2005 Annual Report 4d.Progress report. This report serves to document research conducted under a specific cooperative agreement between ARS and Cornell University. Additional details of this research can be found in the report for the parent project 1907-21000-015-00 D, Conservation and Utilization of the Genetic Resources of Apples, Grapes, and Tart Cherries. This SCA has the goal of determining the resistance of apple germplasm collected in its centers of origin and of diversity to three major diseases of apple: apple scab (caused by the ascomycete, Venturia inaequalis), cedar apple rust (caused by the basidiomycete, Gymnosporangium juniperi-virginianae), and fire blight (caused by the endobacterium, Erwinia amylovora). In this reporting period, we continued working on Malus sieversii accessions that had been collected in the several USDA-funded expeditions to Kazakhstan, the center of origin of M. sieversii.. Over 1200 seedlings are now in the eighth or ninth growing season and over 50% of these have been characterized for horticultural traits since the majority of the seedlings have begun to fruit. These seedlings had all been previously screened in this program for apple scab resistance. Over 40 % of the 1200 seedlings growing at PGRU are resistant to Venturia inaequalis (apple scab). The staff at PGRU has been keeping an accurate record of the natural infection with Erwinia amylovora (fire blight) in these seedlings using a scale 1-5 ( 1 = no infection to 5 = severe infection). Nearly 50% of the seedlings were rated moderately to severely infected (3-5) indicating that these seedlings are susceptible to fire blight. For the 8-yr-old seedlings, 234 out of 600 had a rating of 1 or 2 as of September 2003. Scions from each of these M. sieversii seedlings were grafted to seedling rootstock and grown in the greenhouse in spring 2004 and 2005. Each of these grafted seedlings were inoculated with the virulent strain Ea273 of E. amylovora. 2004 results showed that 40% of those seedlings that did not show fire blight symptoms in the field are indeed resistant. These grafted seedlings following fire blight inoculations, were given a 3-month cold treatment, and then regrown. In 2005 they were inoculated with V. inaequalis (apple scab) to confirm the results that were obtained when the seedlings were inoculated at the very young 2-leaf seedling stage several years ago. Data were collected on scab reactions of the grafted seedling plants and were analyzed in comparison with the data on inoculation of the plants as young seedlings. Overall there was a 66% agreement between the two inoculations. More plants appeared susceptible in the first inoculation than in the second, than plants appearing resistant in the first inoculation and susceptible in the second. This is to be expected since, in general, very young seedlings are more susceptible to scab than grafted plants. The scab-inoculated plants were grown on and inoculated with E. amylovora (fire blight) in three batches depending on their vigor of growth. As in previous fire blight resistance trials with this material, a diversity in resistance was observed. In other activity, we have initiated the disease resistance screening of 12 accessions of M. orientalis collected in the Republic of Georgia in 2004. This material will be tested for resistance to the three diseases described above.