Abstract
This paper addresses the general concern in plant pathology that the introduction of quantitative resistance in the landscape can lead to increased pathogenicity. Hereto, we study the hypothetical case of a quantitative trait loci (QTL) acting on pathogen spore production per unit lesion area. To regain its original fitness, the pathogen can break the QTL, restoring its spore production capacity leading to an increased spore production per lesion. Or alternatively, it can increase its lesion size, also leading to an increased spore production per lesion. A data analysis shows that spore production per lesion (affected by the resistance QTL) and lesion size (not targeted by the QTL) are positively correlated traits, suggesting that a change in magnitude of a trait not targeted by the QTL (lesion size) might indirectly affect the targeted trait (spore production per lesion). Secondly, we model the effect of pathogen adaptation towards increased lesion size and analyse its consequences for spore production per lesion. The model calculations show that when the pathogen is unable to overcome the resistance associated QTL, it may compensate for its reduced fitness by indirect selection for increased pathogenicity on both the resistant and susceptible cultivar, but whereby the QTLs remain effective.
Highlights
What are the evolutionary consequences of deploying quantitative resistance? Cultivar resistance is an efficient, environmentally benign, method of disease control that could allow for a reduction in the use of fungicides in agriculture
There is experimental evidence that lesion size, defined as the area of the spore producing surface, and spore production capacity, defined as the amount of spores produced per unit lesion area, have independent genetic support and can evolve separately in Puccinia triticina (Pariaud et al 2009b; Lannou 2012) and in other plant pathogens (Carlisle et al 2002)
The idea that quantitative traits of the host–pathogen interaction can be under independent genetic control is sustained by many studies showing variety by isolate interactions for such traits (see Pariaud et al (2009a,b) for a review) and is reinforced by recent studies on the genetic support of quantitative resistance (Chung et al 2010)
Summary
What are the evolutionary consequences of deploying quantitative resistance? Cultivar resistance is an efficient, environmentally benign, method of disease control that could allow for a reduction in the use of fungicides in agriculture. What are the evolutionary consequences of deploying quantitative resistance? The recurrent deployment of such major resistance genes over large areas has in most cases led to the rapid breaking of resistance and the development of new virulent pathogen strains due to mutation and deletion events (McDonald and Linde 2002; Deacon 2006). Breeders see quantitative resistance as an alternative approach for developing durably resistant cultivars. Quantitative resistance, less efficient, is considered more durable than qualitative resistance, mainly because its genetic determinism is more complex. Quantitative resistance is under the control of multiple genes (Kuo and Wang 2010; Gonzalez et al 2012), often referred to as minor-genes, such that the pathogen requires multiple mutations and/or recombinations to overcome the resistance. About the consequences for pathogen evolution of deploying quantitative resistance at a large scale
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