Replant Disease Development on the Fine-root System of Apple (Malus domestica Borkh.)

Apple replant disease (ARD) is the stunting and poor growth of apple (Malus domestica Borkh.) when planted on sites where apple has previously grown.  Described as a disease of the fine-root system, ARD has a complex etiology with multiple causal organisms.   As a result, disease development on the root system is poorly understood.  The goal of this study is to clarify disease dynamics by determining the susceptibility of fine-roots of different developmental stages to commonly implicated ARD pathogens.   Hierarchical branching order, as a correlate of root development, is used to investigate shifts in plant defenses and pathogen abundance in response to root development.   We hypothesize that root development and branching order may limit pathogen distribution through shifts in the availability or quantity of habitat or through the distribution of plant defenses.  In separate greenhouse assays, seedlings and clonal rootstocks were grown in soil from a known ARD site and the same soil steam pasteurized for control.   Tissue culture derived plantlets of a rootstock previously shown to be susceptible (M.26) and one that has performed well in replant trials (CG.6210) were grown in the second assay.  Following harvests at three, six, and nine weeks post planting, root systems were analyzed for growth parameters, root system development and appearance of visual symptoms.  Following dissection by hierarchical branching order, targeted root anatomical and chemical defenses were quantified.  Pathogen abundance was measured through quantitative polymerase chain reaction.  Restriction of pathogen populations to lower order roots suggest that secondary development of the root and loss of cortical tissues fundamentally restricts the quantity and quality of habitat available to replant pathogens.  Distribution of defenses within the hierarchical system reflects greater defenses at higher orders, which may further serve to protect downstream branching modules.  Finally, the root system growth and development of M.26 and CG.6210 will be discussed in relationship to relative resistance or tolerance to common replant pathogens.