Vegetative compatibility relationships among weakly pathogenic isolates (pathotype E) of Verticillium dahliae

Twenty-two isolates of Verticillium dahliae, which were isolated from green soybean (Glycine max), udo (Aralia cordata), horseradish (Cochlearia armoracia), sweetpea (Lathyrus odoratus), or a weed (Chenopodium album) were used in this study. Conidia and microsclerotia of these isolates were morphologically identical with those of V, dahliae but did not coincide with V. Iongisporum. Pathogenicity tests showed that these isolates were of weak pathotype. Eleven of the 22 isolates, which were obtained from green soybean and udo, were pathogenic to green soybeans. Thus pathotype E was composed of two groups:’soybean pathotype’ which was pathogenic to green soybeans; and isolates nonpathogenic to green soybeans. The latter were defined as isolates of pathotype E in the narrow sense. Selected representative nit1 and NitM mutants of each V. dahliae isolate were paired with VCGJ testers. Fourteen isolates of V. dahliae (Sol, So22, So23, So27, So28, So39, So40, So41, U54, U68, U69, U90, U95, and U115) showed complementary reactions with subgroups J1 and J3 and were assigned to subgroup J3. Isolate U108 was assigned to subgroup J2. Isolate HR1 was not compatible with any testers of VCGJ. With this exception, isolates of pathotype E in the narrow sense and those of ‘soybean pathotype’ were thus assigned to known VCGJ subgroups and did not form a unique group corresponding to their pathotype. ‘Soybean pathotype’ could not be distinguished among isolates of pathotype E by vegetative compatibility.