During March 2012, 95% of 24-month-old plants of crested molded wax agave (Echeveria agavoides), a succulent plant belonging to the Crassulaceae family, showed symptoms of a basal stem and leaf rot in a commercial farm near Ventimiglia (northern Italy). Affected plants showed extensive chlorosis from the crown level to the stem apex, followed by yellowing and by the appearance of a water-soaked aspect of stem and leaf tissues. As disease progressed, leaves became brown, wilted, and rotted. Wilting was at first unilateral and later affected the entire plant. Brown discoloration was observed in the vascular system of cut stems and leaves. In some cases, leaves were covered by a whitish-orange mycelium. This produced 3-septate, slightly curved macroconidia with a foot-shaped basal cell and a short apical cell, measuring 27.4 to 39.6 × 3.0 to 4.1 (average 34.2 × 3.7) μm and unicellular, ovoid to elliptical microconidia measuring 4.8 to 11.6 × 1.5 to 3.7 (avg. 7.2 × 2.7) μm. A fungus was consistently isolated from discolored vascular leaf tissues on Komada selective medium. Cultures on potato dextrose agar (PDA) and carnation leaf-piece agar (CLA) were incubated at 24 to 29°C. On PDA, a thin growth of whitish mycelium without pigments in the agar was observed. On CLA, sparse macroconidia, 18.9 to 30.7 × 3.0 to 4.2 (avg. 23.9 × 3.6) μm, microconidia, 4.7 to 7.7 × 1.7 to 3.1 (avg. 6.0 × 2.4) μm, and abundant chlamydospores that were single or paired, terminal and intercalary, rough walled, and 6.8 to 9.5 (avg. 7.7) in diameter were produced. Such characteristics are typical of Fusarium oxysporum (2). Amplification of the internal transcribed spacer (ITS) of the rDNA using primers ITS1/ITS4 (3) yielded a 486-bp band (GenBank Accession No. JX441893). Sequencing and BLASTn analysis of this band showed 100% identity and an E-value of 0.0 with the ITS sequence of F. oxysporum (JN232163). To confirm pathogenicity, five 3-month-old healthy plants of E. agavoides were inoculated by dipping unwounded roots in a conidial suspension (1.0 × 107 CFU/ml) of one isolate of F. oxysporum obtained from affected plants, grown for 10 days in potato dextrose broth. Plants were transplanted into pots filled with steam-sterilized substrate (sphagnum peat-perlite-pine bark-clay 50:20:20:10) and maintained in a glasshouse at 28 to 33°C. Five non-inoculated plants served as a control. Chlorosis and yellowing developed on the inoculated plants 15 days after the inoculation. Basal stem rot and vascular discoloration in the crown and stem developed within 30 days on inoculated plants. A whitish-orange mycelium producing macroconidia covered the affected leaves. Non-inoculated plants remained healthy. F. oxysporum was consistently reisolated from symptomatic plants. The pathogenicity test was conducted twice. A Fusarium sp. has been reported as the causal agent of a stem rot on Echeveria sp. in the U.S. (1). To our knowledge, this is the first report of F. oxysporum on E. agavoides in Italy. The disease is currently present in few nurseries, although it could spread, causing significant economic losses due to the increasing cultivation of E. agavoides in Italy.
References: (1) D. F. Farr et al. Fungi on Plants and Plant Products in the United States. APS Press, St. Paul, MN, 1989. (2) J. F. Leslie and B. A. Summerell. The Fusarium Laboratory Manual. Blackwell, Ames, IA, 2006. (3) T. J. White et al. PCR Protocols: A Guide to Methods and Applications. M. A. Innis et al., eds. Academic Press, San Diego, 1990.