Future climate scenarios drive the northward shift and habitat loss of <i>Marssonina coronaria</i> in Shaanxi Province, China’s core apple-growing area

Apple Marssonina blotch (AMB), caused by Marssonina coronaria, endangers apple production on China's Loess Plateau. Climate change has increased its outbreaks, but future distributional impacts remain unclear. Using 832 field records and an eight-model BIOMOD2 (a species distribution modeling platform) ensemble (True Skill Statistic, TSS = 0.90; Area Under the Receiver Operating Characteristic Curve, AUC = 0.98), we projected AMB suitability under Shared Socioeconomic Pathways (SSP) 1_2.6 (low greenhouse gas emission scenario), SSP3_7.0 (medium greenhouse gas emission scenario) and SSP5_8.5 (high greenhouse gas emission scenario) for 2041-2070. Schoener's D (an ecological niche overlap index, 0 = no overlap, 1 = complete overlap) < 0.45 indicates a niche shift toward regions with intensified precipitation seasonality and elevated warm-season precipitation. Key climatic constraints shaping AMB's distribution included the mean diurnal temperature range (39.7%), precipitation seasonality (21.4%), isothermality (15.6%), warm-season precipitation (12.8%), and annual mean temperature (10.5%). Currently, high-suitability habitats for AMB span 2.36 × 10⁴ km² in western Guanzhong. By 2070, the high-suitability areas of this pathogen are projected to decline by 67%, 78% and 86% under SSP1_2.6, SSP3_7.0 and SSP5_8.5, respectively, and will retreat to fragmented, high-elevation refugia in the northern Loess Plateau. Only the SSP5_8.5 achieves a 12% net gain in total suitable area, predominantly from low-suitability zones. Schoener's D (a metric quantifying ecological niche overlap, ranging from 0 = no overlap to 1 = complete overlap) < 0.45 indicates a niche shift toward regions with intensified precipitation seasonality and elevated warm-season precipitation, with the range centroid shifting northeast by up to 109 km. Future warming may favor northern habitats, but spring drought and summer heat stress will restrict colonization, creating a "warming-drying trade-off." Overall, climate change will compress AMB's optimal habitat into isolated, high-altitude patches, reducing disease stability in traditional apple-growing regions. These projections provide scientific support for developing early warning systems and climate-adaptive management strategies for AMB.