Fine-root Dynamics of Apple (Malus domestica Borkh.) Rootstocks under Replant Condition

Apple replant disease (ARD) is a soil-borne disease that affects young apple trees in many replanted orchards around the world. Symptoms of ARD are observed soon after planting and include, among others, stunted growth above ground, reduction in root biomass, and root tip necrosis. Due to the complex etiology and multiple causal agents of ARD, there is a limited understanding of the disease development on the fine-root system, as well as the dynamics that lead to effective plant defense or susceptibility of rootstocks in replant sites. The goal of this study is to identify morphological and physiological differences between susceptible and resistance/tolerant rootstocks and induced changes in the traits when grown in ARD soil. We hypothesized that rootstock resistance/tolerance to ARD is supported by higher root growth rate, higher turnover of tissues, and higher number of growing tips. In an outdoor nursery, seedlings of M.26 rootstock (previously shown to be susceptible to ARD) and clonal rootstock liners of CG.6210 (previously shown to perform well in replant sites) were grown in soil from an ARD site (FS) and in pasteurized soil (PS) from the same site for control. Above and below ground biomass was determined by harvesting a subset of plants per soil treatment and rootstock at 11, 17, and 23 weeks after planting. A subset of plants was used to track root dynamics during the growing season (13 weeks).  Root observation windows were digitally photographed at 7-day intervals from shortly after planting to the final harvest. Rootstocks grown in PS grew significantly more than those in FS, but there were no differences in growth above ground between rootstocks. Total root biomass, first and second order roots, and second-to-first order root ratio were higher in CG6210 rootstocks than in M26 in both soil treatments. First order roots of M26 were larger in diameter and had higher N content than those of CG.6210 under FS and PS. Roots of M26 and CG.6210 were larger in diameter and had lower C content under PS than FS. Under FS, lifespan of M26 roots was significantly longer than those of CG.6210, and the mortality risk of M26 roots was 56% that of CG6210 roots. However, there were no differences on root lifespan or root mortality risk between rootstocks under PS. Differences in root production and lifespan might be linked to differences in resources allocation to production of defense compounds versus to plant growth.