Analysis of the Leaf Structure and Physiological Response of Two Peach Genotypes in Relation to Powdery Mildew Resistance

Powdery mildew is one of the major diseases of peach. To clarify the relationships between peach powdery mildew resistance and leaf structure and defensive enzyme activities, this study used the resistant genotype ‘Zhou Xing Shan Tao’ (Prunus davidiana) and the susceptible genotype ‘Shache Lürou No.3’ (Prunus ferganensis) as a control. Resistance was evaluated through artificial inoculation with powdery mildew conidia (Podosphaera tridactyla). Key indicators including leaf wax content, stomatal characteristics, leaf transverse structure, and the activities of three defense enzymes were assessed and the relationship between these traits and resistance was analyzed. The results showed that ‘Zhou Xing Shan Tao’ exhibited stronger resistance to powdery mildew than ‘Shache Lürou No.3’. ‘Zhou Xing Shan Tao’ leaves exhibited a significantly higher (p < 0.01) wax content and a significantly lower stomatal density relative to ‘Shache Lürou No.3’. However, the size of individual stomata (long diameter, short diameter, circumference, area) did not differ significantly between the two genotypes. Microstructurally, ‘Zhou Xing Shan Tao’ leaves had significantly greater fenestrated tissue, lower epidermis, palisade cell layers, and overall leaf thickness. Its leaf tissue structure was also significantly more compact, with a higher compactness-to-openness ratio, and significantly less cellular laxity than ‘Shache Lürou No.3’. Following inoculation with powdery mildew, ‘Zhou Xing Shan Tao’ leaves accumulated hydrogen peroxide (H2O2) and malondialdehyde (MDA) more rapidly and to a greater extent. The activities of the superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) defense enzymes also increased faster and were significantly higher than in ‘Shache Lürou No.3’. In conclusion, leaf structural traits such as wax content and stomatal density combined with pathogenicity assessments serve as effective phenotypic indicators for evaluating powdery mildew resistance in Prunus species.