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The USDA-National Plant Germplasm System apple (Malus) collection has traditionally been conserved by maintaining orchards in Geneva, NY and cryopreserving dormant buds of clones. The orchard Malus collection includes hundreds of M. sieversii and M. orieutalis individuals. These wild relative species are valued primarily for their diverse alleles, rather than for their specific genotypes. We have developed core collections based on both phenotypic and genotypic (microsatellite) characters to identify a limited number of individuals that represent the diversity of the wild collections. In addition to maintaining selected individuals in orchard core collections, we propose that wild species should be conserved as seeds from strategically selected crosses. This would ensure that many diverse individuals are available for future selection and physiology programs, rather than a limited number of specific genotypes. Crosses have been performed among core collection individuals for two of the three complementary core collections proposed for the M. sieversii collection to produce seeds that can be stored for long-term storage. This conservation approach that promotes preservation of selected clones as well as diverse sets of seeds for vegetatively propagated collections has several advantages. It ensures that valuable alleles in wild collections are conserved, reduces the labor involved in cryopreserving vegetative propagules, and also maintains diverse individuals in field settings that are available for pollen and dormant budwood distribution.

Ma/us 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 i1 x dornestica. Nearly 130,000 seeds from 950 wild M. siei'ersii trees were collected in eight distinct sites, mostl y 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 Veuturia iizaequalis (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 Erit'inia 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. macqua/is was re-assessed on those grafted plants. Those with original reactions similar to the Vr gene showed 100% agreement and those similar to Vp 50% agreement. A large subset was also screened as young seedlings for resistance to G)'!nnosporangium juniJ)eri-I'irginianae (cedar apple rust), with 55% showing resistance. We are in the process of evaluating the heritability of resistance to PIn'tophtlwra, 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 (Penjcj//jii,n 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 macqua/is, 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.

Malus sieversii (Lebed.), a wild apple species native to Central Asia, has been recognized as the major progenitor of the domestic apple. This investigation summarizes the natural infection by Erwinia amylovora (fire blight) on 2590 M. sieversu seedlings grown as own-rooted trees. At a USDA orchard in Geneva, New York (NY), 1180 seedlings (106 populations) representing germplasm from eight sites in Kazakhstan were planted in 1997 and 1998. At a University of Minnesota (MN) orchard in Excelsior, MN, 1410 seedlings representing 124 populations from two climatically diverse sites in Kazakhstan were planted in 1998. Individuals in each population are half-sibs of each other from seeds collected from the same mother tree of wild M. sieversii. One of the Kazakh sites (site 9) selected for the MN trial was a mesic site at 47° N latitude chosen for its potential for extreme cold hardiness. The other group was collected from site 6, a relatively warm, xeric site. To compare results, 32 of the populations grown in MN were also grown in NY including 10 from site 6 and 22 from site 9. Natural infection by fire blight was recorded annually using a 5-point scale (1= no infection to 5= very severe). This rating was cumulative over years. In both MN and NY, we observed a 20% greater incidence of fire blight in the trees from Kazakh site 9 than from site 6, with many infected trees dying outright. Populations grown only in NY from the other six Kazakh sites had results similar to those from site 6. Where infection was medium to severe (3-5 rating), we defined seedlings as susceptible. We determined that 535 (45%) of the seedlings grown in NY were rated 1 or 2 and potentially resistant. To determine if these genotypes were truly resistant, scions from the field trees were grafted to seedling rootstock, and shoot tips of the resulting grafted plants were inoculated in a greenhouse. Of 289 of the genotypes checked to date, nearly 60% of this group that escaped infection in the orchard were resistant.

The United States National Plant Germplasm System has sponsored several explorations to collect a diversity of apple germplasm from Asia and Europe, in order to conserve it for posterity, and after evaluation, to make it available to breeders and geneticists worldwide. Germplasm was acquired from Sichuan Province, China (Malus hupehensis, M. kansuensis, M. prattii, M. sieboldii, M. toringoides, M. transitoria, M. yunnanensis, and M. zhaojiaoensis); Russian Caucasus (M. orientalis); Turkey (M. orientalis); and Germany (M. sylvestris). Samples of germplasm were evaluated as seedlings for resistance to fire blight, by inoculating potted plants in the greenhouse with Erwinia amylovora strain Ea273. Plants with < 20 % of shoot length blighted were characterized as resistant. Considerable variability in resistance was observed in seedlings from almost all collections of all species from individual sites. Seedlings from small-fruited Sichuanese species were also evaluated by inoculation of 2nd leaf plants in the field. Greenhouse and field evaluations of the Sichuanese species were in close agreement. Proportion of resistant seedlings ranged, in the different sites collected, from 0 - 3 % of M. yunnanensis to 0 - 100 % of M. hupehensis. Most populations of M. hupehensis, M. sieboldii, M. toringoides, and M. transitoria had a high frequency of resistant seedlings. Seedlings from different populations of trees with ~ 30 mm fruit size (M. orientalis) from Russian Caucasus were 50 - 93 % resistant, whereas M. orientalis seedlings from Turkey were only 6 - 50 % resistant. Seedlings from small-fruited M. sylvestris from Germany were almost all susceptible. The resistant seedlings from the Sichuanese species may have breeding potential especially for rootstocks and ornamentals. However, Russian M. orientalis may convey resistance to fire blight accompanied by less sensitivity to latent viruses, than small-fruited Malus species. This would be especially useful for breeding of fire blight resistant rootstocks.

Malus sieversii is the main ancestor of the cultivated apple (M. domestica) and is mainly endangered in its center of diversity in Kazakstan. Seeds from 35 wild Malus sieversii across 7 locations (1089 seedlings in total) were selected and germinated for evaluation at the fruit genebank at Dresden-Pillnitz. The seeds were supplied by the USDA-ARS Plant Genetic Resources Unit, Geneva, NY, USA, which obtained them during expeditions to Central Asia. The assessment of the genetic diversity of these seedlings should give assistance in preserving (in situ) the original habitats and for further collecting strategies. The main practical objective of this research is to find new sources of disease resistances and improved horticultural qualities for breeding new apple varieties. The broad range of variability within the plant material can already be observed after 2 to 3 years of evaluation (e.g. start and length of the vegetation period, plant height). Evaluations of apple scab and powdery mildew symptoms, caused by natural infections, showed significant differences within and between offsprings. A total of 64 individuals showed complete absence of both diseases. Therefore, new sources of resistance for use in breeding programs are expected to be identified. With regard to apple scab, this assumption was supported by initial PCR experiments employing closely linked SCAR and RAPD markers for the Vf -gene. Neither of 21 preselected M. sieversii individuals from 6 different sites displayed the respective marker bands, although being scored resistant to the disease. In PCR analyses aimed at determining the genetic diversity of 26 M. sieversii individuals from the different populations, a very good correlation between genetic similarity and the origin of the different individuals could be observed. The evaluation of this material and its characterization by molecular markers is planned to be intensified in future years.

The center of diversity of the genus Malus is situated in East Asia. The diversity of wild and cultivated apples, as a whole represent a great pool of traits for multiple use. This irretrievable diversity is to be preserved now and in the future by genebanks. This is to be done by: 1) keeping the trees in situ at the natural site (wild species) or on farm; 2) ex situ as grafted trees (cultivars), or as seedlings, and seed lots in the genebanks. For decades, classical collections of wild and cultivated apples have existed. Mostly, the material was collected a long time ago, exchanged between different arboreta, often as open-pollinated seeds. Usually, the original passport data is entirely incomplete. Additionally in collections, many representative Malus species are restricted to only a few accessions. Therefore they are neither representative for the extent of variability within a species nor for usable traits. As a result, in recent years collecting of wild specimens has been revived. For multiple use of genebank material, evaluation and characterization are necessary. The most important traits to be defined are for resistances to biotic and abiotic factors so that they may be used in breeding new cultivars.