Resistance to infection by stealth: Brassica napus (winter oilseed rape) and Pyrenopeziza brassicae (light leaf spot)

Abstract

Light leaf spot (Pyrenopeziza brassicae)is an important disease on winter oilseed rape crops (Brassica napus) in northern Europe. In regions where economically damaging epidemics occur, resistance to P. brassicae in commercial cultivars is generally insufficient to control the disease without the use of fungicides. Two major genes for resistance have been identified in seedling experiments, which may operate by decreasing colonisation of B. napus leaf tissues and P. brassicae sporulation. Much of the resistance present in current commercial cultivars is thought to be minor gene-mediated and, in crops, disease escape and tolerance also operate. The subtle strategy of the pathogen means that early colonisation of host tissues is asymptomatic, so a range of techniques and molecular tools is required to investigate mechanisms of resistance. Whilst resistance of new cultivars needs to be assessed in field experiments where they are exposed to populations of P. brassicae under natural conditions, such experiments provide little insight into components of resistance. Genetic components are best assessed in controlled environment experiments with single spore (genetically fixed) P. brassicae isolates. Data for cultivars used in the UK Recommended List trials over several seasons demonstrate how the efficacy of cultivar resistance can be reduced when they are deployed on a widespread scale. There is a need to improve understanding of the components of resistance to P. brassicae to guide the development of breeding and deployment strategies for sustainable management of resistance to P. brassicae in Europe.

Host–pathogen interactions in relation to management of light leaf spot disease (caused by Pyrenopeziza brassicae) on Brassica species

“…They then colonise the subcuticular niche, where extensive hyphal networks can be observed microscopically (Fig. 1b), although there are few visual symptoms on leaves of crops during this endophytic, apoplastic phase in the epidemic cycle (Rawlinson et al 1978b;Boys et al 2007Boys et al , 2012. Early infections during autumn and winter can kill seedlings, decrease plant vigour and increase susceptibility to frost damage (Fitt et al 1998b).…”

“…The asymptomatic growth phase of the pathogen starts with the formation of a hypomycelium, followed by proliferation of fungal hyphae to produce mycelial plates within the subcuticular space (Rawlinson et al 1978b). Prior to and during this stage, twoway communication occurs between the pathogen and the host plant in which the pathogen attempts to utilise the host metabolism for its growth and reproduction, whereas the host may defend itself against the pathogen after recognition of pathogen signals (Boys et al 2007). It has been shown that extracellular cutinases (Pbc1) (Li et al 2003), extracellular proteases (Psp1) (Batish et al 2003) and cytokinins (Ashby 1997) are key pathogenicity determinants involved during the penetration and subcuticular growth of the pathogen.…”

“…rapifera), Chinese cabbage (B. rapa ssp. pekinensis) and black mustard (B. nigra) Simons and Skidmore 1988;Boys 2009;Karolewski 2010). Spread of light leaf spot between different Brassica host species has been suggested (Wafford et al 1986) but there has been little work on this.…”

“…The potential mechanisms of B. napus resistance in relation to the P. brassicae life cycle have been reviewed by Boys et al (2007). Subcuticular colonisation by the pathogen during its asymptomatic growth phase can be a key trigger for host resistance, which may operate to delay the accumulation of pathogen biomass and prevent production of asexual spores (Boys et al 2007(Boys et al , 2012.…”

“…Host resistance can serve as an effective disease management strategy, provided sufficient diversity is present within commercial cultivars (Boys et al 2007(Boys et al , 2012 and variation of P. brassicae populations is considered. Study of genes responsible for resistance against P. brassicae can greatly improve understanding of this pathosystem and help to identify new sources of resistance.…”