Crossing the Best with the Best to Establish a New Legacy Tree Orchard at Meadowview
By Jared Westbrook, TACF Director of Science
For the past three breeding seasons, TACF’s Meadowview staff has been performing controlled crosses between American chestnut backcross trees previously identified to have inherited resistance to chestnut blight or Phytophthora root rot from Chinese chestnut. Trees were selected for controlled crosses because their open pollinated progeny had the least severe chestnut blight cankers or root rot lesions as compared with the progeny of approximately 700 other mother trees that have been screened for resistance to these diseases since 2011. Controlled crosses were performed because these highly resistant trees are planted in seed orchards amongst approximately 5000 other American chestnut backcross trees which have unknown disease resistance and are likely to be more susceptible to blight or root rot. Until we finish culling all but the most disease-resistant trees, we expect progeny of these controlled crosses to be more resistant than progeny from open pollination of the same parents used in these crosses.
In 2017, Meadowview staff performed 8 crosses between 16 American chestnut BC3-F2 trees that are most resistant chestnut blight. We plan on planting the progeny of these crosses at orchard locations away from other susceptible American chestnut backcross trees. One of these orchards will be a legacy tree orchard in commemoration of donors that have supported TACF’s mission. The legacy tree orchard will be composed of approximately 500 progeny from 16 controlled crosses conducted in 2015 and 2016. The trees will be planted in a secluded section of TACF’s Glenn C. Price Research Farm. In 3 to 5 years, we will inoculate these trees with the fungus that causes chestnut blight to perform additional selection for blight resistance. As these trees reach reproductive maturity in 5-10 years, they will cross via open pollination to reliably generate blight resistant American chestnut seed from the backcross breeding program.
We also performed 3 crosses between 6 BC3-F2 trees that are most resistant to Phytophthora root rot. The progeny from these crosses will be screened for root rot resistance as seedlings and the survivors will be planted at orchard locations previously infested with Phytophthora cinnamomi. The root rot resistant American chestnut backcross selections may be used in future breeding with blight resistant transgenic American chestnut to combine resistance to these diseases.