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Experiments were established to evaluate the suitability of growing degree-day (GDD, Tbase = 0 8C) models for predicting emergence, tip dieback, and flowering of lowbush blueberry ramets in Nova Scotia, Canada. Data for model development were collected from quadrats established in several non-bearing and bearing blueberry fields throughout the dominant blueberry production areas in northern and central Nova Scotia. Blueberry ramets emerged between 222 and 265 GDD (6 May to 14 May) and reached 90% emergence between 619 and 917 GDD (7 June to 5 July). Emergence continued to slowly increase until late summer or early fall. Tip dieback began between 598 and 792 GDD (14 June to 21 June) and duration of this phase depended on whether lateemerging ramets developed to tip dieback. A four-parameter Weibull and a three-parameter Gompertz equation adequately explained cumulative blueberry ramet emergence and cumulative ramets at tip dieback as functions of GDD in the non-bearing year, respectively. The four-parameter Weibull function also explained the relationship between cumulative flowering ramets and GDD in the bearing year. Flowering ramets were first observed between 376 and 409 GDD (19 May to 30 May) in the bearing year. Model predictions for initiation of emergence, tip dieback, and flowering were 243, 692, and 389 GDD, respectively. Models were validated with independent data sets collected throughout northern and central Nova Scotia. The relationship between the percentage of open flowers on individual ramets and GDD in the bearing year was well described by a Gaussian model at two sites with a predicted peak number of open flowers between 552 and 565 GDD. The lowbush blueberry is a native, perennial berry species in Nova Scotia. Commercial fields are not planted but are developed on abandoned farmland or cleared woodland where native blueberry stands already exist (Agriculture and Agri-Food Canada, 2005; Barker et al., 1964). These stands are composed of distinct and variable clones that spread by underground rhizomes (Glass and Percival, 2000; Hall and Aalders, 1961). The crop is managed on a 2-year cycle. Plants are pruned to ground level to promote vegetative growth in the first, or non-bearing, year. Flowering, fruit development, and harvest occur in the second, or bearing, year. Growth and development in the non-bearing year consists of ramet emergence from underground rhizomes and growth of these ramets until abortion of the apical meristem (tip dieback) stimulates development of floral buds in late summer and fall (Aalders and Hall, 1964; Barker and Collins, 1963; Hall and Ludwig, 1961). Ramets generally emerge rapidly from mid-May until late June (Eggert, 1957). Initiation of emergence is largely determined by temperature and date of spring pruning (Eaton and White, 1960; Trevett, 1962). Modern blueberry production systems in Nova Scotia typically use fall pruning after harvest in the bearing year instead of spring pruning in the non-bearing year as described by Trevett (1962). Because pruning does not actually occur in the non-bearing year, initiation of ramet emergence in this phase of the production cycle in Nova Scotia is likely regulated primarily by air or soil temperatures in early spring. Vegetative growth of emerged ramets is most rapid in warm temperatures and long days (Hall and Ludwig, 1961; Kender, 1967). Tip dieback is not thought to be induced by external stimuli (Barker and Collins, 1963), but temperature is an important factor affecting apical shoot abortion in some woody species (Millington, 1963; Suzuki, 1991). Abortion of the apical meristem in lowbush blueberry is a prerequisite to flower bud initiation in the non-bearing year (Bell, 1950; Bell and Burchill, 1955). Therefore, the time period between ramet emergence and tip dieback is crucial in terms of establishment of both crop density and biomass that will support the development of flower buds in the bearing year. Extensive stands of lowbush blueberries in certain regions of Nova Scotia regularly fail to produce acceptable fruit yields, primarily as a result of limitations imposed on growth and development by low temperatures in both the non-bearing and bearing years (Hall et al., 1964, 1970). Therefore, rates of ramet emergence and development to tip dieback in the non-bearing year may simply be a function of temperature within a given growing season. Growth and development in the bearing year are comprised of leaf expansion and flowering in spring (Bell and Burchill, 1955; Hall et al., 1979) and fruit development and harvest by late summer. Flowering is delayed by cool temperatures (Bell, 1953), but flower buds typically swell in early May when air temperatures exceed 10 C for more than 4 d (Hall et al., 1979). The bloom period is from 1 to 3 weeks (Chiasson and Argall, 1996; Wood, 1961) and timely movement of honeybee hives during bloom is important for ensuring adequate pollination. Most management practices in lowbush blueberry production are governed by the timing of ramet emergence or specific ramet growth stages. Blueberry growers rely on calendar dates (e.g., Aalders et al., 1972) or general references to crop development stage (Anonymous, 2011) for timing these management practices. Relating blueberry ramet emergence and growth stage to GDDs should reduce the variability in timing of these processes observed between field locations and growing seasons (Bell, 1950, 1953; Wood, 1961). GDD models have been useful for predicting harvest dates of highbush blueberry cultivars in Michigan (Carlson and Hancock, 1991) and cumulative flowering of ‘rabbiteye’ blueberries (NeSmith and Bridges, 1992). Received for publication 1 Mar. 2012. Accepted for publication 15 May 2012. Funding for this work was provided by the Natural Sciences and Engineering Research Council Industrial Postgraduate Scholarship and DuPont. We acknowledge the assistance of summer students in the Vegetation Management Research Program at the Nova Scotia Agricultural College, Truro, Nova Scotia, Canada. We also acknowledge Bragg Lumber Company and Purdy Resources for providing field sites. This article is a portion of a thesis submitted by Scott White in partial fulfillment of the requirements for a PhD degree at the University of Guelph. To whom reprint requests should be addressed; e-mail nboyd@nsac.ca. 1014 HORTSCIENCE VOL. 47(8) AUGUST 2012 Similar models have not been developed for lowbush blueberries in Nova Scotia, but GDD models to predict maturity of perennial forage crops in this region have been successful (Bootsma, 1984). The effects of weather conditions have proven inconsistent in terms of predicting lowbush blueberry fruit production in Eastern Canada (Hall et al., 1982), but no attempts have been made to model growth and development of this species as functions of GDDs. The lowbush blueberry acreage in Nova Scotia increased by 31% between 1992 and 2003 and is expected to exceed 40,000 acres by 2013 (Strik and Yarborough, 2005). This increased acreage, in combination with management practices driven by the growth and development of a native species in response to environmental conditions, gives immediate applicability to predictive models in crop management. The objectives of this study were to 1) develop GDD models to predict emergence and tip dieback of lowbush blueberry ramets in the non-bearing year and flowering in the bearing year; and 2) validate these models with independent data sets collected throughout northern and central Nova Scotia. Materials and Methods Site selection. Blueberry ramet emergence and development were monitored in six commercial lowbush blueberry fields in Nova Scotia between 2009 and 2011. Site locations were chosen based on proximity to large blueberry acreages in northern and central Nova Scotia. All sites were initially established in fields that had been pruned by flail mowing at the end of the previous season. Two sites were established in the non-bearing year near Collingwood in 2009 (Purdy and Wyvern) with an additional non-bearing year site near Collingwood in 2010 (Pigeon Hill) (Table 1). Non-bearing year sites were also established near Londonderry in 2010 and Mount Thom and North River in 2011 (Table 1). Sites established in non-bearing fields in 2009 and 2010 were retained for bearing year data collection in 2010 and 2011, respectively (Table 1). Weather data. Hourly air temperature at each site was monitored using temperature loggers (HOBO Pro V2; Onset Computer Corporation, Cape Cod, MA). Data loggers were attached to wooden stakes and were located 0.5 m above the soil surface. Regional air temperature data from the nearest Environment Canada weather station were used to supplement field-based temperature data so that GDDs could be calculated starting on 1 Apr. (day of year 91). Cumulative GDDs were calculated using the formula:

hortscience

Growing Degree-day Models for Predicting Lowbush Blueberry (Vaccinium angustifolium Ait.) Ramet Emergence, Tip Dieback, and Flowering in Nova Scotia, Canada