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Progress in evaluating Malus sieversii for disease resistance and horticultural traits

Fazio, G., Aldwinckle, H.S., Volk, G.M., Richards, C.M., Janisiewicz, W.J., Forsline, P.L.
Acta horticulturae 2009 v. no.814 pp. 59-66
plant pathogenic bacteria, Erwinia amylovora, apples, plant pathogenic fungi, blight, genetic resistance, plant collections, scab diseases, disease resistance, Malus domestica, wild relatives, Venturia inaequalis, bacterial diseases of plants, pathogenicity, USDA, germplasm screening, agronomic traits, germplasm conservation, Kazakhstan, Kyrgyzstan, New York
Malus sieversii, a wild apple species native to Central Asia, has been recognized as the major progenitor of the domestic apple. Collection trips to Central Asia have verified that M. sieversii is very diverse and has all the qualities present in M. × domestica. Nearly 130,000 seeds from 950 wild M. sieversii trees were collected in eight distinct sites, mostly in Kazakhstan, on four collection trips from 1989 to 1996. The investigation of 1480 seedlings from these diverse sites is summarized. All seedlings were screened as young seedlings for resistance to Venturia inaequalis (apple scab) and planted as own-rooted seedlings in a high-density orchard in Geneva, New York. Resistance was recorded in 41% of the seedlings (7%-66% among sites). Natural infection by Erwinia amylovora (fire blight) was also recorded, with 45% exhibiting no infection (26%-75% among sites). Seedlings with no fire blight symptoms were grafted, and replicates were inoculated in the greenhouse and 60% of that group was resistant. Resistance to V. inaequalis was re-assessed on those grafted plants. Those with original reactions similar to the Vr gene showed 100% agreement and those similar to Vf, 50% agreement. A large subset was also screened as young seedlings for resistance to Gymnosporangium juniperi-virginianae (cedar apple rust), with 55% showing resistance. We are in the process of evaluating the heritability of resistance to Phytophthora, Rhizoctonia and other components of apple replant disease in these accessions. In another collaborative project, fruit from a subset of the collection are being screened for resistance to the post harvest disease, blue mold (Penicillium expansum) with some indication of resistance. Seven of the scab resistant elite accessions were crossed with the susceptible cultivar Royal Gala. By screening >200 seedlings/population with V. inaequalis, 10% to 67% of each population were observed to be resistant. Correlation of the markers with phenotype indicated some M. sieversii parents likely had known resistance genes but, in some cases, exhibited patterns suggesting they also contained novel resistance loci. Seven highly diverse microsatellite markers were selected to assess the genetic relationships among the trees in the field collections that represent the populations and the collection sites. We characterized trees and fruit for horticultural traits using multiple descriptors on 746 seedlings. Among the many descriptors measured, individual fruit sizes (17 g-158 g) were quite variable among the trees and sites. We anticipate this germplasm will offer useful genetic diversity for crop improvement.