Disease Resistance Response of Korla Fragrant Pear Branches to Potassium Fertilizer Application

The impact of potassium fertilization on disease resistance in Korla fragrant pear trees was evaluated under drip irrigation to determine the optimal application rate. Seven- to eight-year-old trees were subjected to four K treatments: the control (K0, 0 kg/hm) and applications of 75 (K75), 150 (K150), and 225 kg/hm2 (K225). Disease resistance indices in current-year shoots and old branches were assessed throughout growth stages, and correlations with branch mineral contents were analyzed. The K75 treatment significantly increased branch Ca and Mg contents and enhanced flavonoid and lignin contents and PAL activity relative to K0. The K150 treatment markedly raised N, P, K, Fe, Mn, and Cu contents, as well as flavonoid, lignin, soluble sugar, PPO, and PAL levels, with optimal effects on flavonoids and old branch PAL activity. The K225 treatment mildly reduced Ca and Mg but strongly elevated total phenols, flavonoids, lignin, soluble sugars, PPO, and PAL, exerting the greatest influence on total phenols, soluble sugars, PPO, and current-year shoot PAL. K rates were significantly correlated with disease resistance indices. Branch mineral contents showed highly significant correlations with resistance indices and yield, but resistance indices were not significantly associated with yield. Potassium directly modulated resistance indices, with mineral elements exerting more pronounced effects in current-year shoots. Application of 150 kg/hm K is proposed as the optimal rate to improve disease resistance, mineral nutrition, tree vigor, survival, and yield in 7–8-year-old Korla fragrant pear orchards.