Impacts of N-P-K-Mg Fertilizer Combinations on Tree Parameters and Fungal Disease Incidences in Apple Cultivars with Varying Disease Susceptibility

Adequate mineral fertilization helps to ensure optimal tree growth, fruit development, and predictable yield of apple trees. This 7-year study (2016-2022) aims to investigate the effect of nitrogen (N), phosphorus (P), potassium (K), and magnesium (Mg) fertilizer combinations (NP, NPK, NPKMg, and control) on eight parameters (trunk cross-sectional area-TCSA; fruit yield-FY; number of fruit per tree-FNT; crop load-CL; fruit diameter-FD; fruit weight-FW; fruit scab incidence-FSI; and powdery mildew incidence on shoot-PMIS) on the cultivars (cvs) 'Golden Reinders' (disease susceptible) and 'Pinova' (scab and mildew tolerant). In the 7-year period, TCSA values continuously increased for both cultivars over the years. Fertilizer treatments showed significant differences on TCSA but the effect varied greatly annually among fertilizer treatments. Fertilizer treatments had increasing effects on FY and FNT in 2018 and 2022, on CL in 2018, on FD in 2018 and 2019, and on FW in 2016 and 2018 in both cultivars compared to the control treatment. FSI values were the lowest in the NPKMg treatment for cv. 'Golden Reinders' in 2016, 2017, and 2022; for cv. 'Pinova' in 2016; PMIS values for cv. 'Golden Reinders' in 2017, 2018, 2021, and 2022; and for cv. 'Pinova' in 2018. Correlation and regression analyses revealed strong and significant (p = 0.05) relationships between FNT versus (vs.) TCSA, FNT vs. FY, FW vs. TCSA, CL vs. FY, FW vs. FD, and FSI vs. FW. In conclusion, our study showed that multiyear application of fertilizer combinations can successfully increase TCSA and yield parameters as well as reduce fungal disease incidences, especially on the disease-susceptible cultivar in sandy soil with moderate fertility, under Central-European continental climate conditions.