2004 Annual Report 4.What were the most significant accomplishments this past year? D. Progress Report: This report serves to document research conducted under a Trust Fund Cooperative Agreement between ARS and Washington Tree Fruit Research Commission. Additional details of this research can be found in the report for the parent project 1907-21220-002-00D entitled, Evaluation of Pest, Disease Resistance, and Stress Tolerance in Grape and Apple Rootstock. In order to identify genetically resistant rootstocks to apple replant disease we have planted three replicated field trials of elite experimental apple rootstocks at three locations (Yakima, Lake Chelan, Wapato) in Washington State. Among the objectives of this work is the search for alternatives to fumigation (methyl bromide, chloropicrin) to combat the devastating effects of the disease. The replicated field trials are set up to test differences and interactions in the way apple rootstocks respond to fumigated and non fumigated field plots known to carry the disease. Yield, and growth will be measured on the scion variety (Brookfield Gala) for several years after planting to identify rootstocks that do well in untreated disease ridden soils. Data gathering will begin in September 2004 and continue at least through 2010. One of the field trial locations (Lake Chelan) is an organic orchard. Knowledge about the rootstock performance in that orchard will provide a primary basis for recommendations to organic growers with regards to apple rootstocks. A second set of trials is being prepared for planting in the spring of 2005. The rootstocks that are being entered in these trials differ from the 2004 set in regard to their dwarfing capacity. Over 1000 rootstock liners of 10 experimental rootstocks were produced in Geneva, NY and budded with the scion variety Fuji at ProTree nursery (Brentwood, CA) to make one year finished trees. These trees will be planted in replicated trials in WA State in the spring of 2005. Knowledge gathered from these experiments will help growers with economically viable alternatives to fumigation. In order to investigate the disease potential, soil biota and soil composition of the replant sites (Yakima, Lake Chelan, Wapato) soil from fumigated and non fumigated treatments was analyzed by Dr. Mazzola (ARS, Wenatchee). The analysis included nematode counts and tests for the presence of root rot causing organisms. This knowledge will help in the genetic dissection of the disease resistance identified in the elite apple rootstocks.