Proper irrigation management is critical for profitable blueberry production in many commercial growing regions, including the Pacific northwestern United States where ≈24,000 t of the fruit is produced annually (U.S. Dept. of Agr., 2006). Without enough irrigation, g S declines quickly in blueberry (Améglio et al., 2000), leading to less CO2 assimilation (Erb et al., 1991) and lower growth and production (Anderson et al., 1979; Cameron et al., 1989). However, too much irrigation often reduces root function (Davies and Wilcox, 1984), increases soil erosion and nutrient leaching (Starr et al., 2004), and potentially causes problems with fungal pathogens such as Botrytis cinerea Pers. and Phytophthora cinnamomi Rands (Caruso and Ramsdell, 1995; de Silva et al., 1999). Typically, 25 to 50 mm of water per week is recommended for blueberry (Strik et al., 1993), although actual water requirements will likely vary not only with the environmental conditions, but also with the cultural practices under which the crop is grown. Two such practices that have received little attention in the literature but have considerable potential to influence plant water use include planting density and choice of cultivar.
Many growers these days are planting highbush blueberry at higher densities (Strik and Yarborough, 2005) to increase yield per hectare and lower production costs (Moore et al., 1993; Strik and Buller, 2002, 2005). It is reasonable to expect that plants spaced close together will require more irrigation in a given area than those spaced further apart because higher plant populations generally produce more leaves and canopy cover per hectare (Holzapfel et al., 2004; Storlie and Eck, 1996). Yuste et al. (2004) recently found that increasing density in 5-year-old grapevines (Vitis vinifera L.) from 2645 to 3953 plants/ha increased water use by up to 6.5% with only a 6.2% increase in total dry matter. They attributed the increase primarily to more leaf development at high density. Depending on how much leaf production is affected by planting density in blueberry, growing at higher density could substantially increase its water requirements. In a 4-year study on ‘Duke’, ‘Bluecrop’, and ‘Elliott’ blueberry, Strik and Buller (2005) reported that total dry weight per hectare of wood, crown, and roots (but no leaves) was 218% to 562% higher when plants were spaced at 0.45 m than at 1.2 m within rows.
Water requirements often differ among cultivars (e.g., Buchanan et al., 1978). These differences may be the result of inherent morphologic and physiological features as well as phenologic factors such as timing of fruit development. Water use appears to be especially high during fruit ripening in blueberry. Mingeau et al. (2001) found that almost 55% of the total seasonal water requirements of ‘Bluecrop’ occurred in June and July during fruit ripening; once fruit were picked, its water requirements decreased to nearly half. Therefore, cultivars that ripen early in the season may have very different water requirements than those that ripen late in the season. Haman et al. (1997) reported that daily water use in an early-season cultivar of highbush blueberry was much higher than in two late-season cultivars of rabbiteye blueberry (Vaccinium ashei Reade). Identifying when and why water is most needed by different cultivars may help growers to better budget their irrigations and maximize production efficiency.
The objective of the present study was to determine the effects of cultivar and planting density on water requirements during irrigation of highbush blueberry. Three cultivars with different fruit-ripening periods were chosen for study. Each cultivar was planted either at a high-density spacing of 0.45 m apart within rows or at a more traditional spacing of 1.2 m.
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