The 12,000 ha of the European hazelnut, Corylus avellana L., in Oregon's Willamette Valley produce 98% of the United States crop and 3% to 5% of the world crop (FAO Production Yearbook, 2003).The Oregon hazelnut industry is seriously threatened by eastern filbert blight (EFB) incited by the pyrenomycete Anisogramma anomala (Peck) E. Müller. The fungus is endemic on the American hazelnut (C. americana Mill.) in eastern North America. On susceptible European cultivars, it causes severe cankers, rapid yield loss, and eventually tree death in 5 to 12 years if control measures are not practiced (Pinkerton et al., 1993). Control practices include pruning of infected branches and fungicide applications. However, because of the expense of fungicide applications and the dramatic yield loss incited by severe pruning of cankers, genetic resistance is the most desirable and economic means of disease control (Mehlenbacher, 1994). Therefore, developing cultivars resistant to EFB is a goal of the Oregon State University (OSU) hazelnut breeding program.
Complete resistance to EFB was first discovered in the obsolete pollinizer ‘Gasaway’ (Cameron, 1976). Genetic studies showed that complete resistance is conferred by a single dominant gene (Mehlenbacher et al., 1991). ‘Gasaway’ has been the major source of resistance used in the OSU breeding program. However, ‘Gasaway’ has low yields and undesirable nut and kernel characteristics, thus requiring considerable effort to combine the resistance gene with the many attributes required of a commercially acceptable cultivar. Furthermore, concern exists about the durability of a single resistance gene because a new race of A. anomala could potentially overcome it (Johnson et al., 1996). The identification of additional sources of genetic resistance would be desirable.
Inoculation of European hazelnut cultivars with the EFB pathogen has revealed additional sources of complete resistance. ‘Zimmerman’, an uninfected tree identified in a hedgerow near a severely infected orchard near Boring, Ore. (Pinkerton, pers. comm.), remained free of disease after greenhouse inoculations (Coyne, 1995). ‘Closca Molla’ and ‘Ratoli’, both superior in many horticultural respects to ‘Gasaway’, displayed no symptoms of EFB after greenhouse inoculations (Lunde et al., 2000). Complete resistance has also been detected in numerous accessions of Corylus species and interspecific hybrids (Coyne et al., 1998; Lunde et al., 2000).
The fungus A. anomala has a 2-year life cycle that includes an incubation period of 12 to 14 months before symptoms are expressed (Gottwald and Cameron, 1980; Johnson et al., 1994; Pinkerton et al., 1995). Thus, evaluation by observing canker development on the field is a slow process. An indirect enzyme-linked immunosorbent assay (ELISA) after greenhouse inoculation shortens the detection time to 6 months and offers a reliable method for evaluation of genotypes for complete resistance (Coyne et al., 1996).
In this study, 58 hazelnut accessions from the collections of the OSU hazelnut breeding program and the U.S. Department of Agriculture, Agricultural Research Service National Clonal Germplasm Repository in Corvallis, Ore., were evaluated for response to EFB inoculation. In a second study, we exposed potted trees to an infection source to quantify susceptibility of genotypes that had been reported as completely resistant during earlier greenhouse tests.
Coyne, C.J. 1995 Genetic resistance to eastern filbert blight Department of Horticulture, Oregon State University Corvallis, Ore PhD diss.
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