Coleus verschaffeltii Lem. (synonym C. blumei Benth., Plectranthus scutellaroides (L.) R. Br., and Solenostemon scutellarioides (L.) Codd), a perennial plant belonging to the Lamiaceae family, is used as a bedding plant for public gardens. The most popular cultivars produce speckled leaves of various colors. In October 2010, severe outbreaks of a previously unknown wilt were observed in a public garden at Torino (northern Italy) on 50 8-month-old plants. Plants were sprinkle irrigated. Initial symptoms were withering of leaves starting from the collar and brown streaks in the vascular tissue of roots, crown, and stem. Subsequently, infected tissues wilted and plants became stunted. Early leaf drop was observed and plants appeared bare, keeping few leaves only at the end of stems. Infected plants did not die but they lost the original ornamental aspect. Seventy percent of the plants were affected. Stems of 10 plants were disinfected with 1% sodium hypochlorite. Cross-sections through symptomatic vascular tissues were plated on potato dextrose agar amended with 25 ppm of streptomycin sulfate. After 10 days at 20 to 23°C, a fungus was consistently recovered from 90% of stems. Irregular, black microsclerotia, 29 to 76 × 14 to 52 (average 49 × 28) μm, developed in hyaline hyphae after 15 days of growth. Hyaline, elliptical, single-celled conidia, 3.9 to 7.2 × 1.7 to 2.8 (average 5.1 × 2.2) μm, developed on verticillate conidiophores with three phialides at each node. On the basis of these morphological characteristics, the fungus was identified as Verticillium dahliae (3). The internal transcribed spacer (ITS) region of rDNA was amplified using the primers ITS1/ITS4 (4) and sequenced. BLASTn analysis (1) of the 491-bp segment showed a 99% homology with the sequence of V. dahliae (Accession No. GU461634). The ITS nucleotide sequence of our isolate has been assigned the GenBank Accession No. JF704205. Pathogenicity tests were performed twice using 45-day-old plants obtained from seeds of C. verschaffeltii grown in 1-liter pots containing a 50:20:20:10 steamed mix of peat moss/pumice/pine bark/clay. Roots of 10 healthy plants were immersed in a conidial suspension (1.7 × 107 ml–1) of one culture of V. dahliae isolated from infected plants. Ten plants immersed in sterile water served as controls. Plants were maintained in a glasshouse at daily average temperatures between 20 and 28°C and relative humidity between 50 and 80%. First wilt symptoms and vascular discoloration in the roots, crown, and stems developed 20 days after inoculation. V. dahliae was consistently reisolated from infected vascular tissues of crown and stems of symptomatic plants. Noninoculated plants remained healthy. To our knowledge, this is the first report of Verticillium wilt on C. verschaffeltii in Italy. Verticillium wilt had been previously reported on S. scutellaroides in the United States (2). At this time, the economic importance of Verticillium wilt on C. verschaffeltii in Italy is limited.
References: (1) S. F. Altschul et al. Nucleic Acids Res. 25:3389, 1997. (2) D. Farr et al. Fungi on Plants and Plant Products in the United States. The American Phytopathological Society. St Paul, MN, 1989. (3) G. F. Pegg and B. L. Brady. Verticillium Wilts. CABI Publishing, Wallingford, UK, 2002. (4) T. J. White et al. Page 315 in: PCR Protocols: A Guide to Methods and Applications. Academic Press, San Diego, 1990.