European hazelnut (Corylus avellana) is an important world agricultural crop, ranking fifth in overall tree nut production. Turkey produced 430,000 t of hazelnuts in 2011, accounting for ≈58% of total world production (742,997 t in 2011), followed by Italy (128,940 t), the United States [34,927 t (≈5%)], Azerbaijan (32,922 t), and the Republic of Georgia (31,100 t) (Food and Agriculture Organization of the United Nations, 2013). Commercial production in the United States takes place almost solely in the Willamette Valley of Oregon, with 99% of U.S. hazelnut crop originating there (Mehlenbacher and Olsen, 1997).
The presence of the disease EFB, caused by Anisogramma anomala, has historically prevented the commercial production of hazelnuts across much of eastern North America (Fuller, 1908; Thompson et al., 1996). A. anomala is an ascomycetous fungus native to east of the Rocky Mountains, where it is harbored by its natural host, the wild american hazelnut [Corylus americana (Johnson and Pinkerton, 2002)]. Unfortunately, whereas the american hazelnut is generally highly tolerant of EFB, most European cultivars are highly susceptible (Capik and Molnar, 2012; Pinkerton et al., 1993; Thompson et al., 1996). The absence of this pathogen combined with a mild climate allowed hazelnut production to flourish in the Pacific Northwest for nearly a century (Thompson et al., 1996). However, EFB was inadvertently introduced into southwestern Washington in the late 1960s. The resulting disease devastated hazelnut orchards in the state, as control methods were not yet established (Cameron, 1976; Davison and Davidson, 1973).
Although it was later learned that scouting for cankers, therapeutic pruning, and copious fungicide applications could keep the disease under control (Johnson et al., 1996), due to their associated expenses, the most cost-effective and sustainable approach for long-term management was considered to be using and developing genetic resistance to the pathogen (Julian et al., 2009; Mehlenbacher, 1994; Thompson et al., 1996). In the 1970s, ‘Gasaway’ european hazelnut, an obsolete, late-blooming pollenizer, was found to be resistant to EFB. It was later shown to transmit this resistance to its offspring in a manner indicative of a dominant allele at a single locus (Mehlenbacher et al., 1991). ‘Gasaway’ has since been widely used in the Oregon State University (OSU) breeding program. To date, a number of cultivars carrying the gene have been released, including Yamhill (Mehlenbacher et al., 2009), Jefferson (Mehlenbacher et al., 2011), and Dorris (Mehlenbacher et al., 2013), as well as various pollenizers (Mehlenbacher and Smith, 2004; Mehlenbacher and Thompson, 1991; Mehlenbacher et al., 2012). These new cultivars have reinvigorated the hazelnut industry in Oregon, which, after decades of decline, has been expanding at a rate of about 1200 ha per year for the past 5 years (S.A. Mehlenbacher, personal communication).
In addition to ‘Gasaway’, a number of other sources of EFB resistance have also been identified at OSU and Rutgers University and are currently being used in breeding (Capik et al., 2013; Chen et al., 2007; Lunde et al., 2000; Molnar et al., 2009, 2010; Sathuvalli et al., 2010). Capik and Molnar (2012) examined the disease response of 190 clonal accessions of hazelnut, including multiple hazelnut species and interspecific hybrids from a wide diversity of origins, in New Jersey. While some plants previously reported as resistant to EFB in Oregon developed disease, including ‘Gasaway’ and some of its offspring, a large number of the accessions remained resistant or highly tolerant to EFB in New Jersey over more than 10 years of exposure.
Today, with access to a multitude of EFB-resistant cultivars and breeding selections, one of the major impediments to developing a commercial hazelnut industry in parts of eastern United States has been greatly diminished. As such, it is important to examine other factors critical to consistent hazelnut production in this region. Because nut production is fully dependent on successful cross pollination, one factor of vital importance is flowering—a topic poorly studied and documented for hazelnut in the eastern United States.
Hazelnuts are monoecious, wind-pollinated, and self-incompatible. Reproduction is restricted by a sporophytic self-incompatibility system, which is controlled by a single locus with various S-alleles determining compatibility (Mehlenbacher, 1997; Olsen et al., 2000; Thompson, 1979). Over 30 S-alleles have been identified to date (S.A. Mehlenbacher, personal communication). Dominance or codominance of the alleles is expressed in the pollen, whereas all known S-alleles are codominant in the pistil (Mehlenbacher, 1997; Mehlenbacher and Thompson, 1988).
Hazelnuts are also dichogamous. Male (catkins, staminate) and female (pistillate) flowers have different chilling requirements to break dormancy, with catkins typically having lower chilling requirements than the female flowers [ranges of 100–860 h and 290–1550 h, respectively (Mehlenbacher, 1991)]. Normal flowering occurs in winter before vegetative budbreak, over a range of dates depending on the genotype, geographic location, and year. In traditional hazelnut-producing regions, most of which are primarily located adjacent to large bodies of water, which moderate their climate hazelnuts can bloom over an extended period from early December through March. In colder regions, bloom is compressed over a much shorter time frame in late winter or early spring in response to warming temperatures (Črepinšek et al., 2012; Germain, 1994; Olsen et al., 2000; Piskornik et al., 2001; Solar and Stampar, 2009; Thompson et al., 1996). Plants are typically either protandrous or protogynous depending on their genetic background and the climate of the region they are grown in. In regions with mild climates, protandry seems to be more common, whereas in regions with long, cold winters, protogynous or homogamous flowering typically occurs (Germain, 1994; Mehlenbacher, 1991; Olsen et al., 2000; Piskornik et al., 2001).
Female flowers are unique in that, if not pollinated, stigmatic surfaces can stay receptive to fertilization for up to 3 months (Thompson, 1979). When compatible pollen reaches a receptive female flower, the pollen grain germinates and develops a germ tube, which grows down to the base of the style where the sperm cell subsequently travels and then rests. At this time, the ovary is not yet fully formed. After ovary formation is complete, usually in late spring, the pollen tubes begin to grow again and fertilization occurs (Beyhan and Marangoz, 2007).
Differences in cold tolerance have also been reported for male and female flowers. In controlled freezing tests, Hummer et al. (1986) showed that female flowers of some european hazelnut cultivars could survive temperatures below −40 °C. However, catkins were shown to be injured at warmer temperatures. The most cold-tolerant, fully dormant catkins tested were hardy to −35 °C, although some cultivars (e.g., Ennis, Tonda Romana) displayed injury at temperatures reaching only −15 °C (Hummer et al., 1986). Catkins elongating before anthesis or fully elongated and shedding pollen were not tested. However, it should be noted that past experience of the authors suggests that elongating or shedding catkins are much more susceptible to cold damage than fully dormant ones (data not shown). Thus, in cold regions with unpredictable winter climates, such as that found across the mid-Atlantic region of the eastern United States, catkin survivability can present a significant challenge for consistent nut production. Once chilling requirements are met, the occurrence of atypical warm winter weather can signal the catkins to elongate prematurely, making them more sensitive to cold damage. This problem can be exacerbated by high winter winds, not uncommon in the eastern United States, which appear to cause desiccation injury (Reed and Davidson, 1958; Slate, 1933). Past reports suggest that hazelnuts may appear to thrive in the eastern United States but fail to produce nuts because of catkin damage and lack of pollination (MacDaniels, 1964).
The density of pollenizers in orchards around the world ranges from 3% to 30%, with 10% pollenizer density as the standard in Oregon (Olsen et al., 2000). Recent recommendations in Oregon include planting at least three different pollenizers that shed pollen at different times during the period that female flowers of the main crop cultivar are receptive to ensure consistent orchard pollination (Mehlenbacher et al., 2009). In respect to meeting this recommendation, very little research has been done to document how fluctuating winter temperatures affect flowering phenology of hazelnut in the eastern United States. For example, over a 10-d period from 22 Dec. 2008 to 1 Jan. 2009, winter temperatures in New Brunswick, NJ, varied from −11.1 to 18.9 °C, then back to −8.3 °C (National Climate Data Center, 2013). Knowledge of how hazelnuts respond under these conditions is vital to developing orchards that produce nuts on a consistent yearly basis.
The objective of this study was to evaluate the flower and budbreak phenology of 19 different EFB-resistant and EFB-tolerant hazelnut cultivars and breeding selections over 4 years to better understand their response to New Jersey’s climate and to provide a benchmark for selecting suitable pollenizers and breeding parents in the future.
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Pistillate, staminate, and vegetative bloom dates for 19 accessions of hazelnut in New Jersey.