Anisogramma anomala (Peck) E. Müller is the incitant of the disease eastern filbert blight (EFB), which causes severe cankering, branch dieback, and the death of most European hazelnuts, Corylus avellana L. It is an obligate biotrophic pyrenomycete native to a wide geographic area east of the Rocky Mountains where it is found associated with its much more tolerant natural host, Corylus americana Marshall (Fuller, 1908; Weschcke, 1954; Farr et al., 1989; Johnson and Pinkerton, 2002). Anisogramma anomala is known to reproduce only by ascospores and has a multiyear lifecycle that requires the host plant to cycle through a dormancy period after infection to express disease symptoms (Stone et al., 1992; Pinkerton et al., 1993). EFB is believed to be the primary reason commercial hazelnut orchards were never successfully established in the eastern United States (Barss, 1921; Thompson et al., 1996). Alternatively, hazelnut production thrived in western Washington and Oregon due to being outside the native range of A. anomala, as well as having a climate well suited for European cultivars (Thompson et al., 1996). Currently, the top hazelnut-producing country in the world is Turkey, which generally produces 60% to 70% of the world's crop (world total was 776,890 tons in 2007). Turkey is followed by Italy, which produces around 17% of the world's total, and then the United States, which produces less than 5% (FAOStat, 2009). Ninety-nine percent of the United States hazelnut crop is produced in the Willamette Valley of Oregon (Mehlenbacher and Olsen, 1997).
The destructive nature of EFB was known and quarantine laws were established in the early 1900s to prevent its introduction into the western United States (Barss, 1921; Lagerstedt, 1979). Despite these precautions, EFB was discovered in a commercial orchard in southwest Washington in the late 1960s (Davison and Davidson, 1973). Since then, it has spread southward throughout the entire Willamette Valley of Oregon where it threatens the long-term viability of the U.S. hazelnut industry (Mehlenbacher, 2005). EFB control measures have been developed, including fungicide sprays and therapeutic pruning; however, they are expensive, yield-reducing, and not entirely effective (Johnson et al., 1996; Julian et al., 2008). Therefore, the development of cultivars with genetic resistance to the pathogen appears to be the most effective means for control (Mehlenbacher, 1994). Breeders at Oregon State University (OSU; Corvallis, OR) have been working on this objective since 1976, when the first controlled pollinations were made with the obsolete pollinizer ‘Gasaway’. ‘Gasaway’ was shown to carry a dominant allele at a single locus that confers complete resistance to EFB (Mehlenbacher and Thompson, 1991a). Since its discovery, the ‘Gasaway’ source of resistance has been widely used in the OSU hazelnut breeding program, leading to the release of EFB-resistant pollinizer cultivars (Mehlenbacher and Thompson, 1991b; Mehlenbacher and Smith, 2004) and cultivars with kernel quality suitable for commercial production (Mehlenbacher et al., 2007, 2009).
Although the ‘Gasaway’ allele continues to provide a high level of EFB resistance in the Pacific Northwest (PNW), breeders and plant pathologists are concerned with the long-term durability of using only one source of single-gene resistance (Osterbauer, 1996; Coyne et al., 1998; Pinkerton et al., 1998; Lunde et al., 2006). Adding to this concern is the question whether the genetic diversity of A. anomala found in the PNW is less than that found across its native range because A. anomala in the PNW is believed to trace back to a single point introduction in southwest Washington (Gottwald and Cameron, 1980; Johnson et al., 1996). Based on the pathogen's wide native range and sexual reproduction, it is likely that genetic diversity and pathogenic variation exists in the species. Consequently, A. anomala may exist outside the PNW with the ability to overcome ‘Gasaway’ resistance. In an attempt to investigate this question, Osterbauer (1996) subjected trees of VR6–28, an OSU selection containing the ‘Gasaway’ allele, to greenhouse inoculations with A. anomala collected from across the eastern United States and Canada. At the conclusion of her experiment, none of the isolates were able to incite typical EFB on VR6–28. However, concern was raised when isolates from Minnesota and Ontario caused development of sunken non-sporulating lesions. While these lesions did not form the conspicuous “football-shaped” stromata typical of the fungus, their presence suggested possible variability in the pathogen. Thus, Osterbauer's findings reinforced the need to maintain quarantine regulations in the PNW and to continue the search for additional sources of genetic resistance to A. anomala.
Lunde et al. (2006) investigated the inheritance of EFB resistance in progeny derived from ‘Zimmerman’, which carries the ‘Gasaway’ resistance allele (Gökirmak et al., 2009). Along with reporting that ‘Zimmerman’ typically confers resistance to progeny in a 3 resistant:1 susceptible ratio, which is different from the 1:1 ratio observed in progeny of ‘Gasaway’, they found several seedlings that expressed small sunken lesions, but with no sporulation. These seedlings amplified the RAPD markers UBC152–800 and UBC268–580 that are closely linked to the ‘Gasaway’ allele (Mehlenbacher et al., 2004), signifying its presence. Their findings suggest that although progeny may not be “completely resistant” or “immune” as previously reported, ‘Gasaway’ and ‘Zimmerman’ still transmit a very high level of resistance to their offspring. In addition, their work suggests that further investigation is needed to elucidate the response of the ‘Gasaway’ allele when expressed in different genetic backgrounds.
Since the discovery of ‘Gasaway’, additional sources of resistance to A. anomala have been identified at OSU, including several C. avellana cultivars and selections as well as other Corylus species and interspecific hybrids (Coyne et al., 1998, 2000; Lunde et al., 2000; Chen et al., 2005, 2007; Sathuvalli, 2007). However, these new sources were identified by challenging them with A. anomala originating in the PNW, and their response to isolates originating across the pathogen's native range is largely unknown. The objectives of this study were to develop a better understanding of A. anomala's pathogenic variation and to assess the broader response of available sources of host resistance by challenging a diverse group of cultivars and selections shown to be completely resistant in Oregon with A. anomala isolates collected across the pathogen's native range.
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