cereus marginatus var. cristata, Cactaceae family, grown in a commercial farm located in Liguria (northern Italy) showed symptoms of a dry or soft rot. In the case of dry rot, affected plants showed on the stem superficial necrosis and dry rot, irregularly shaped, 1 to 10 mm, while epidermal and cortical tissues were wounded. Affected plants survived but they lost ornamental value. In the case of soft rot, associated with conditions of higher relative humidity, rots on the stem extended as far as 4 cm in width. The internal part of bark, cambium, and xylem tissues as far as about 3 cm in depth was rotted. Vascular tissues were not discolored. Plants died in about 20 days. A Fusarium sp. was consistently isolated from symptomatic tissue on Komada selective medium (2) from plants showing soft rot. The isolates were purified and subcultured on potato dextrose agar (PDA). On PDA, the cultures produced a thick and soft growth of white to light pink mycelium and pale pink pigments in the agar. On Spezieller Nährstoffarmer agar (SNA), cultures produced short monophialides with unicellular, ovoid-elliptical microconidia measuring 3.7 to 8.2 × 1.7 to 3.5 (average 5.4 × 2.5) μm. On carnation leaf-piece agar (CLA), chlamydospores were abundant, terminal or intercalary, single or paired, but frequently also aggregated. On the same medium, at temperatures ranging from 20 to 24°C (14 h daylight, 10 h dark), cultures produced light orange sporodochia with macroconidia. These were 3 to 4 (sometimes 5) septate, nearly straight with a foot-shaped basal cell and a short apical cell, and measured 28.5 to 41.4 × 3.3 to 4.9 (average 35.0 × 4.0) μm. Such characteristics are typical of Fusarium oxysporum Schlechtendahl emend. Snyder & Hansen (3). Amplification of the internal transcribed spacer (ITS) of the rDNA using primers ITS1/ITS4 yielded a 504-bp amplicon (GenBank Accession No. KJ909935). Sequencing and BLASTn analysis of this amplicon showed a 100% homology with the sequence of F. oxysporum KC304802. To confirm pathogenicity, two Fusarium isolates were tested. For each isolate, three 2-year-old healthy plants of C. marginatus were inoculated by introducing into lesions (4 lesions/plant) artificially produced on the stem sterile needles contaminated with the pathogen (4). Inoculum was obtained from pure cultures grown on PDA. Control plants were punctured with sterile needles without inoculum. All the plants were placed in a greenhouse, at temperatures ranging between 16 and 24°C. For both tested strains, the first necrosis of stem tissues developed around the needles 7 days after the artificial inoculation, while non-inoculated plants remained healthy. Then, necrosis extended causing soft rot on plants maintained at relative humidity ranging from 55 to 65%. F. oxysporum identified by morphological characteristics was consistently isolated from symptomatic plants. The pathogenicity test was conducted twice. F. oxysporum has been reported on Cereus sp. in the United States and on C. peruvianus monstruosus in Italy (1). Currently, this disease is present in a few commercial nurseries in Liguria, although it could spread further and cause important economic losses.
References: (1) A. Garibaldi et al. Plant Dis. 95:877, 2011. (2) H. Komada. Rev. Plant Prot. Res. 8:114, 1975. (3) J. F. Leslie and B. A. Summerell. The Fusarium Laboratory Manual. Blackwell, Ames, IA, 2006. (4) V. Talgø and A. Stensvand. OEPP/EPPO Bulletin 43:276, 2013.
