In inoculation tests with E. cru c ife ra ru m only members of the
families Cruciferae and Papaveraceae were infected: no infection
occurred on plants selected from seven other families. Various cruciferous weed species showed a variety of susceptible to resistant responses in infection studies. From tests on a comprehensive range of
cultivated cruciferous hosts with different isolates of E. c ru c ife ra ru m ,
collected from different hosts and sources, differences in disease levels
following inoculation were evident both between and within species.
Levels of resistance or susceptibility of particular hosts tended to be
reflected over the whole range of isolates, although some variation
in the overall level of disease development associated with particular
isolates occurred. Variation among isolates was generally quantitative
rather than qualitative. Specific interactions with large effects occurred
only occasionally within host species at a cultivar level. The development
of different isolates on the susceptible cultivar Doon Major (6 . napus)
was similar and no relationship was found between the rate of colony
development and the level of early production of conidia. The mean
length of conidia of different isolates ranged from 42 to 45 pm and the
mean breadth from 14 to 20 pm.
Rates of germ tube and appressorium formation were similar for
all pathogen/cruciferous host combinations. Doon Major (6 . napus )
was recognised as a universally susceptible host and all other hosts
tested exhibited some degree of resistance expressed at complete or
partial levels. Complete resistance prevented growth beyond the appressorial stage although it was never found to occur with all infection
units. Partial resistance emerged as causing a delay or restriction in
colony development and a reduction in spore production. Resistance
may also be related to reduced haustorial production and efficiency.
Various tissue responses to infection were observed. Early cell necrosis
was associated with restricted infection, while in susceptible plants
necrosis occurred at advanced stages of infection due to cell exhaustion.
Callose deposition occurred with all hosts as a generalised response
to penetration, but in some cases encapsulation of haustoria by callose
was observed and associated with host resistance. Lignification occurred
at sites of penetration and in lateral walls but no correlation between
lignification and resistance was found. Factors which influenced leaf
surface characteristics were found to affect the predisposition of host
tissues to infection by E. cruciferarum .