Field surveys were made in several central and southern Italian tomato-growing areas for Pyrenochaeta lycopersici, the cause of corky root of tomato. In addition to P. lycopersici, a different fungus was frequently isolated from roots showing typical corky root symptoms, even after disinfestation of diseased roots with 0.1% (vol/wt) mercury chloride water solution for 1 min. The fungus was isolated from primary and secondary tomato roots in 8 of 21 fields visited. The isolates were grown on potato dextrose agar (PDA), with morphological features such as color and shape of mature conidia and pycnidia, type of conidiogenesis, presence of microsclerotia, and color of colony underside noted. Preliminary identification of the fungus was Rhizopycnis vagum Farr. To confirm the identification, the internal transcribed spacer (ITS) region of rDNA of one isolate (maintained at the ISPaVe collection at the authors' address and available on request as isolate ER 940) was amplified with two universal primers, ITS5 and ITS4. The ITS fragment was sequenced, and the nucleotide sequence compared with that of R. vagum deposited in GenBank (Accession No. AF022786). Both sequences were identical supporting the identification. R. vagum is a recently described species associated with the vine decline syndrome of melon in the United States, Guatemala, Honduras (2), and Spain (3). Eight isolates were tested for pathogenicity both on tomato (five cultivars) and melon (three cultivars) using two methods. In method 1, plantlets at the cotyledonary stage were grown on blotter in petri dishes and tested by placing a 6-mm plug of colonized PDA on the tap root (1). After 7 days, the plug was removed, and the roots were checked for symptoms. In method 2, 20-day-old seedlings were transferred to pots with infested soil (50,000 CFU/g of soil) and grown for 45 days before the roots were checked for each isolate-cultivar combination. Eight and four plants were used in tests 1 and 2, respectively. With the first method, rotten, pinkish lesions with different extensions from the inoculation point were observed on all the melon cultivars tested (Pamir, Cantalupo di Charentais, and Charme). On tomato, three of eight isolates caused root necrosis of limited extent, without pinkish discolorations at the inoculation site on cvs. Monalbo and Bonnie Best, the former showing the larger lesions. The tests on plants grown in infested soil confirmed pathogenicity on both host species, although the symptoms were of minor intensity (light, small brown lesions on secondary roots, no pinkish discoloration). The symptomatic plantlets ranged from 0 to 100% on both hosts in the petri dish tests and from 0 to 100% and 0 to 50%, respectively, for tomato and melon in the pot tests, varying according to the cultivar-isolate combination. The fungus was consistently reisolated from all symptomatic plants. To our knowledge, this is the first report of R. vagum associated with tomato roots. Although the isolates showed varying degrees of virulence with respect to host species (all being pathogenic at least on one host), the virulence of R. vagum on tomato was certainly low. Nevertheless, tomato may maintain or possibly increase inoculum for melon, which often follows tomato in Italian crop rotations.
References: (1) M. Clerjeau and M. Conus. Annu. Rev. Phytopatol. 5:143, 1973. (2) D. F. Farr et al. Mycologia 90:290, 1998. (3) J. García-Jiménez et al. EPPO Bull. 30:169, 2000.