Population structure of Venturia inaequalis, a causal agent of apple scab, in response to heterogeneous apple tree cultivation.

BACKGROUND: Tracking newly emergent virulent populations in agroecosystems provides an opportunity to increase our understanding of the co-evolution dynamics of pathogens and their hosts. On the one hand host plants exert selective pressure on pathogen populations, thus dividing them into subpopulations of different virulence, while on the other hand they create an opportunity for secondary contact between the two divergent populations on one tree. The main objectives of the study were to explore whether the previously reported structure between two Venturia inaequalis population types, virulent or avirulent towards Malus x domestica cultivars carrying Rvi6 gene, is maintained or broken several years after the first emergence of new virulent strains in Poland, and to investigate the relationship between 'new' and 'native' populations derived from the same commercial orchards. For this purpose, we investigated the genetic structure of populations of the apple scab fungus, occurring on apple tree cultivars containing Rvi6, Rvi1 or Rvi17 resistance gene or no resistance at all, based on microsatellite data obtained from 606 strains sampled in 10 orchards composed of various host cultivars. RESULTS: Application of genetic distance inferring and clustering methods allowed us to observe clear genetic distinctness of the populations virulent towards cultivars carrying Rvi6 gene from the Rvi6-avirulent populations and substructures within the Rvi6-group as a consequence of independent immigration events followed by rare, long-distance dispersals. We did not observe such a structuring effect of other genes determining apple scab resistance on any other populations, which in turn were genetically homogenous. However, in two orchards the co-occurrence of strains of different virulence pattern on the same trees was detected, blurring the genetic boundaries between populations. CONCLUSIONS: Among several resistance genes studied, only Rvi6 exerted selective pressure on pathogens populations: those virulent toward Rvi6 hosts show unique and clear genetic and virulence pattern. For the first time in commercial Malus x domestica orchards, we reported secondary contacts between populations virulent and avirulent toward Rvi6 hosts. These two populations, first diverged in allopatry, second came into contact and subsequently began interbreeding, in such way that they show unambiguous footprints of gene flow today.

Emergence of new virulent populations of apple scab from nonagricultural disease reservoirs.

Plant pathogens adapt readily to new crop varieties in agrosystems, and it is crucial to understand the factors underlying the epidemic spread of new virulent strains if we are to develop more efficient strategies to control them. In this study we used multilocus microsatellite typing, molecular epidemiology tools and a large collection of isolates from cultivated, wild and ornamental apples to investigate the origin of new virulent populations of Venturia inaequalis, an ascomycete fungus causing apple scab on varieties carrying the Rvi6 resistance gene. We demonstrated a common origin at the European scale of populations infecting apples (Malus × domestica) carrying the Rvi6 resistance and Malus floribunda, the progenitor of the Rvi6 resistance. Demographic modeling indicated that the Rvi6-virulent lineage separated several thousands of years ago from populations infecting non-Rvi6 hosts, without detectable gene flow between the two lineages. These findings show that 'breakdowns' of plant resistance genes can be caused by the selection and migration of virulent genotypes from standing genetic variation maintained in environmental disease reservoirs, here ornamental crabapples. This work stresses the need to take better account of pathogen diversity in resistance screenings of breeding lines and in resistance deployment strategies, in order to enhance sustainable disease management.

Multilocus Sequence Analysis of Selected Housekeeping- and Pathogenicity-Related Genes in Venturia inaequalis.

The relationship between housekeeping and pathogenicity-related genes and virulence or avirulence towards the primary Malus resistance genes (R) has not been previously studied for Venturia inaequalis fungus, the causal agent of apple scab. In this study, the sequences of two housekeeping genes encoding elongation factor alpha (EF-1α) and ß-tubulin and two previously unstudied effector genes of V. inaequalis from mannosidase and glucosidase families of 100 strains collected from apple cultivars with Rvi6, Rvi1, and Rvi17 and without known scab resistance genes were submitted to the analyses. Based on the phylogenetic and diversity data, as well as recombination analyses of the sequenced regions, we assessed the phylogenetic relationships and genetic structure of the pathogen within the species and the evolutionary forces that are currently acting upon this microorganism. The topology of the obtained phylograms demonstrates the lack of a relationship between the phylogenetic position of the strain and the host cultivar and the geographical origin or race of the strain. The isolates from different hosts were differentiated but did not form diagnosable, distinct phylogenetic groups. These results suggest that the analyzed genes may be too conserved to reflect the adaptation of pathogens to apple genotypes with different R genes; thus, they do not adequately reflect race discrimination. In contrast, based on variation and gene flow estimation, genetic divergence was observed among strains virulent to apple trees containing Rvi6. The results of this study confirmed a lack of free recombination between strains and demonstrated that the analyzed regions are in linkage disequilibrium and contain non-random polymorphisms associated with the strain.