Mitigating apple replant disease with biocontrol soil treatments

Apple replant disease (ARD) can increase apple tree (Malus domestica Borkh) mortality, delay production, and reduce yield, resulting in losses of up to $60 K/ha over an orchard’s lifespan. Common fumigation treatments can harm human and environmental health, have variable effectiveness, and disrupt beneficial soil microbial activity and processes. An experiment was conducted at the Simcoe Research Station in Norfolk County, Ontario to assess the effectiveness of commercially available plant growth-promoting (PGP) microbial biocontrols to treat ARD. Five treatments were replicated in-field five times as a randomized block design. Treatments included: untreated control, fumigation control (chloropicrin (FC)), PGP fungi (PGP-F), PGP rhizobacteria (PGP-R), and a combination of PGP-F and PGP-R (PGP-M). Trees growth and health and rhizosphere microbial diversity was assessed at three points over two years. PGP-R produced the greatest mean root mass followed by the chemical fumigation, which was 32% and 10% more root mass than the untreated control, respectively. Chemical fumigation resulted in the greatest above-ground biomass tree growth followed by the PGP-R, accumulating 30% and 6% more biomass than the untreated control. However, PGP-F accumulated less biomass than the untreated control. FC and PGP-R both resulted in strong growth but impacted the microbial community differently. PGP-R increased rhizosphere bacterial diversity and decreased fungal diversity while FC did the opposite. PGP-R changes to bacterial communities persisted while FC soils resembled the untreated control most closely after two years. These results indicate PGP-R biocontrol treatments are viable alternatives to fumigation for apple growers facing ARD.