Abstract/Summary

Apple cultivars differ in their resistance to the fungal pathogen Venturia inaequalis, the causal agent of apple scab. Mixed cultivar orchards, where the cultivars present have differing resistance to V. inaequalis, have been shown to reduce the levels of scab compared to monoculture. To maximise the mixture effect on reducing scab development, cultivars need to be selected with maximum differences in their scab resistance. One indirect, yet efficient, method of selecting such cultivars is to quantify population differences of scab from different cultivars, which are expected to largely reflect the differences in the resistance to the pathogen. We sampled early season scab lesions from different cultivars in two mixed cider cultivar orchards in the Southwest of England and from various desert and cider cultivars in an orchard in the Southeast of England. Using Simple Sequence Repeat (SSR) markers we compared the scab populations sampled from the different cultivars. Scab populations from different cultivars differed significantly, depend ng on specific pairs of cultivars; however, larger differences appear to be among fungal populations from different sites. The results demonstrate that certain cultivars likely share much of their genetic resistance factors to V. inaequalis. For dessert apple the scab populations on Cox, Gala, Bramley and Fiesta were not different and therefore it is not advisable to plant these cultivars in the same orchard with a view to reducing scab development. On the other hand, a reduction in scab would be more likely if one of the above cultivars were planted together with Golden Delicious, Red Falstaff or Spartan. There was a very large difference in the scab population from cv. Three Counties and populations from all other cider cultivars. This was particularly surprising given the shared parentage (Dabinett x James Greave) between Three Counties and all but one of the other cider cultivars sampled, suggesting considerable differences in the resistance to V. inaequalis between the two parents. It also indicates that selection of cultivars for inclusion in mixed orchards cannot be reliably made based on pedigree information alone.