Twenty Finnish isolates of Rhynchosporium secalis (Oud.) J.J. Davis, t
he causal agent of scald, were taken from infected barley (Hordeum vul
gare L.) plants and inoculated on to seedlings of a differential serie
s of barley containing a range of major genes for resistance to the fu
ngus, as well as on to six Nordic B-row spring barleys and three winte
r ryes (Secale cereale L.). These fungal isolates derived from four si
tes and three host varieties. Disease development was monitored on two
leaves of seedlings in the greenhouse employing a standard scale, and
on adult plants in the field by assessing the diseased area on the th
ree uppermost leaves. A comparison was also made between the pathogeni
city and virulence of ten Finnish and ten Canadian R. secalis isolates
. The Finnish isolates varied in virulence, but with the exception of
Algerian (CI 1179) seedlings and adult La Mesita (CI 7565) all seedlin
gs and adult plants of the entire differential series were resistant t
o all isolates. Canadian isolates were, on average, less virulent than
Finnish isolates. All the Nordic checks were susceptible to all Finni
sh and seven Canadian isolates, but differences in the degree of susce
ptibility were evident. Isolates of R. secalis from barley were non-pa
thogenic on rye, isolates from Elymus repens L. were non-pathogenic on
barley and rye, and isolates from rye were only pathogenic on rye. Fi
nnish R. secalis isolates contain no redundant pathogenic diversity. T
he differential series represents a useful, but as yet untapped, sourc
e of resistance to R. secalis for Finnish barley breeders.