We are examining the ability of an alternative fumigant to methyl bromide, the use of compost, tolerant rootstock genotypes and cultural control to reduce the impact of apple replant disease (ARD) on apple tree growth and yield in a replanted orchard site in Ithaca, NY.

Apple replant disease is a disease complex that often affects young apple trees in replanted orchards. Up to 70% of replanted orchard sites in New York State are affected by ARD. ARD commonly reduces fruit production, especially during the early years of orchard growth. Thus, apple growers who take on loans to replant their orchards may have difficulty repaying them. The most common treatment for ARD is soil fumigation with methyl bromide, which will banned within the next few years. Moreover, soil fumigation proved helpful in only about 50% of tested NY orchard sites. Alternatives to the use of methyl bromide for ARD control are clearly needed.

In 2001, a 20 year old orchard in Ithaca, NY, was replanted. The old trees were removed and preplant soil treatments of fumigation with Telone C-17, compost application or a combination of the two were applied. Five dwarfing or semi-dwarfing rootstocks were planted into two different positions: the old tree row (OR) or the grass lane (GL) of the previous orchard. Replant tree growth in the OR was reduced compared that in the previous GL. DNA fingerprinting revealed significant differences in the bacterial community composition between OR and GL. However, there were no significant differences in the community composition of oomycetes, a pathogen often associated with replant disease. This "old row effect' was detrimental to trees on the rootstocks M7, M26 and CG16, while the growth of trees on CG30 and CG6210 rootstocks, chosen for their ARD tolerance, was similar in both positions. Three projects are running currently: 1. A comparison of the performance and the microbial rhizosphere community composition of five different rootstocks in a replanted orchard site, starting one year after replanting through the first harvest. (2) A comparison of the performance and the microbial rhizosphere community composition of fourteen different rootstocks in replant soils collected in three major apple growing regions in NY-State. (3) An evaluation of a possible connection between ARD severity and ground-cover management strategies in the previous orchard using seedling biotests.

Results from these three projects should yield a better understanding of the mechanisms behind ARD and generate improved disease management strategies for use by growers. Current data suggest that use of disease tolerant rootstocks, followed by replanting outside of the old tree row are the most promising approaches for minimizing the impact of ARD in replanted orchards in New York State.

• Ian Merwin (Horticulture)
• Gennaro Fazio (Geneva Station)
• Angelika Rumberger (Crop and Soil Sciences)
• Gregory Peck (Horticulture)
• Eric Nelson (Plant Pathology)