Genetic variation in Alternaria brassicae, A. brassicicola, and A. rap
hani collected from geographically diverse regions of the world was st
udied with RAPD and RFLP markers. Twenty 10-mer primers of arbitrary n
ucleotide sequences were tested for amplification of genomic DNA of A.
brassicae using PCR. Of these, five primers amplified the genomic DNA
from 20 A. brassicae isolates and produced reproducible RAPD profiles
. UPGMA analysis of RAPD data showed that isolates collected from geog
raphically distinct regions could be broadly classified into four grou
ps. Intra-regional variation between isolates was less apparent. Varia
tion was, however, higher in A. brassicicola, as based on RAPD analysi
s. Two isolates (from Canada and France) of A. raphani also showed var
iability with different RAPD profiles generated by all five primers te
sted. Five polymorphic, distinct RAPD products were used as hybridizat
ion probes for RFLP analysis to detect inter- and intra-specific varia
tion. Variation among A. brassicae, A. brassicicola, and A. raphani wa
s evident. Non-radioactive probes were also used to hybridize with Sou
thern blots of A. brassicae, A. brassicicola, Leptosphaeria maculans,
Rhynchosporium secalis and Brassica juncea for the selection of A. bra
ssicae-specific probe(s). Probe AbP3 specifically hybridized with rest
riction digests of A. brassicae but not with those of A. brassicicola
or other tested species. This probe should, therefore, be useful for d
istinguishing between two important pathogens of crucifers, i.e. A. br
assicae and A. brassicicola both in culture and infected tissues.