A study of scab resistance in 16 apple progenies using parents with partial scab resistance

Abstract

A study was conducted to examine the inheritance of partial resistance to apple scab [Venturia inaequalis (Cke.) Wint.] using as parents a set of apple (Malus × domestica Borkh.) cultivars that show a range of field resistance to the disease, but are not known to carry any major-gene resistance. Akane, Alkmene, Antonovka, Carola, Chehalis, Goldgelbe, Katja and Summerred were used as pollen parents, and Spartan and an unnamed selection of Splendour × Gala parentage were used as seed parents in a factorial 2 × 8 mating design. One hundred seedlings of each family were planted in a randomized complete block design at a site with heavy annual natural scab infections. Each seedling was rated for infection severity for 3 consecutive years using a six-point categorical scale. A χ2 procedure was used to estimate the contribution of each parent to field resistance (analogous to general combining ability, GCA) and the contribution of the particular male-female combination (analogous to specific combining ability, SCA). Infection severity tended to increase over the 3 yr of the study. Antonovka and Chehalis had the highest analogous GCA for resistance (defined here as absence of sporulating lesions). Specific combining ability was present in 2 of 3 yr, but was small in magnitude relative to GCA. The field resistance of a given parent tended to be a poor predictor of its breeding value. However, 13 of the 16 families had at least 30% of seedlings with a potentially useful degree of partial resistance (low severity of infection). The results are discussed in the context of breeding apples with durable scab resistance.Key words: Fruit breeding, Malus, partial resistance, Venturia inaequalis