Long-term field evaluation of Ziyang Xiangcheng and trifoliate orange rootstocks for tree vigor, crop load and fruit quality in ’Cuimi’ kumquat

’Cuimi’ kumquat (Fortunella crassifolia) is a high-value fresh-market cultivar in southern China, but suitable rootstock and nursery plant strategies for rapid orchard establishment and stable production remain unclear. This study compared trifoliate orange (TO) and Ziyang Xiangcheng (ZX) rootstocks for ’Cuimi’ kumquat through a continuous field evaluation from nursery propagation to the fourth fruiting year. Graft establishment, nursery growth, post-planting performance of three nursery plant types, fruit retention and growth dynamics, multi-year early-bearing fruit quality, and bearing-stage tree and fruit performance were evaluated. Both rootstocks supported successful grafting, while ZX increased graft survival, established seedling rate and nursery-stage growth. During early orchard establishment, 2-year-old large-container grafted trees showed greater trunk diameter and tree height increments than 1-year-old small-container and field-grown grafted seedlings, indicating that nursery plant type strongly affected early tree development. Rootstock effects on fruit retention were cohort-dependent, whereas fruit enlargement and fruit-shape development were generally maintained. During the early-bearing years, TO showed advantages in fruit weight and several sugar-related traits in some years. In the fourth fruiting year, ZX significantly increased trunk diameter, tree height and fruit number per tree, while fruit weight, fruit diameter, total soluble solids and peel color were comparable between rootstocks. These results indicate that ZX is a promising vigor-promoting rootstock for increasing fruit number per tree without detectable loss of major mature-fruit quality traits under the tested orchard conditions.