Differences in Valsa Canker Resistance and Its Physiological Mechanisms Among Various Apple Rootstock–Scion Combinations

To compare resistance to apple Valsa canker across scion–rootstock combinations and to identify key evaluation indicators and superior resistant combinations, we established 15 scion–rootstock pairings using ‘Yueyanghong’, ‘Harlikar’, and ‘Qiufuhong’ as scions and Qingzhen No.1, Qingzhen No.2, M. baccata Borkh., M. robusta Rehd., and M. hupehensis Rehd. as rootstocks. We assessed resistance under natural field infection conditions. We measured plant growth indices, leaf photosynthetic parameters, anatomy of one-year-old shoots, and mineral nutrient contents for each combination. We then performed correlation analysis with the disease index to identify relationships between these indicators and disease occurrence under diseased conditions and to clarify their physiological basis. Finally, we comprehensively evaluated grafting compatibility and disease resistance among scion–rootstock combinations to identify combinations that might reduce Valsa canker severity and the associated predictive indicators. Results indicated that combinations using ‘Yueyanghong’ as the scion had the lowest overall disease index. In particular, ‘Yueyanghong’/Qingzhen No.2 showed the best graft compatibility (FCC = 2.10, RSR = 1.10) and the most favorable comprehensive disease-resistance profile. ‘Yueyanghong’/M. hupehensis Rehds exhibited the highest photosynthetic rate. By contrast, ‘Harlikar’/Qingzhen No.2 and ‘Qiufuhong’/M. baccata Borkh displayed relatively high disease indices and were classified as highly susceptible under the experimental field conditions. Lower disease index correlated with higher leaf net photosynthetic rate and with higher intercellular CO2 concentration. Combinations with low relative electrical conductivity retained greater membrane-system stability. A lower disease index was positively associated with higher potassium (K) content, while higher iron (Fe), manganese (Mn), and zinc (Zn) contents correlated with increased disease index. Finally, lower disease index corresponded to greater shoot phloem thickness and higher cell density. In conclusion, using the disease index as a quantitative measure and integrating multidimensional indicators, plant growth, photosynthetic physiology, mineral nutrition, and shoot anatomical structure, allows preliminary evaluation of compatibility and disease resistance across scion–rootstock combinations.