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23Caffier, V., Patocchi, A., Expert, P., Bellanger, Durel, M., 24 Broggini, G.A.L, Groenwold, R., and Bus V.G.M. 2014. Virulence characterization of 25 Venturia inaequalis reference isolates on the differential set of Malus hosts. Plant Dis. 26XX:XX-XX.27 28 A set of differential hosts has recently been identified for 17 apple scab resistance genes in an 29 updated system for defining gene-for-gene (GfG) relationships in the Venturia inaequalis- 30 Malus pathosystem. However, a set of reference isolates characterized for their 31 complementary avirulence alleles is not yet available. In this paper, we report on improving 32 the set of differential hosts for h(7) and propose the apple genotype LPG3-29 as carrying the 33 single major resistance gene Rvi7. We characterized a reference set of 23 V. inaequalis 34 isolates on 14 differential apple hosts carrying major resistance genes under controlled 35 conditions. We identified isolates that were virulent on at least one of the following defined 36 Sixteen different virulence patterns were observed. In general, the isolates carried one to 38 three virulences, but some of them were more complex, with up to six virulences. This set of 39 well-characterized isolates will be helpful for the identification of additional apple scab 40 resistance genes in apple germplasm and the characterization of new GfG relationships to 41 help improve our understanding of the host-pathogen interactions in the V. inaequalis-Malus 42 pathosystem.43 44 45 3 Plant DiseaseDurable pest and disease resistance is a major breeding objective for most crops, but 46 achieving it has proven to be a challenge for breeders in the absence of truly non-race-47 specific resistances. Many genes involved in effector-triggered immunity (20) code for highly 48 polymorphic proteins containing nucleotide binding (NB) and leucine-rich repeat (LRR) 49 domains that instigate a resistance reaction in the host following recognition of the 50 complementary pathogen effector protein(s) in this specific gene-for-gene (GfG) relationship 51 (13). However, the high evolutionary potential of the pathogen effector arsenal negatively 52 affects the durability of resistance genes. To counteract, or at least delay the breakdown of 53 resistance genes by virulent races of the pathogen, breeders search for new resistance genes 54 (major genes and quantitative trait loci (QTLs)) that are effective against the already known 55 virulent races (23) and pyramid them (including 'spent' ones) to improve their durability as 56 well as increase the efficacy of the combined genes. This is based on the principle that for a 57 multiple virulent strain to develop, it will have to acquire these virulences simultaneously 58 (11). The effectiveness of this strategy relies on the availability of a set of differential hosts 59 and reference isolates to understand the genetics of the arsenal of both effectors in the 60 pathogen and resistances in the host, and their GfG relationships (34). 61 Apple scab, caused by Venturia inaequali...

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Atypical scab-like symptoms were reported for the first time in 2007 in the south of France on fruits of apple cultivars carrying the Rvi6 (=Vf) major resistance gene to Venturia inaequalis. With microscopic observations, nucleotide sequence data and pathological tests, it was shown that the causal agent was Venturia asperata. Scanning electron microscopy was used to compare its infection process and conidiogenesis to those of Venturia inaequalis on apple and Venturia pirina on pear. Venturia asperata produced fewer hyphae and fewer spores than the two other Venturia species, and resulted in weaker symptoms. This fungal species was previously described as a saprotroph on apple leaf litter. This is the first report of damage on apple fruits caused by V. asperata. Changes in host and cultural practices may have created a new context favourable for the emergence of this pathogen. It was also detected on symptomless leaves and on overwintered leaves on the ground. Pseudothecia developed on overwintered leaves and released ascospores over a 2-month period from the end of March until the end of May, suggesting that the fungus is able to survive from season to season. However, it is not yet known if this new disease will establish over coming years and become an emergent disease.

Understanding evolutionary dynamics of pathogens during domestication of their hosts and rise of agro-ecosystems is essential for durable disease management. Here, we investigated changes in life-history traits of the fungal pathogen Venturia inaequalis during domestication of the apple. Life traits linked to fungal dispersal were compared between 60 strains that were sampled in domestic and wild habitats in Kazakhstan, the center of origin of both host and pathogen. Our two main findings are that transition from wild to agro-ecosystems was associated with an increase of both spore size and sporulation capacity; and that distribution of quantitative traits of the domestic population mostly overlapped with those of the wild population. Our results suggest that apple domestication had a considerable impact on fungal characters linked to its dispersal through selection from standing phenotypic diversity. We showed that pestification of V. inaequalis in orchards led to an enhanced allocation in colonization ability from standing variation in the wild area. This study emphasizes the potential threat that pathogenic fungal populations living in wild environments represent for durability of resistance in agro-ecosystems.

Secondary contact between crops and their wild relatives poses a threat to wild species, not only through gene flow between plants, but also through the dispersal of crop pathogens and genetic exchanges involving these pathogens, particularly those that have become more virulent by indirect selection on resistant crops, a phenomenon known as "pestification." Joint analyses of wild and domesticated hosts and their pathogens are essential to address this issue, but such analyses remain rare. We used population genetics approaches, demographic inference and pathogenicity tests on host-pathogen pairs of wild or domesticated apple trees from Central Asia and their main fungal pathogen, Venturia inaequalis, which itself has differentiated agricultural and wild-type populations. We confirmed the occurrence of gene flow from cultivated (Malus domestica) to wild (Malus sieversii) apple trees in Asian forests,

Hybridization and adaptation to new hosts are important mechanisms of fungal disease emergence. Evaluating the risk of emergence of hybrids with enhanced virulence is then key to develop sustainable crop disease management. We evaluated this risk in Venturia inaequalis, the fungus responsible for the common and serious scab disease on Rosaceae hosts, including apple, pyracantha and loquat. Field isolates from these three hosts and progenies obtained from five crosses between formae speciales isolates collected from pyracantha (f.sp. pyracantha) and apple (f.sp. pomi) were tested for their pathogenicity on the three hosts. We confirmed a strict host specificity between isolates from apple and pyracantha, and showed that most isolates were able to cause disease on loquat. None of the 251 progeny obtained from five crosses between V. inaequalis f.sp. pyracantha and V. inaequalis f. sp. pomi could infect apple. If confirmed on more crosses, the inability of the hybrids to infect apple could lead to a novel biocontrol strategy based on a sexual hijacking of V. inaequalis f.sp. pomi by a massive introduction of V. inaequalis f.sp. pyracantha in apple orchards. This strategy, analogous to the sterile insect approach, could lead to the collapse of the population size of V. inaequalis and dramatically reduce the use of chemicals in orchards.

1 7 1 8 1 9 3 1agricultural-type pathogen in wild forests. We detected a SNP predicting the ability of the 3 2 fungus to parasitize the different host populations, which induced an early stop codon in a 3 3 gene coding for a small secreted protein in the agricultural-type fungal population, thus 3 4 representing a putative avirulence gene which function loss would enable to parasitize 3 5 cultivated apples. Pathogenicity tests in fact revealed the pestification of V. inaequalis, with 3 6 higher virulence of the agricultural-type population on both wild and domesticated trees. Our 3 7 findings highlight the threat posed by cultivating a crop near its center of origin, with the 3 8invasion of a pestified pathogen on wild plants and introgression in the wild-type pathogen.