Recycling mushroom residue as a soil amendment: pH-associated attenuation of blueberry crown gall severity diverging from enhanced rhizosphere denitrification

Abstract

Cold region blueberry production faces constraints from unsuitable soils and crown gall disease caused by Agrobacterium tumefaciens. To investigate whether amendment-mediated soil changes regulate this disease, a 3 × 3 factorial pot experiment was conducted with three amendments [CK (none), T1 (peat + sulfur), and T2 (peat + mushroom residue + sulfur)] under three water regimes, using artificial stem-wound inoculation with A. tumefaciens. We quantified rhizosphere properties, denitrification genes (nirS/nirK), the pathogenicity marker virD2, potential denitrification rate (Dr), and crown gall severity assessed on a 0–9 scale at 30 days post-inoculation. T2 resulted in the lowest pH (4.99) and the highest nutrient pools. nirS abundance followed T2 > T1 > CK, whereas nirK showed no significant response. Dr followed T1 > T2 > CK (1.66–31.33 mg⋅kg−1⋅h−1). virD2 and gall severity decreased in the order CK > T1 > T2. Although disease incidence reached 100% at the treatment-combination level, and the disease index declined from 65.4 (CK) to 23.5 (T2). The severity pattern aligned with pH and virD2 but diverged from Dr, indicating that T2 attenuated symptoms primarily through pH-associated reduction in virD2 abundance rather than through enhanced denitrification potential. The contrasting response patterns between enhanced denitrification (T1) and reduced severity (T2) further imply that these two effects were not directly linked but may represent independent responses to the amendments.