Six Races of Venturia inaequalis are Found Causing Apple Scab in Spain

Apple scab, caused by the ascomycete Venturia inaequalis, is an important disease of apple (Malus domestica) worldwide and is generally managed with intensive applications of fungicides or, for specific markets, by planting of resistant cultivars. Currently, eight races of the pathogen have been defined on the basis of their ability to overcome sources of resistance used by plant breeders (2). We undertook the identification of apple scab races present in Spain as a necessary step for the deployment of resistant cultivars. Spore suspensions (2.5 × 10⁵ spores per ml) were prepared from fresh lesions of apple scab collected from apple trees in commercial or domestic orchards located in the northeastern Spanish provinces of Gerona, Guipúzcoa, and Navarra. To identify races 1 to 7, the suspensions were used to inoculate cv. Gala and six differential apple clones with five replicates. Cultivar Gala was used as a universal susceptible control. The differential apple clones included X2250, X2253 (Geneva), X2249, X2225, X2596 (Prima), and X6518 (M. floribunda 821) (1,4). All leaves were assessed on each inoculated shoot 17 days after inoculation, and the type of symptoms were scored using the scale of Chevalier et al. (3). Sporulation was evaluated on seven replicate leaf discs (1 cm in diameter) for each combination of inoculum and plant host (4). All the inocula tested produced typical apple scab symptoms and sporulation (>10⁶ conidia per cm²) on cv. Gala. Races 2, 3, 5, and 6 were identified on the basis of the elicitation of typical disease symptoms with abundant sporulation (>3 × 10⁶ conidia per cm²) on clones X2250, X2253, X2225, and X2596, respectively. Races 2 and 6 were present in inocula from Gerona, Guipúzcoa, and Navarra, whereas races 3 and 5 were identified only in an inoculum from Tudela, Navarra. An inoculum from La Tallada, Gerona, was assigned to race 1 because it produced typical disease symptoms and sporulation only on cv. Gala, while the differential clones were scored as resistant. In Santesteban, Navarra, a monosporic isolate of V. inaequalis, designated VIP28, was obtained in the field from a single lesion in a 2-year-old tree of clone 2.1.1, which is resistant to race 1. VIP28 produced lesions typical of weak virulence in clone X6518 and produced a significant degree of sporulation (0.8 × 10⁶ conidia per cm²), whereas the differential clones for races 2 to 6 were scored as resistant, indicating that this isolate can be classified as race 7 (1). To our knowledge, this is the first report of races of V. inaequalis present in Spain. References: (1) G. Bénaouf and L. Parisi. Phytopathology 90:236, 2000. (2) V. G. M. Bus et al. New Phytol. doi:10.1111/j.1469-8137.2005.01395.x, 2005. (3) M. Chevalier et al. Plant Pathol. 40:249, 1991. (4) L. Parisi et al. Phytopathology 83:533, 1993.