Three new methods for inoculating brassicas with
Plasmodioohora brassicae were developed for glasshouse
clubroot resistance tests. The ,1:modified slurry method"
produced markedly higher and more consistent disease
levels than the widely used "root dip procedure". It
was furthermore simpler and required less inoculum. Improved techniques for single spore manipulation and inoculation were also devised.
The resting spore maturation rate in diseased tap
roots depended upon host species and P.brassicae population. Under glasshouse conditions infective spores were
first detected 6 or 7 weeks after the sowing date, maximum
infectivity being reached in 8- to 10-week-old roots,
usually in galls which were firm and white and, in some
cases, still swelling. Numbers (log10) of detectable
spores per unit fresh tissue weight appeared to be almost
constant and independent of plant age, h o s t 'species,
pathogen population and gall size.
The pathogenicity and virulence of inocula stored as
concentrated resting spore suspensions for up to 6-12
months at -18 °C or h °C showed no decrease, but occasional
marked increase. Loss of infectivity was eventually rapid
and unpredictable and depended upon the P.brassicae population used. Inocula stored as frozen galls retained pathogenicity and virulence for at least 2 years.
Differences in pathogenicity and virulence were
recorded between spore populations from roots and soil at
the same site. There were also marked differences between inocula from different host species, different plants
of the same cultivar and different galls on the same plant.
Eighty P.brassicae populations occurring naturally in
South East Scotland were tested for physiologic specialisation on the European Clubroot Differential set. Most
were virulent on the Brassica napus and B .oleracea differentials but avirulent on the stubble turnips (B.camnestris). One population carried virulence factors corresponding to all the differential hosts. The preponderance
of intermediate disease indices was probably caused by
pathogen, and to a lesser extent host, heterogeneity.
The influence of an arbitrarily selected resistance/susceptibility cut-off value on population classification was
often considerable. Many combined virulence pairs
occurred more frequently than would have been expected from
the individual virulence frequencies. This was mainly
attributable to the high degree of similarity amongst resistance factors in the ECD set, but may also have been due
partly to host selection on the soil population, particularly in the case of interspecific virulence pairs.
The frequencies of the reaction patterns recorded on
the B .nanus differentials suggested that this group is more
likely to carry five resistance genes than three. Virulence to B.nanus occurred most frequently in populations
from the Borders region.
As unit doses of mature resting spores from different
populations apparently differed in their capacity to cause
disease, an assay could not be used for the accurate determination of absolute concentrations of infective spores in
field soils. However, the test devised provided a reliable and simple means for evaluating comparative soil