Molecular evidence that Verticillium ablo-atrum Grp 2 isolates are distinct from V-albo-atrum Grp 1 and V-tricorpus

Verticillium species-specific primers were used in a polymerase chain reaction to differentiate between Verticillium albo-atrum groups and V tricorpus. Amplification with species-specific primers identified 21 isolates from the 64 V albo-atrum isolates tested as Grp2. Genome analysis using RAPDs and restriction fragment length polymorphism (RFLP) of the intergenic (IGS) region of the rDNA showed that V albo-atrum Grp2 isolate were genetically distinct from either V albo-atrum Grp1 or V tricorpus, demonstrating a significant differentiation between these species. The sizes of the amplified IGS fragment were different, with Grp1 isolates having a smaller fragment (approximate to 2.1 kb) than either Grp2 (approximate to 2.3 kb) or V tricorpus (approximate to 2.7 kb). Based on RAPID analysis, the average similarity coefficients between Grp1 and Grp2 were 35% and 34% between Grp2 and V tricorpus. Multiple correspondence analysis separated the isolates into three major groups corresponding to Grp1, Grp2 and V tricorpus. Surprisingly, isolates collected from Pisum sativa were distinct from other Grp1 V. albo-atrum isolates. The observed low levels of genetic similarity, the differences in sizes of IGS fragments, IGS-RFLP profiles and the RAPID patterns point to the possibility of Grp2 isolates comprising a different species of Verticillium than those occupied by either V albo-atrum Grp1 isolates or V tricorpus.