Assessment of leaf spot disease-resistant hydrangea cultivars and the role of epidermal wax underlying in their resistance to leaf spot disease

Hydrangea (<i>Hydrangea macrophylla</i>), a globally significant ornamental species, suffers severe economic losses from leaf spot disease caused by <i>Corynespora cassiicola</i>. Through detached-leaf assays screening 40 commercial cultivars, this study identified seven resistant cultivars ('Rivet', 'White Angel', 'Blue Danube', 'Flora', 'Fire Works', 'Spring Bird', and 'Inspiration') with distinct phenotypic clustering. Comparative analysis of resistant 'White Angel' (WA) versus susceptible 'Ocean Heart' (OH) revealed multilayered resistance mechanisms: WA exhibited 19.33% thicker leaves and significantly smoother epidermis (<i>P</i> < 0.05), alongside 3.11-fold higher leaf wax content dominated by C28-C32 fatty acids, primary alcohols, and alkanes. Scanning electron microscopy at 24 h post-inoculation (HPI) demonstrated WA's structural defenses via significantly lower hyphal colonization and smaller stomatal apertures. Transcriptomic profiling identified six cuticular wax biosynthesis genes upregulated in OH at 24 HPI, whereas WA prioritized constitutive defenses with only two induced wax transporter genes (TRINITY_DN9458_c1_g1 and TRINITY_DN1615_c0_g1). These findings provide insights into the role of leaf tissue epidermal waxes in the hydrangea resistance to leaf spot disease, laying a foundation for understanding disease resistance mechanisms and breeding for disease-resistant hydrangea cultivars.