Relations Among Agronomic Traits of Commercial Blackberry (Rubus subg. Eubatus Focke) Cultivars Under the Climatic Conditions of the Moscow Region

Blackberry (Rubus subg. Eubatus Focke) ranks among the four most commercially valuable berry crops globally, alongside raspberry, strawberry, and blueberry, owing to its high antioxidant content—particularly flavonoids, anthocyanins, and polyphenols. Compared to other berry crops, blackberry cultivation requires lower labor and financial inputs, with plantations remaining productive for 12–15 years. In Russia, total blackberry area is limited (~100 ha), and the Moscow Region is particularly suited for trailing and semi-trailing cultivars with early-to-mid-season ripening. This three-year study (2021–2023) conducted at the Tsitsin Main Botanical Garden (RAS) evaluated ten promising blackberry cultivars to (i) assess interrelationships among phenological, morphological, and fruit quality traits; and (ii) identify optimal market niches for each genotype. Cultivars were grouped by ripening time: early (‘Karaka Black’, ‘Loch Tay’, ‘Natchez’) and medium (‘Columbia Sunrise’, ‘Hall’s Beauty’, ‘Caddo’, ‘Columbia Giant’, ‘Victoria’, ‘Brzezina’). Morphologically, ‘Columbia Giant’, ‘Columbia Star’, ‘Columbia Sunrise’, ‘Hall’s Beauty’, and ‘Loch Tay’ exhibited the most balanced architecture. For fresh-market retail, ‘Hall’s Beauty’ (650.3 gf), ‘Loch Tay’ (632.0 gf), and ‘Victoria’ (882.2 gf) stood out for high fruit firmness, whereas ‘Columbia Giant’ (11.5 g fruit mass, 354.1 gf) is recommended for direct consumer sales due to its large fruit size and acceptable firmness. Key trait associations included flowering duration and drupelet number (r = −0.83); fruiting onset and lateral length (r = 0.75); central leaflet length and fruiting laterals per shoot (r = −0.86); fruit number per lateral and Soluble Solids Content (SSC, r = 0.83); and lateral length (r = 0.84). These findings indicate the importance of proper variety selection for establishing blackberry plantations in the specific climatic conditions of the Moscow Region.