Establishment of American chestnuts (Castanea dentata) bred for blight (Cryphonectria parasitica) resistance: influence of breeding and nursery grading

Abstract

European and American chestnut species (Castanea) have been decimated by exotic species, most notably chestnut blight (Cryphonectria parasitica), since the early nineteenth century. Backcross breeding programs that transfer blight disease resistance from Chinese chestnut (C. mollissima) into American chestnut (C. dentata) offer promise for chestnut restoration, particularly for the American chestnut which was a keystone species in eastern North America. Nursery prescriptions and conformity to desired American chestnut traits following planting must be tested, however, before blight resistance can even be evaluated. We tested early field performance of American and Chinese chestnut and hybrid seedlings from the third backcross generation (e.g., BC3F3) in two-aged regeneration harvests on highly productive sites in the southern Appalachians, USA. We also tested a common nursery prescription of grading seedlings by size prior to planting. BC3F3 seedlings had similar 4-year survival to American chestnut seedlings, but generally had smaller stem heights and ground-line diameters (GLD). Although blight had not yet substantially challenged some sites, the BC3F3 seedlings had blight incidence similar to the Chinese chestnut which was lower than the American chestnut. Visual seedling grading affected planting shock and stem height and GLD by the end of year 4. Large size-class seedlings had more stem dieback and 5 % lower survival compared to small size-class seedlings, but larger trees exhibited the same height in year 3 as small trees in year 4. Advanced breeding material (BC3F3) was successfully established during the stand initiation phase of forest development on highly productive sites, but deviations in desired growth rate of the American chestnut was evident. Visual grading of seedlings affected establishment of breeding material, and should be considered in the restoration process.