Effects of biochar and Rhizophagus irregularis on Malus domestica, variety Honeycrisp, in the presence of apple replant disease pathogens

Apple replant disease (ARD) inhibits the development of young apple trees, leading to tree death. Recent research has focused on biocontrol methods to combat ARD following the ban on chemical soil fumigants. This growth chamber experiment used Rhizophagus irregularis, a beneficial arbuscular mycorrhizal fungus (AMF), as a bioinoculant and biochar, pyrolyzed biomass, as an amendment to reduce ARD symptoms on Honeycrisp (Malus domestica). Four pathogens isolated from apple rootstocks were also inoculated in certain treatments to determine their role in the ARD complex. After a six-week soil incubation with the presence/absence of biochar and pathogens in pasteurized and non-pasteurized soils, Honeycrisp seedlings previously inoculated with the presence/absence of AMF were potted in the incubated soils. Honeycrisp seedling growth increased with AMF inoculation and was significantly higher (p0.05) in pasteurized than non-pasteurized soils. Honeycrisp root colonization of AMF was greater (p0.05) in treatments with biochar and AMF present (80%) compared to other treatments. Phosphorus and potassium content in plants indicated a positive correlation (R>0.66) between AMF and plant growth, while in soils, phosphorous and potassium were negatively correlated (-0.48<R<-0.74) with those parameters. Adding pathogens slightly decreased (p>0.05) plant growth in pasteurized (control) and non-pasteurized (control); however, no negative effects were noted on plant growth for soils containing biochar and AMF. Long-term field trials must be considered to confirm these beneficial results. Overall, this controlled study shows promising results in helping farmers mitigate ARD symptoms in their first years after planting young apple trees by adding biochar amendment and an AMF inoculant.