A NEW FORMA SPECIALIS OF FUSARIUM OXYSPORUM ON CRASSULA OVATA
G. Ortu, D. Bertetti, M.L. Gullino, A. Garibaldi
Symptoms of wilting were observed in Crassula ovata cvs Mini and Magical tree in northern Italy during summer 2010. Fourteen fungal isolates recovered from diseased tissues were analyzed by ITS sequences and identified as Fusarium oxysporum. For pathogenicity assays, roots of both cultivars plants were dipped into a conidial suspension. Inoculated plants showed typical symptoms of Fusarium wilt, confirming the virulence of all isolates. For phylogenetic analysis three genomic regions [endopolygalacturonase gene (pg5), exopolygalacturonase gene (pgx1) and Mat alpha (Mat 1-1)] were amplified by PCR, sequences were aligned with those of other formae speciales of Fusarium oxysporum from GenBank and used for constructing phylogenetic trees. Regardless of the sequenced region Crassula ovata isolates grouped together in a cluster clearly separated from other known formae speciales. Recently, a wilt disease of Cactaceae caused by F. oxysporum f. sp. opuntiarum has been reported from Italy, whose presence among the C. ovata isolates was checked by sequencing the elongation factor EF1a. The occurrence of several gaps in the sequence alignment excluded the presence of F. oxysporum f. sp. opuntiarum. Based on the results of biological and molecular investigations, it seems plausible to conclude that the F. oxysporum infecting C. ovata plants represents a novel forma specialis, for which the name F. oxysporum f. sp. crassulae, f. sp. nov. is proposed.