Cultivated Vaccinium species require specific soil conditions for establishment, vigorous plant growth, and fruit production. Soil adaptation of blueberry plants has been the subject of research over many years (Burkhard et al., 2009; Krewer et al., 2009; Strik and Buller, 2014). Soils suitable for blueberry production must be acidic (pH 4.0 to 5.5), well aerated with good drainage, high in organic matter (Williamson et al., 2012), and have readily available iron (Fe) and ammonium (Darnell and Hiss, 2006). To grow cultivated blueberry, including SHB, on typical agricultural soils (i.e., higher pH and lower organic matter), soil amendments are required.
In the southeastern United States, pine bark beds or pine bark incorporated into the top layer of the soil and sulfur application (Williamson et al., 2012) are used to increase organic matter, lower pH, and increase Fe and nitrogen (N) availability. Without soil amendments, plants exhibit decreased growth (Krewer et al., 2009), possibly due to decreases in Fe and/or N uptake (Poonnachit and Darnell, 2004). Increased N uptake by blueberries grown in amended soil promotes greater growth and yield compared with plants grown in nonamended soil (Burkhard et al., 2009). However, soil amendments are expensive (Julian et al., 2012) and may not be ecologically sound. The high costs associated with soil amendments decrease the profitability and sustainability of the blueberry industry.
Grafting is a well-established horticultural practice commonly used in perennial fruit crops and more recently in annual horticulture crops. Grafting has several benefits to plants, such as control of biotic and abiotic stresses (Liu et al., 2014), increased soil adaptation (Penella et al., 2015), and improved horticultural traits (Marra et al., 2013). Rootstocks may potentially impart several advantages to blueberries. Galleta and Fish Jr. (1971) reported that highbush cultivars grafted onto rabbiteye blueberry (Vaccinium virgatum Ait.) rootstocks exhibited greater plant height, flower bud number per shoot, and survival compared with own-rooted highbush cultivars (V. corymbosum L.). Kunitake et al. (2006), testing different combinations of blueberry rootstocks (Vaccinium bracteatum Thunb. and V. virgatum) and scions (V. corymbosum), found high survival rates in grafted plants and no signs of incompatibility between rootstock and scion, and speculated that blueberry grafting is possible for several varieties. Xu et al. (2014) found that ‘Sharpblue’ SHB grafted onto Wufanshu (V. bracteatum) had greater plant height and yield than nongrafted plants; however, no differences were found for berry weight or fruit quality.
Vaccinium arboreum is a wild blueberry relative that is native to the southeastern United States. It grows in calcareous, sandy or sandy clay upland soils and tolerates soil pH up to 6.5, low organic matter (Lyrene, 1997), low Fe availability, and nitrogen in the form of nitrate (NO3−) (Darnell and Hiss, 2006)—conditions that V. corymbosum tolerates poorly. Studies have found that NO3− and Fe uptake are greater in V. arboreum compared with V. corymbosum (Darnell and Hiss, 2006; Poonnachit and Darnell, 2004), especially at high pH (Darnell, unpublished data). In addition, V. arboreum also has a deep root system, which increases drought tolerance (Lyrene, 1997).
Because of the wide soil adaptation and poor overall fruit quality of V. arboreum (Vander Kloet, 1988), previous studies have examined the use of V. arboreum as a rootstock for highbush blueberry. Ballington (1996) compared ‘Premier’ rabbiteye blueberry own-rooted vs. grafted onto V. arboreum and found that grafted plants had greater yield and fruit size than own-rooted plants, with no differences in fruit quality. In a later study also using V. arboreum, Ballington (1998) reported “superior” plant growth, fruit yield, and berry size of grafted ‘Premier’ compared with own-rooted plants; however, no significant differences were found in fruit flavor.
Vaccinium arboreum has the potential for use as a rootstock to expand blueberry production to soils with higher pH and low organic matter. Although there is information about soil adaptation of V. arboreum, as well as information on the use of V. arboreum as a rootstock in research trials, little is known about its performance when used as a rootstock in commercial field settings, under amended vs. nonamended soil conditions.
The hypothesis tested in this research was that SHB plants grafted onto V. arboreum rootstocks exhibit increased growth and yield compared with own-rooted SHB plants when grown in nonamended soil. The specific objectives were to evaluate the effects of soil treatment (pine bark–amended or nonamended soil) and root system (own-rooted or grafted plants) on plant growth, flower bud number, fruit number per inflorescence, fruit yield, berry weight, and fruit quality in two SHB cultivars.
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