Breakdown of resistance to grapevine downy mildew upon limited deployment of a resistant variety

Abstract

Background: Natural disease resistance is a cost-effective and environmentally friendly way of controlling plant disease. Breeding programmes need to make sure that the resistance deployed is effective and durable. Grapevine downy mildew, caused by the Oomycete Plasmopara viticola, affects viticulture and it is controlled with pesticides. Downy mildew resistant grapevine varieties are a promising strategy to control the disease, but their use is currently restricted to very limited acreages. The arising of resistance-breaking isolates under such restricted deployment of resistant varieties would provide valuable information to design breeding strategies for the deployment of resistance genes over large acreages whilst reducing the risks of the resistance being defeated. The observation of heavy downy mildew symptoms on a plant of the resistant variety Bianca, whose resistance is conferred by a major gene, provided us with a putative example of emergence of a resistance-breaking isolate in the interaction between grapevine and P. viticola.

Results: In this paper we describe the emergence of a P. viticola isolate (isolate SL) that specifically overcomes Rpv3, the major resistance gene carried by Bianca at chromosome 18. We show that isolate SL has the same behaviour as two P. viticola isolates avirulent on Bianca (isolates SC and SU) when inoculated on susceptible plants or on resistant plants carrying resistances derived from other sources, suggesting there is no fitness cost associated to the virulence. Molecular analysis shows that all three isolates are genetically closely related.

Conclusions: Our results are the first description of a resistance-breaking isolate in the grapevine/P. viticola interaction, and show that, despite the reduced genetic variability of P. viticola in Europe compared to its basin of origin and the restricted use of natural resistance in European viticulture, resistance-breaking isolates overcoming monogenic resistances may arise even in cases where deployment of the resistant varieties is limited to small acreages. Our findings represent a warning call for the use of resistant varieties and an incentive to design breeding programmes aiming to optimize durability of the resistances.

Figure 1

A P. viticola isolate showing vigorous growth on the resistant variety Bianca. P. viticola isolates SL, SC and SU were inoculated on leaf discs of the resistant grapevine variety Bianca (top) and the susceptible grapevine cultivar Chardonnay (bottom). Pictures were taken at 6 dpi.

Figure 2

Isolate SL overcomes the main resistance QTL from Bianca. P. viticola isolates SL, SC and SU were inoculated on leaf discs of resistant (R) and susceptible (S) individuals from a cross between Chardonnay and Bianca. Pictures were taken at 4 dpi (A) and 6 dpi (B). Pictures show results from a single individual of each class and are representative of other individuals. (C) Sporulation of P. viticola SL, SC and SU isolates on leaf discs of Bianca (Bia), Chardonnay (Cha), three resistant (blue) and three susceptible (red) individuals from the progeny Chardonnay × Bianca. Spore counting was performed at 6 dpi. Results are the average of two independent experiments with two replicates each. Bars show standard errors.

Figure 3

Isolate SL specifically overcomes Bianca-derived resistance. P. viticola isolates SL, SC and SU were inoculated on leaf discs of the resistant species Vitis rupestris (Vrup) and the susceptible grapevine cultivar Muscat Ottonel (VvMO). Pictures in (A) and (B) were taken at 4 and 6 dpi respectively. (C) Sporulation of P. viticola SL, SC and SU isolates on leaf discs of the resistant species Vitis riparia (Vrip) and Vitis rupestris (Vrup), the resistant hybrids 0125G, 7042H and 7050H, the susceptible hybrid 7054H and the susceptible cultivar Muscat Ottonel (VvMO). Spore counting was performed at 6 dpi. Results are the average of two independent experiments with two replicates each. Bars show standard errors.

Figure 4

QTL mapping with isolates SL and SU. QTL analysis on chromosome 18 for resistance to downy mildew using 30 individuals from the cross Chardonnay × Bianca. (A) Inoculation performed with P. viticola isolate SU. (B) Inoculation performed with P. viticola isolate SL. Grey bar shows threshold LOD score of 3. Note graphics are at different scale.

Figure 5

Phylogenetic analysis of P. viticola isolates. NJ tree based on allele shared distance (D_AS) calculated with eight microsatellite loci for the three isolates of this study (SU, SC, and SL) and 18 P. viticola isolates collected in Italy (IT), France (FR), Czech-republic (CZ), Germany (DE) and in the United States of America (US). Bootstrap percentages > 50% are reported above the branches of the tree. For French isolates the star (*) indicates isolates collected in Colmar.