In this preliminary study, sixteen strains of V. dahliae designated as either tomato races 1 or 2 were tested for vegetative compatibility (Table 1). Compatibility was assessed by pairings of complementary nitrate-nonutilizing (nit) mutants. Nit mutants were induced from wild-type strains as follows. Agar-disks were cut from the edge of each wild-type strain of V. dahliae growing on minimal medium agar (MM) (1), and transferred to plates (4 disks/plate) containing corn meal agar with dextrose (Difco) amended with 25 g/L of potassium chlorate. After 8-12 days at 22-26 C, plates were inspected for fast growing fan-like chlorate-resistant sectors. When subcultured onto MM, sectors usually exhibited a sparse but expansive colony morphology in contrast to the dense mycelial growth of the wild type. All colonies showing a thin mycelial growth response on MM were referred to as nit mutants. Two phenotypically distinct nit mutants (nitl and nitM) were identified by growth tests on MM containing one of several nitrogen sources (1) from tester strains previously assigned to VCG's by Puhalla and Hummel (2). The following strains from their collection were used: V-44 and T-9 (VCG 1), WM and PH (VCG 2), 115 (VCG 3), and BB (VCG 4). In addition, Ohio strains S-92 (VCG 3) and S-39 (VCG 4) were employed. Vegetative compatibility was assessed by pairings of nit mutants derived from each tomato strain with nit mutants of tester strains. A nit mutant of each tomato strain was placed in the center of a plate containing MM. Two phenotypically distinct nit mutants of each tester strain of each VCG were plated 1.0-1.5 cm apart to either side. After 21-day incubation, strains were considered vegetatively compatible when heterokaryons were produced between the respective nit mutants as evidenced by the dense prototrophic growth at the line of mycelial contact. Preliminary results revealed that all 12 r ace 1 strains from Ohio, North Carolina, Japan, Canada and Australia were compatible with strains WM and PH (VCG 2) (Table 1). The remaining four race 2 strains from North Carolina and Australia were compatible primarily with S- 39 (VCG 4) and either compatible or partially-compatible with BB (VCG 4). If this trend proves to be consistent and since strains in 2 and 4 are highly incompatible, it would seem that vegetative compatibility analysis would be a useful tool to separate races 1 and 2 without resorting to the laborious and time consuming pathogenicity tests.
Table 1. Strains of Verticillium dahliae used in this study.
______________________________________________________________ Strain Host plant Geographical origin Donor^a Assigned designation of origin State Country VCG^b ______________________________________________________________ 395 Tomato (Race 1) OH U.S.A. 1 2 425 Tomato (Race 1) OH U.S.A. 1 2 442 Tomato (Race 1) OH U.S.A. 1 2 443 Tomato (Race 1) OH U.S.A. 1 2 450 Tomato (Race 1) OH U.S.A. 1 2 461 Tomato (Race 1) OH U.S.A. 1 2 410 Tomato (Race 1) OH U.S.A. 1 2 20-B Tomato (Race 1) NC U.S.A. 2 2 TC Tomato (Race 1) -- Canada 3 2 77-10C Tomato (Race 1) -- Australia 4 2 58 Tomato (Race 1) -- Australia 4 2 LE-8601 Tomato (Race 1) -- Japan 5 2 RG#2 Tomato (Race 2) NC U.S.A. 2 4 RGG Tomato (Race 2) NC U.S.A. 2 4 2011 Tomato (Race 2) -- Australia 4 4 1035 Tomato (Race 2) -- Australia 4 4 ______________________________________________________________a Isolate furnished courtesy of: 1 = R.C. Rowe, Ohio; 2 = P.B. Shoemaker, North Carolina; 3 = J.E. Puhalla, California; 4 = R.G. O'Brien, Australia; 5 = T. Iijima, Japan.
b VCG = vegetative compatibility group.
Literature Cited:
Correll, J. C., Klittich, C. J. R., and Leslie, J. F. 1987. Nitrate nonutilizing mutants of Fusarium oxysporum and their use in vegetative compatibility tests. Phytopathology 77:1640-1646.
Puhalla, J. E., and Hummel, M. 1983. Vegetative compatibility groups within Verticillium dahliae. Phytopathology 73:1305-1308.