Monitoring of Venturia inaequalis virulences

Establishing a network of orchards of differential hosts

Selection of the differential hosts
A first set of differential hosts used to assign specific Venturia inaequalis isolates to a certain race, developed over time from the works of Shay and Williams (1956), Shay et al. (1962), Williams and Brown (1968), Parisi et al. (1993), Bénaouf and Parisi (2000) and Bus et al. (2005) (Table 1). The more the apple-apple scab interaction was studied, more evident become the problem of the current nomenclature to assign to a race an isolate able to grow on two differential hosts. To solve these conflicts Bus et al. (2008) proposed a new nomenclature which is better suited to describe the increasing complexities of combinations of genes (from scab and the apple) involved in both race-specific and race-nonspecific interactions. In this attempt the “old” set of differential hosts was revisited. Previously not considered accessions carrying R-genes that could be interesting for breeding purposes have been included and differential host of the “old” set known to carry more than an R-gene have been substituted with selections carrying only one of these known R-genes. In order to coordinate the genetic studies of the Venturia inaequalis-Malus pathosystem at laboratory scale with the monitoring of apple scab virulences in the field, the same differential hosts proposed by Bus et al. (2008) with few additions have been chosen for the monitoring (Table 2).
This set of differential host reflects the knowledge of the V. inaequalis-Malus pathosystem of June 2008. The set will be up-dated each time new apple scab R-genes are discovered and sufficient plant material is available. Currently Rvi16 discovered in MIS op 93.051 G07-098 (Bus et al. 2010) and Rvi17 of Antonovka APF22 (Dunneman and Egerer 2010) are missing in the set. As soon plant material of these genotypes will be available, they will be added to the set.

Verification of the genotypes selected as differential hosts
All the selected genotypes have been molecularly checked. The genotypes and reference DNAs (requested to different institutes) have been analyzed with 14 SSRs selected by ECPGR to molecularly characterize apples. The fingerprints have been compared to verify if the genoytpes available at ACW are “true-to-type”. The second check consisted in testing the genotypes with the molecular markers associated to their R-genes (if markers were available) to confirm the presence of the expected R-genes. Plants of genotypes found “not-true-to-type” have been replaced.

Table 1: Set of “old” differential hosts (adapted from Gessler et al. 2006)

Table 2: New set of differential hosts (adapted from Bus et al. 2008)