American beech (Fagus grandifolia) is a common, slow-growing deciduous tree native to most of the eastern United States. American beech is monoecious and generally outcrossing (Koch et al., 2010a), but root sprouts are also a common method of regeneration in the forest and may lead to clonal clumps within stands. Mature trees on good sites can reach 50 cm in diameter at breast height and attain heights of over 24 m and generally produce good nut crops on 2- to 8-year intervals after reaching 40 years of age (Tubbs and Houston, 1990). American beech provides food and habitat for over 40 different species of birds and mammals and is an important component of hardwood and mixed hardwood forests in eastern North America (Gysel, 1971; McCullough et al., 2001). It is a significant tree species in urban forests, identified as the species of greatest importance in Washington, DC (Nowak et al., 2006) and listed as one of the most important park trees in Boston (Welch, 1994). Increased public awareness of the value of planting native species has resulted in interest in the use of American beech as a landscape tree as well, recently being named a “problem-free tree for Virginia landscapes” by the Virginia Cooperative Extension (Hansen, 2009).
Beech bark disease has had a devastating impact on American beech in both natural and urban settings and is the most important health problem of beech, significantly limiting its life and use. BBD is a slow-spreading invasive disease complex consisting of the beech scale insect, Cryptococcus fagisuga, and either Neonectria ditissima or Neonectria faginata as the fungal component. Mortality levels in the initial wave of the disease are reported as high as 50%, and surviving trees are left deformed, resulting in the loss of merchantable timber, overall stand health, many wildlife and ecosystem services in forests, and reduced aesthetic value in landscape settings (Ehrlich, 1932; Houston, 1983; Morin et al., 2007). However, some trees are resistant to the insect portion of the complex, which is sufficient to prevent disease and this resistance is heritable (Koch et al., 2010b). Efficient vegetative propagation of scale-resistant American beech genotypes is required for seed orchard establishment for urban and natural forest reforestation, for cultivar development for the landscape and nursery industries, and for conserving germplasm.
Attempts at propagating American beech using vegetative techniques such as rooting of cuttings and micropropagation produced plantlets, but they failed to overwinter (Barker et al., 1997; Loo et al., 2005; Pond, 2008). Top grafting was more successful with take rates of 30% reported in the first year by Ramirez et al. (2007). However, this rate declined to 12% in the second year of their study. In 2002–04, our attempts at traditional grafting of American beech using both side veneer and top cleft graft methods yielded an overall take rate of 19%. In 2002, we contracted a nursery to graft scion that we supplied, and their graft success rate was 17% in the first year with second-year survival of less than 1%. For grafting to be feasible for seed orchard establishment and cultivar propagation, success rates need to be improved. Hot callus grafting (Lagerstedt, 1981), which heats the graft union while keeping the rootstock and scion cool, can significantly increase graft success of woody plants (Avanzato and Tamponi, 1987; Lagerstedt, 1984). Our objective was to use a hot callus system to improve the success rate of grafting BBD-resistant American beech trees and identify factors influencing success rates.
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