First Report of Crown and Root Rot Caused by Fusarium oxysporum on Sweet Pepper (Capsicum annuum) in Italy

Abstract

During August 2018, sweet pepper (Capsicum annuum L.) cv. Cuneo, grown in 3,000 m² of commercial greenhouses near Cuneo in Piedmont, northern Italy, exhibited symptoms of reduced growth and sudden wilting. Crown and roots were severely affected, showing a brown necrosis and slight rot. No internal discoloration of vascular tissues was observed. Eventually, affected plants collapsed. Symptoms developed on approximately 10% of the plants 3 months after transplanting. Tissue fragments of 1 mm² were excised from the crown and roots of 10 symptomatic plants, dipped in a 1% sodium hypochlorite solution, washed in sterile water, and placed on potato dextrose agar. Fungal colonies were isolated from all samples incubated under constant fluorescent light at 22 ± 1°C for 6 days. Thirty-day-old single-spore cultures of four selected isolates (FOC1, FOC2, FOC3, and FOC4) grown on carnation leaf agar generated short monophialides with unicellular, ovoid-elliptical microconidia, measuring 5.6 to 13.5 × 2.5 to 3.5 µm. Sparse macroconidia were 3-septate, slightly curved, with a foot-shaped basal cell and a short apical cell, and measured 19.4 to 38.3 × 2.9 to 4.5 µm. Chlamydospores were terminal and intercalary, single, in pairs, clusters, or in chains, and measured 6.7 to 10.6 µm in diameter. Such characteristics are typical of Fusarium oxysporum (Leslie and Summerell 2006). DNA was extracted from the four isolates and molecular identification was carried out by sequencing the portions of the elongation factor-1 alpha (TEF-1α) gene (O’Donnell et al. 1998) and the intergenic spacer (IGS) region (GenBank accession nos. MK061116–MK061119 for TEF-1α and MK061120–MK061123 for IGS). A BLASTn search showed that the TEF-1α sequences (625 bp) were 99 to 100% similar to the corresponding sequences HG916993–HG916995 of F. oxysporum f. sp. radicis-capsici from sweet pepper (Lomas-Cano et al. 2014), while the IGS sequences (1187–1274 bp) were 99% similar to that of F. oxysporum (FJ985621). The four isolates were used for pathogenicity tests conducted in 2-liter pots filled with steamed peat substrate in a greenhouse at 25 to 28°C. Twenty-day-old sweet pepper plants (cv. Cuneo) at the three true leaf stage were inoculated by dipping roots in a conidial suspension (1 × 10⁶ conidia/ml) obtained from a 10-day-culture in potato dextrose broth. Twenty plants (5 plants/pot) per treatment were used in two repeated trials for each isolate, and 20 noninoculated plants were used as controls. First wilt symptoms developed 8 to 10 days after inoculation, and 70 to 100% of inoculated plants were dead 15 to 20 days after inoculation, showing crown and root rot. Tomato plants (Solanum lycopersicum) cv. Cuore di bue, inoculated with the four isolates under the same conditions, did not develop symptoms after 50 to 60 days. All noninoculated control plants remained healthy. A fungus morphologically identified as F. oxysporum was consistently reisolated from all the symptomatic pepper plants and confirmed by the TEF-1α and the IGS analysis. To our knowledge, this is the first report of F. oxysporum on sweet pepper in Italy. Although F. oxysporum f. sp. capsici is reported on sweet pepper (Pernezny et al. 2003), symptoms observed in this study were very similar to those caused by F. oxysporum f. sp. radicis-capsici described by Lomas-Cano et al. (2014, 2016) on sweet pepper seedlings in Spain. The disease, although limited at present in the greenhouses surveyed in northern Italy, could expand to other areas where sweet pepper is grown intensively.