Genetic variability and preliminary heritability estimates of resistance to scab (Venturia inaequalis) in an apple genetics population

The genetic diversity of scab (Venturia inaequalis (Cke.) Wint.) resistance was studied on two of the four sublines of the HortResearch Apple Genetics Population. Subline 91 consisted of 214 families and was evaluated in its 6th leaf, and subline 94 consisted of 75 families and was evaluated in its 3rd leaf at Havelock North. A total of 253 families were derived from open-pollinated seed from apple cultivars (Malus x domestica Borkh.), as well as crabapples (Malus spp.) from various countries, including 52 families derived from seed collected in Kazakstan. The other 36 families resulted from controlled crosses. Seedlings at two sites (Havelock North and Riwaka) were individually rated on a 10-point scale (from 0 = not infected to 9 = very highly infected) for the amount of scab on the leaves (two sites) and fruit (one site only) after natural infection in orchards where no fungicides were applied in the season of evaluation. The overall average scab score was 1.61 for the open-pollinated families with known provenance in subline 9 1, with the lowest score for the families from the Netherlands (1.03), followed by Denmark (1.27) and Sweden (1.30). For subline 94, the average scab score for the Kazak families (2.60) was much lower than that for the families derived from cultivars (3.57). There was a fair relationship between the scab levels on the leaves and the fruit (R-2=45.8%). The estimated heritability of scab resistance was moderate (c. 0.30) for some subsets of the Apple Genetics Population, which is sufficient for the efficient development of new apple cultivars with durable resistance. However, heritabilities were much lower for the full-sib families of subline 91 and the Kazak collection in subline 94. Based on their mean scores, 31 families were identified as having the highest potential for improving scab resistance in apple.