Many techniques have been used to propagate chestnut trees in Europe, including chip, patch, and t-budding as well as whip and tongue, cleft, and side veneer grafting (Balta et al., 1993; Bazzigher et al., 1984; Pereira-Lorenzo and Fernanez-Lopez, 1997; Serdar and Soylu, 2005). In the United States, commercial nurseries generally use chip-budding or whip and tongue grafting to propagate Chinese chestnut (Castanea mollissima) trees (W. Lovelace, personal communication), whereas small chestnut producers often use the three-flap method on field-planted seedling rootstock, which is grafted 1 year after establishment (Hunt et al., 2009; Reid, 2010).
Regardless of the technique used, the grafting success rate is often low with chestnut and has limited the availability of trees for commercial production in the United States (Warmund, 2011; Weber and MacDaniels, 1969). Early researchers speculated that graft failure was caused by scion/rootstock incompatibility with restricted flow of nutrients through the union (MacDaniels, 1955; Weber and MacDaniels, 1969). Alternatively, it was hypothesized that low winter temperatures injured the developing tissues in the graft union (MacDaniels, 1955). Others suggested that poor grafting technique or chestnut blight infection caused union failure, especially when using different scion and rootstock species (McKay and Jaynes,1969). Chip-budding ‘AU-Super’ scions onto ‘AU-Cropper’ seedling rootstocks in late summer also resulted in lower grafting success than budding in mid-September (Warmund and Coggeshall, 2009).
Excessive sap flow or stem bleeding caused by strong root pressure is problematic when field-grafting chestnut during rainy seasons (K.L. Hunt, unpublished data; Weber and MacDaniels, 1969). Excessive sap flow in walnut and grape interferes with healing after grafting but can be reduced by making small cuts below the graft union or by withholding water from container-grown plants (Coggeshall and Beineke, 1997; Hartmann et al., 2011).
Huang et al. (1994) investigated graft compatibility of various chestnut species. Grafting success of various American (Castanea dentata Borkh.) or Japanese chestnut (C. crenata Sieb. & Zucc.) scions on ‘AU-17’ Chinese chestnut seedlings ranged from 6% to 93%. For intraspecific grafts of Chinese chestnut, few unions were formed and 6% to 10% of the trees survived after 6 months when side veneer graft cuts were made through phloem fibers in rootstock stems. Because grafting success of chestnut has been problematic with commonly used nursery methods, experiments were conducted to: 1) examine the stem anatomy of Chinese chestnut rootstocks; 2) determine if the presence of secondary phloem fibers in the stem tissue is associated with unsuccessful graft unions; and 3) investigate the effect of rootstock stem and growing medium moisture content on whip and tongue grafting success.
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