Forty-seven isolates of Verticillium albo-atrum, 35 from
hop (Humulus lupulus), seven from lucerne (alfalfa,
Medicago sativa) and five
from four other hosts, were analysed for DNA polymorphisms. Restriction
fragment length polymorphisms (RFLPs) were detected in
ribosomal RNA genes (rDNA) using Southern hybridization. Polymorphisms
in mitochondrial DNA (mtDNA) were detected in
ethidium bromide stained gels after digestion of total genomic DNA
with restriction enzymes which recognize four bases containing
only G and C. Amplified polymorphic DNA (APD) was analysed using primers
based on rDNA sequences from the intergenic
spacer (IGS) and 25S regions. These data were used to construct
phenograms using either squared Euclidean dissimilarity coefficients
(SEDC) and cluster analysis, or unweighted pair grouping with arithmetic
averaging (UPGMA). rDNA RFLPs revealed one group
with 44 isolates, a second group with two atypical hop isolates, and a
third group containing a single avirulent lucerne isolate.
mtDNA RFLPs separated rDNA group one into two subgroups, one group
containing 38 isolates from different hosts and the other
containing all six virulent lucerne isolates. APD analysis divided the
isolates into 16 phenotypes, 12 of which contained most of the
hop isolates, but there was no correlation with origin, hop cultivar,
pathogenicity or year of isolation. One APD phenotype
contained all six virulent lucerne isolates, indicating the genetic
differentiation between hop and lucerne isolates. Two further APD
phenotypes coincided with the second atypical group containing two hop
isolates and a distinct avirulent lucerne isolate,
respectively. The three methods revealed that three isolates differed
markedly from those of the main group.