Ice Nucleation Activity by Fusarium tricinctum Can Enhance Cold Stress: A Potential Contributor to Cold Injury in Apple Trees

Cold stress is a major abiotic factor that adversely affects apple trees and associated dwarf rootstocks, increasing the susceptibility of apple trees and dwarf rootstocks to secondary biotic infections. Among these biotic factors, certain Fusarium species exhibit ice nucleation activity (INA), which can exacerbate cold injury. Therefore, this study aimed to evaluate the effects of INA-positive Fusarium tricinctum KNUF-21-F29 and INA-negative F. solani KNUF-21-F27 on the growth of in vitro apple dwarf rootstocks (cultivars Malling 9 and Malling 26). Apple shoots treated with KNUF-21-F29 culture filtrates and exposed to low temperatures showed significantly reduced growth compared with those treated with KNUF-21-F27, suggesting that the INA of F. tricinctum contributes to growth inhibition. Gene expression analysis revealed that KNUF-21-F29 filtrates upregulated genes associated with ethylene biosynthesis, antioxidant activity, and cold response. Furthermore, Malling 9 rootstocks treated with KNUF-21-F29 filtrates for 3 months had lower survival rates than both potato dextrose broth (PDB)-treated and untreated controls. These findings demonstrate that fungal INA can negatively affect apple growth and represent a critical biotic factor in the development of cold injury.