Hazelnuts (Corylus avellana) are a major tree nut crop ranking fifth in world production behind cashews (Anacardium occidentale), almonds (Prunus dulcis), walnuts (Juglans regia), and chestnuts (Castanea sp.). The top hazelnut-producing country in the world is Turkey, which produces ≈70% of the world’s crop (888,328 t in 2010). Turkey is followed by Italy (≈15%) and the United States (≈5%) (Food and Agriculture Organization of the United Nations, 2012), where production occurs primarily in the Willamette Valley of Oregon. Cultivated forms of C. avellana, of which several hundred have been described, produce the largest and highest quality nuts of the genus. Recent taxonomic revisions suggest that Corylus holds 11 to 13 polymorphic species placed in four subsections (Erdogan and Mehlenbacher, 2000a, 2000b; Mehlenbacher, 1991; Thompson et al., 1996).
Although current regions of commercial hazelnut production have mild, Mediterranean-like climates, attempts have been made since colonial times to produce hazelnuts in the eastern United States with little recorded success. It was eventually understood that the fungal disease eastern filbert blight (EFB), caused by Anisogramma anomala, an obligate biotrophic ascomycete in the order Diaporthales, was the main limiting factor in this region (Fuller, 1908; Halsted, 1892; Johnson and Pinkerton, 2002; Thompson et al., 1996). Eastern filbert blight is found naturally occurring on the wild American hazelnut, C. americana, which is native to a wide swath of eastern North America, from Maine in the northeast to Minnesota and southern Manitoba in the northwest, extending south to northern Florida, and westward as far as eastern Oklahoma (Drumke, 1964; Gleason and Cronquist, 1998). Although EFB typically results in inconsequential damage to C. americana (Capik and Molnar, 2012; Fuller, 1908; Weschcke, 1954), in C. avellana, the disease causes perennial cankers, branch dieback, and eventually death of most plants (Johnson and Pinkerton, 2002). Previously, EFB was only found east of the Rocky Mountains. Unfortunately, in the 1960s, it was inadvertently spread west and can now be found throughout the Willamette Valley, where its control measures add considerable expense to commercial-scale hazelnut production (Davison and Davidson, 1973; Johnson et al., 1996; Julian et al., 2008, 2009).
In comparison with cultivated forms of C. avellana, C. americana produces very small nuts (typically under 1.5 cm in diameter) with thick shells as well as fleshy husks (involucres) that tightly clasp the nuts. This tight involucre creates an impediment to harvesting because nuts do not fall freely to the ground at maturity. Furthermore, their extensive production of basal sprouts (suckers) is detrimental to standard orchard management in the United States, where trees are maintained with single stems. Despite these limitations, positive traits such as EFB resistance, cold-hardiness, and stress tolerance exist in the species (Capik and Molnar, 2012; Mehlenbacher, 1991; Molnar, 2011a). It is also cross-compatible with C. avellana in both directions (Erdogan and Mehlenbacher, 2000b), allowing it to act as a donor of these traits in a genetic improvement program. Both C. avellana and C. americana exhibit sporophytic incompatibility (Erdogan and Mehlenbacher, 2001; Mehlenbacher, 1997).
Starting in the early 1900s, efforts were made to hybridize C. americana and C. avellana to develop better-adapted, EFB-resistant plants. The pioneer was J.F. Jones of Lancaster, PA, who in 1919 crossed the local Pennsylvania C. americana selection ‘Rush’ with several C. avellana cultivars including Barcelona, Cosford, Daviana, Italian Red, and DuChilly. His work was continued by C.A. Reed of the U.S. Department of Agriculture (USDA) at Beltsville, MD, and G.H. Slate of the New York Agricultural Experiment Station in Geneva, NY, both of whom used ‘Rush’ in their hybrid breeding programs (Crane et al., 1937; Reed, 1936; Slate, 1961). Additional hybrid breeding work was performed by S.A. Graham of Ithaca, NY, using seedlings of the ‘Rush’ hybrids. Graham also used C. americana ‘Winkler’ (from Iowa) in crosses with C. avellana in his breeding program (Graham, 1936; Slate, 1961, 1969).
Further breeding using ‘Winkler’ was conducted by Weschcke (1954) in River Falls, WI. ‘Winkler’, along with several wild selections from the surrounding area, was crossed with cold-hardy selections of C. avellana, although detailed parental records are not available. Germplasm from Weschcke’s program was later used at Badgersett Research Corporation, Canton, MN, which also included plant material related to ‘Rush’ and other wild C. americana and C. cornuta (beaked hazelnut) accessions (Rutter, 1987, 1991). Seedlings from Badgersett have been planted across many states in the upper Midwest region of the United States. Plants were purchased from Badgersett by the National Arbor Day Foundation (NADF), Nebraska City, NE, to establish their 9-acre orchard, from which many thousands of subsequent seedlings, also derived from open pollination, have been further distributed around the United States and Canada (Hammond, 2006; Molnar, 2011b).
Eastern filbert blight-resistant hybrids were successfully developed from this body of early work, as discussed in Capik and Molnar (2012), Chen et al. (2007), Coyne et al. (1998), Lunde et al. (2000), and Rutter (1991), and clones or seedlings from these early efforts are still available today. However, despite the development of these resistant plants, little has been documented on the inheritance and expression of EFB resistance in seedlings from interspecific cross of C. americana and C. avellana. In fact, it was reported that Weschcke (1970) and Graham (Slate, 1961, 1969) eventually lost much of their breeding material to EFB, which provides some insight into the complex nature of the system. Current efforts are complicated by the lack of genetic diversity used in past breeding. Sathuvalli and Mehlenbacher (2011) used simple sequence repeat marker analysis to characterize 67 C. americana × C. avellana hybrid hazelnut accessions held in the USDA Agricultural Research Service National Clonal Germplasm Repository, Oregon State University (OSU) (both in Corvallis, OR), and NADF collections. They discovered that nearly all of them grouped with plants related to ‘Rush’ or the ‘Winkler’/Weschcke hybrids. Furthermore, of the 23 hybrid accessions examined for response to EFB in New Jersey, only 13 remained free of signs or symptoms of EFB, and all of these traced back to the ‘Rush’ or ‘Winkler’/Weschcke hybrids (Capik and Molnar, 2012). Thus, the inheritance of EFB resistance in seedlings from crosses of C. americana and C. avellana remains unclear.
Besides the early hybrid breeding work described above, hazelnut breeding efforts to date have been focused primarily on improving nut and kernel characteristics and increasing yield of Corylus avellana grown in existing production regions (Thompson et al., 1996). The world’s largest hazelnut breeding program has been ongoing at OSU since the late 1960s (Mehlenbacher, 1994). With the introduction of A. anomala in the Willamette Valley, breeding for resistance to EFB became an additional objective of the OSU program. The early identification of C. avellana ‘Gasaway’, a cultivar transmitting a dominant gene for EFB resistance (Mehlenbacher et al., 1991), has supported the use of intraspecific hybridization as a breeding option, leading to the recent release of improved, EFB-resistant C. avellana cultivars (Mehlenbacher et al., 2007, 2009, 2011). Furthermore, a number of other additional C. avellana sources of EFB resistance have also been identified at OSU (Chen et al., 2005, 2007; Coyne et al., 1998; Lunde et al., 2000) that are being incorporated into intraspecific breeding efforts (S.A. Mehlenbacher, personal communication).
Today, interest in developing hazelnuts as a commercial crop for regions outside of Oregon is rising (Braun et al., 2009, 2011; Hybrid Hazelnut Consortium, 2012; Molnar, 2011b; Olsen, 2011; Upper Midwest Hazelnut Development Initiative, 2012). In these regions, especially the Midwest and Upper Midwest, hybrid hazelnuts will be important not only for their resistance to EFB, but also their ability to tolerate cold temperatures. Hybrid plants adapted to these regions have been identified that are both EFB-resistant and high-yielding (Capik and Molnar, 2012; Hammond, 2006; Rutter, 1987). However, their nut size and kernel characteristics are generally poor compared with cultivars of C. avellana (Molnar, unpublished data; Xu and Hanna, 2010). Consequently, further breeding work is necessary to combine the cold-hardiness and EFB resistance from C. americana with the excellent nut and kernel quality of C. avellana. The lack of knowledge of inheritance of EFB resistance in crosses of C. americana with susceptible C. avellana makes reaching this breeding goal very challenging.
To gain a better understanding of the inheritance of EFB resistance from C. americana, 17 controlled crosses were made using pollen of susceptible C. avellana with resistant C. americana and advanced-generation hybrids. The seedlings were planted in the field in New Jersey. The plants were evaluated for their response to the disease after five years.
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