Variability of the Pathogenicity of Venturia inaequalis in Europe

In the frame of the DARE (Durable Apple Resistance in Europe) project, a collection of strains of Venturia inaequalis was established. It comprises 319 strains from 8 European countries, isolated from 48 different Malus species and M. x domestica cultivars. The pathogenicity of 39 strains from this collection was tested on a set of 8 differential hosts, under controlled conditions (in a growth-chamber). The results provided information on the virulence of the fungus in Europe, with special attention to virulence to the Vf gene. The most virulent strains originated from Northwest Europe. No significant correlation between aggressiveness and the number of cultivars to which a strain was virulent was found. The results also provided information on the susceptibility of the 8 cultivars of the host range, and permitted the selection of 5 representative strains of the fungus for the test of the apple cultivars and progenies in the greenhouse. The geographical distribution of Vf virulent strains is displayed on a map of Europe, including information from previous publications. INTRODUCTION The overall objective of the DARE project was to improve the knowledge on the genomic organisation of resistance factors in apple, and to select cultivars with a broad spectrum of resistance. Hence, an important topic of the project was the study of the variability and the geographical distribution of the pathogenicity of V. inaequalis in Europe, with a strong focus on partial resistances. However, the Vf gene was given special attention because it is the main source of scab resistance used in apple breeding programmes. Since the first detection of strains of V. inaequalis virulent to this gene in Germany and England (Parisi et al., 1993; Roberts and Crute, 1994), several reports on the presence of these strains in Europe were published. Race 7 was detected in the Netherlands (Schouten and Schenk, 1997) in France (Parisi et al., 2000) in Belgium (Lateur et al., 2002) and in Spain (Dapena and Blazquez, 2004); and races 6 and 7 in Denmark (Bengtsson et al., 2000) and Sweden (Sandskär, 2003). The results obtained during the DARE project, added to the reports of previous publications, allowed the distribution of these strains in Europe to be mapped. XIth Eucarpia Symp. on Fruit Breed. & Genetics Eds. F. Laurens and K. Evans Acta Hort. 663, ISHS 2004 108 MATERIALS AND METHODS Isolate Collection and Inoculum Production All the strains of the collection were obtained after the isolation and culture of single conidia, in a malt agar medium, at 17°C. Then, the monoconidial strains were grown in tubes and stored at 4°C. Inoculum of each of 39 isolates for use in the experiments was obtained by inoculating ‘Golden Delicious’ x ‘Idared’ seedlings in a growth chamber with primary inoculum grown on media (Keitt and Palmiter, 1938). The scabbed leaves of each strain were collected, dried, and conserved at -18°C until the experiments. Test of the Pathogenicity of a Set of Strains Ten experiments were performed between August, 1998 and May, 2000, in the same growth chamber. The 39 strains were tested on a host range of 8 cultivars or Malus species: ‘Gala’, ‘Golden Delicious’, ‘Fiesta’, ‘Discovery’ TN 10-8, ‘Durello di Forli’, ‘Prima’ and M. floribunda 821. The plants of the host range were grafted on M9 rootstocks in spring, and grown in a greenhouse. For each strain, six shoots per cultivar were evaluated. The last unfolded leaf was labelled on each shoot just before the inoculation, and the number of leaves counted. Leaves were sprayed with a suspension of 100 000 conidia/ml. A wetness period of 48 hours was obtained with a humidifier, in the dark, at 18°C. The plants were then incubated at 17-19°C, under a 16-h photoperiod of 70 μE sm with relative humidity 85 to 95%. Scoring of the symptoms was performed every 2 or 3 days between 11 and 25 days after the inoculation. The number of leaves grown above the label, the number of scabbed leaves, the number of lesions on each scabbed leaf and the class of the symptoms (following the scale of Chevalier et al., 1991) were assessed. Statistical Analysis Statistical analysis was performed using the statistical software package Genstat. Establishment of a Core-Orchard Network In each of the 8 countries, an experimental orchard was planted, comprising 19 cultivars of M. x domestica and one clone of M. floribunda. From this host range, 3 cultivars were considered as susceptible (but at different levels), 3 were Vf resistant genotypes (‘Prima’, ‘Priscilla’ and M. floribunda 821) and 14 cultivars carried putative partial resistances to scab, based on the information provided by the partners in each country. The set of the 3 Vf resistant genotypes, plus ‘Gala’ and ‘Golden Delicious’ allowed the detection of the presence of the races 6 and 7. They are both virulent to the Vf gene, but differ in pathogenicity on ‘Golden Delicious’ (Vg gene) and M. floribunda 821 (Parisi and Lespinasse, 1996; Bénaouf and Parisi, 2000; Durel et al., 2000) (Table 1). In this paper, we only take into account the presence/absence of scab on these 5 hosts to characterize the virulent V. inaequalis races. RESULTS AND DISCUSSION Constitution of the Core-Collection of V. inaequalis The collection comprises of 319 strains from 8 European countries, isolated from 48 different Malus accessions. The hosts included the most cultivated clones in Europe, but also polygenic resistant cultivars, Vf resistant cultivars, and several Malus species considered as sources for scab and mildew resistance. This collection represents a great diversity of geographic and genetic origins. Test of the Pathogenicity of a Set of Strains 1. Virulence of the Strains. All the strains were virulent to ‘Gala’ and 87% of the strains were virulent to the Vg gene of ‘Golden Delicious’ (Bénaouf and Parisi, 2000) (Table 2A).