Study of the influence of five rootstocks on the growth and productivity of three plum cultivars grown in Bulgaria

The rootstocks ‘G×N15’ (‘Garnem’), ‘GF677’, Mirobalan seedling, ‘Ishtara’ and ‘Wavit^®’ were studied for their influence on the growth and productivity characteristics of three well-known plum cultivars (‘Čačanska Lepotica’, ‘Stanley’ and ‘Jojo’) in Bulgaria. The study was conducted during three years (2017-2019) in southern Bulgaria near the town of Plovdiv on relatively low fertile soil with drip irrigation provided. In the autumn of 2015 one-year-old nursery trees were planted at the density of 500 trees ha^‑1 (5.0×4.0 m). Trees were formed as free crown. In terms of growth characteristics, the peach-almond hybrid rootstocks ‘G×N15’ and ‘GF677’ formed the biggest trunk-cross sectional area and crown volume in all three cultivars. During the first four vegetations the most commonly used rootstock in this region, Mirobalan seedling, induced the formation of trees with similar sizes as those on ‘Ishtara’ and ‘Wavit^®’ rootstocks. The trees on ‘GF677’ produced the largest cumulative yield tree^‑1. The rootstock ‘Ishtara’ induced the highest productivity per trunk-cross sectional area and crown volume of cultivars ‘Čačanska Lepotica’ and ‘Stanley’. Mirobalan seedling and ‘Wavit^®’ occupied an intermediate position while peach-almond hybrid rootstocks were often inferior in order of mentioned parameter. The results concerning productivity of cultivar ‘Jojo’ budded on different rootstocks cannot be reliably commented due to frost during flowering period in 2019, which occurs significantly earlier in this cultivar than in the other two cultivars. At the end of the fourth vegetation 20% of the trees on ‘Ishtara’ rootstock were strongly inclined by winds and this combination definitely needs a support construction, while there is no such problem with other rootstocks.