Tree growth and fruit quality of “Fortune” plum grafted on 17 different rootstocks: results for 10 years after replant.

‘Fortune’ plum trees were grafted in 2005 on 17 different rootstocks at 4.3×2.0 m and trained as free central axis. The first four years of growth and production were reported in the previous ISHS meeting in California. Hereafter the authors report the last three years of full production. The experimental layout was randomized blocks with 4 trees rootstock-1 plot-1 and 4 replications. The trial was managed with drip irrigation in replant conditions applying soil organic amendments along the row at planting and in the 5th year. Integrated pest management was applied regularly for all the experimental period. Yield, fruit size, fruit soluble solids content (SSC), and firmness were recorded every year. Tree mortality related to grafting affinity, phytoplasma symptoms (otherwise sensitivity) and pruning weight (representing plant vigor) were also recorded. The high plant vigor was positively related with big fruit size (20% higher) and high firmness while low vigor with high SSC (1° Brix higher) and 3-5 days of anticipated ripening. Mortality was very high with Felinem rootstock (75%), followed by Barrier 1, VVA1, GF677, and Cadaman (30-60%), due to both disaffinity and phytoplasma. The yield in the last three years confirms what was yielded in the first four years and generally was positively related to plant vigor. The optimum plant density to plan the ‘Fortune’ plum replant in relation to the expected vigor of the rootstock, its affinity with the tested cultivar and the required quantity and quality of the fruit has been discussed. Under the tested conditions vigorous rootstocks can be planted at 4×2 m as free central axis with long pruning for the first 3-4 years and then with branch head back, getting high fruit quality. Using intermediate rootstocks, head back can start from the 3rd year to maintain the high fruit quality, in particular fruit size. With low vigor rootstocks, the planting distance can be reduced to 1-1.5 m along the row and branch head back must be practiced from the second year.