First Report of Fusarium oxysporum Causing Wilt on Astrophytum myriostigma in Italy

Astrophytum myriostigma is a succulent plant in the family Cactaceae that is commercialized in Italy as a potted plant. During fall 2014 in a commercial farm close to Ventimiglia (Liguria region, northern Italy), approximately 50% of 4,000 30-month-old plants of A. myriostigma showed symptoms of a previously unknown wilt. Symptoms consisted of chlorosis and wilting of stems. At the base of stems, a whitish, light-orange mycelium appeared in discolored vessels and produced sporodochia containing unicellular, ovoid-elliptical microconidia measuring 5.7 to 11.0 × 1.8 to 3.2 (mean 7.6 × 2.5) µm. Macroconidia were 3-septate, slightly curved, with a foot-shaped basal cell and a short apical cell, and measured 19.8 to 32.1 × 2.4 to 4.1 (mean 27.5 × 3.3) µm. Eventually, affected tissues rotted, collapsed, and plants died. A fungus was consistently isolated from affected tissues on potato dextrose agar (PDA) and single-spore isolates were obtained. On carnation leaf-piece agar (CLA), single-spore cultures generated short monophialides with unicellular, ovoid-elliptical microconidia measuring 3.5 to 7.2 × 1.8 to 2.9 (mean 5.4 × 2.4) µm. Macroconidia produced in sporodochia were 3-septate, slightly curved, with a foot-shaped basal cell and a short apical cell, and measured 23.4 to 37.4 × 2.7 to 4.1 (mean 30.9 × 3.6) µm. Chlamydospores were terminal and intercalary, single, in pairs, clusters, or in chains, and measured 7 to 10 µm in diameter. Such characteristics are typical of Fusarium oxysporum (Leslie and Summerell 2006). DNA was extracted from a culture of the single-spore isolate DB14OTT07M1. Amplification of the elongation factor 1 alpha gene (EF1α) with primers EF1/EF2 (O’Donnell et al. 1998) yielded a 683-bp amplicon (GenBank Accession No. KT183483). BLASTn analysis of the sequence exhibited 100% similarity with the sequence of F. oxysporum DQ837689 in the NCBI database and 100% similarity with the F. oxysporum sequence FD_00638_EF-1a in the FusariumID database (http://isolate.fusariumdb.org). The ITS region was amplified with primers ITS1/ITS4 and the BLAST analysis confirmed the identification of F. oxysporum with 99% similarity with the sequence HQ379656. In pathogenicity tests, three 27-month-old plants of A. myriostigma were inoculated. Small wounds were made on stems (3 lesions/plant) using needles carrying spores and mycelium of the isolate DB14OTT07M1 (Talgø and Stensvand 2013). Inoculum was obtained from PDA cultures. Three control plants were wounded with sterilized needles without the pathogen. All plants were maintained at 20 to 34°C. First symptoms of Fusarium wilt appeared in stems of inoculated plants 6 days after inoculation. All inoculated plants died approximately 20 days after the inoculation. F. oxysporum was consistently reisolated from affected plants. Noninoculated plants remained healthy. Reisolations attempted from controls failed. The pathogenicity test was conducted twice with the same results. This is the first report of F. oxysporum in A. myriostigma in Italy and elsewhere. In Italy, F. oxysporum was reported on other species belonging to Cactaceae, including Echinocactus grusonii (Polizzi and Vitale 2004). The disease could potentially cause significant economic losses in A. myriostigma cultivated in Italy. © 2016 The American Phytopathological Society.