About 6000 ha of blackberry (Rubus L. subgenus Rubus, Watson) were harvested in the United States in 2012, with 42% of this production located in Oregon (U.S. Department of Agriculture, 2014). Oregon is the leading producer of trailing blackberry in the United States but also produces erect and semierect types for the fresh market. The growth habit and fruiting season of trailing, erect, and semierect blackberry differ considerably (Strik and Finn, 2012). In Oregon, the fruiting season of trailing cultivars ranges from late June through July with erect and semierect types fruiting from late July through August and early August through early October, respectively. All of these blackberry types produce biennial canes. The primocanes of these floricane-fruiting cultivars are vegetative in their first year of growth. In their second year, when they are called floricanes, they flower, fruit, and then senesce. In trailing types, primocanes are not self-supporting and are trained along the ground under the floricane canopy until trained to the trellis after fruit harvest and floricane pruning. By contrast, the primocanes of erect and semierect types are self-supporting and are summer pruned (tipped) to encourage branching. Summer pruning of primocanes affects leaf nutrient levels in primocane-fruiting blackberry (Strik, 2015).
Commercial blackberry growers are encouraged to develop fertilization programs based on general guidelines for nitrogen (N) fertilizer, in which rates increase from the planting year to maturity (Hart et al., 2006). Adjustments of fertilizer N and other macronutrients and micronutrients are based on the periodic soil nutrient analysis, observations of plant growth, and annual leaf tissue analysis (Bolda et al., 2012; Bushway et al., 2008; Fernandez and Ballington, 1999; Hart et al., 2006; Krewer et al., 1999). In floricane-fruiting blackberry and raspberry, leaf sampling of primocanes in mid to late-season informs growers of plant nutrient requirements for fruit production the following season, when the primocane becomes a floricane.
Leaf sampling for tissue nutrient analysis is recommended for primocanes from May to August (Bolda et al., 2012), “following harvest” (Fernandez and Ballington, 1999), the first week of August (Bushway et al., 2008), or late July to early August (Hart et al., 2006). The recommended nutrient sufficiency levels are similar among these currently available nutrient management guides and all have the same standards and sampling time recommendations regardless of the blackberry type. Strik (2015) recommended that primocane-fruiting blackberry be sampled at the early green-fruit stage (about 8 weeks after summer pruning) than a particular calendar date and suggested the leaf sufficiency range for phosphorus (P) and potassium (K) may need to be lowered for this crop.
Primocane leaf nutrient levels have been shown to vary over the growing season in erect (Clark et al., 1988) and trailing (Mohadjer et al., 2001) floricane-fruiting blackberry, primocane-fruiting blackberry (Strik, 2015), and floricane-fruiting raspberry (Hughes et al., 1979; John and Daubeny, 1972; John et al., 1976; Kowalenko, 1994; Wright and Waister, 1980). Although floricane leaves in blackberry have been shown to change over the fruiting season, standards for this leaf tissue type have only been developed in Brazil (Pereira et al., 2015).
Cultivars of blackberry (Fernandez-Salvador et al., 2015a, 2015b, 2015c; Dixon et al., 2016; Harkins et al., 2014; Strik, 2015) and raspberry (John and Daubeny, 1972; John et al., 1976) differed in primocane leaf nutrient levels when sampled in midseason. By contrast, Clark et al. (1988) found no difference among three erect blackberry cultivars in leaf nutrient levels and speculated that this was because of their similar parentage.
The objective of this study was to evaluate the impact of sample date on primocane leaf nutrient concentration in trailing, erect, and semierect blackberry cultivars with a goal of establishing the ideal sampling time for these crops. In addition, we studied the impact of sampling time during fruiting lateral development and fruiting on the nutrient concentration in the floricane to assess whether this might offer an alternative sampling method in these types of blackberry.
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