Pathogen-host relationships between Erysiphe cruciferarum and members of the family Cruciferae

Abstract

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 .