Durability of Resistance and Cost of Virulence

Abstract

A seasonal model, where a growing season is defined as the time between sowing and harvest and alternates with an inter-crop period, was derived to study the effects of the 'cost of virulence' and cropping ratio on durability of resistance. We assumed a single strain of virulent pathogen, a single strain of avirulent pathogen and two cultivars (one resistant and one susceptible) and studied two measures of durability of resistance ('take-over time' and 'usefulness time'). Take-over time is defined as the time needed for the virulent strain of the pathogen to reach a preset threshold and predominate over the previous pathogen population. Usefulness time is the time needed before the estimated gain in green canopy area duration per plant through the use of the resistant cultivar becomes negligible. The model suggested that, although it could take several seasons before the virulent strain of the pathogen predominated over the previous pathogen population, the usefulness time of the resistant cultivar was always much shorter. Furthermore, increasing selection for the virulent strain of the pathogen (through increasing the cropping ratio of the resistant cultivar) caused the virulent strain of the pathogen to invade the system more rapidly. Cost of virulence, reflecting differences in pathogen infection rates between the four possible combinations of cultivar/pathogen strain, significantly affected durability of resistance, with the dynamics of the virulent and avirulent strains ranging from a case where the virulent strain of the pathogen died out to a case where the virulent strain of the pathogen invaded the resident pathogen population. An intermediate state, where the system reached equilibrium and the virulent strain of the pathogen neither became predominant nor died out, was defined as 'coexistence' of both strains of the pathogen. Occurrence of coexistence was directly related to the cost of virulence since it did not occur when virulence of the pathogen did not have a fitness cost. Two methods to include cost of virulence in the model gave similar results in relation to the two measures of durability of resistance studied.

Fitness cost of virulence differs between the AvrLm1 and AvrLm4 loci in Leptosphaeria maculans (phoma stem canker of oilseed rape)

“…napus pathosystem provide experimental evidence for a fungal pathogen that it may be possible to predict durability of R genes introduced into new cultivars by assessing the fitness cost of virulence. This supports theoretical evidence and experimental evidence with bacterial pathogens (Leach et al 2001;Pringle and Taylor 2002;Pietravalle et al 2006;Bent and Mackey 2007). The evidence that fitness cost of virulence at the AvrLm4 locus is generally greater than that at the AvrLm1 locus suggests that the corresponding resistance gene Rlm4 may be more durable than Rlm1 when it is deployed in commercial cultivars (Fig.…”

“…have a fitness cost). Evidence in support of this hypothesis comes from theoretical modelling (Pietravalle et al 2006) and from experimental work with bacterial pathogens (Leach et al 2001) but there has been little experimental work done with fungal pathogens.…”

World-Wide Importance of Phoma Stem Canker (Leptosphaeria maculans and L. biglobosa) on Oilseed Rape (Brassica napus)

“…The durability of major gene resistance may be increased by diversification schemes, which classify the current commercial cultivars by the resistance genes they carry, to guide strategies for deployment of these genes (Gladders et al, 2006). Modelling the effects of different deployment strategies in both space (pattern of areas sown to cultivars with different genes) and time (seasonal pattern of deployment), in relation to different measures of durability of resistance (van den Bosch and Gilligan, 2003), can be used to guide advice on effectiveness of different proposed deployment strategies (Pietravalle et al, 2006).…”

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

“…It is thought that loss or mutation of avirulence alleles by pathogens to render resistance ineffective usually carries a fitness cost, and although it has been demonstrated less frequently for fungal pathogens than for bacteria (White et al 2000), this phenomenon has been demonstrated in the L. maculans--B. napus pathosystem (Huang et al 2006). It is useful to know whether this is the case for a particular resistance gene when deciding on the best resistance management strategies by which to deploy it (Pietravalle et al 2006) with the aim of improving resistance to P. brassicae in commercial oilseed rape cultivars and decreasing fungicide use.…”