The American Chestnut Back Cross Breeding Program

Dr. Charles Burnham, a plant geneticist at the University of Minnesota was one of the founders of The American Chestnut Foundation. He applied his knowledge from successful corn and soybean breeding to the blight problem of the American chestnut tree. The concept of TACF's backcross breeding program began in 1981 to transfer the resistance of the Chinese chestnut to the American chestnut and retain the American timber type, growth habit, cold hardiness, nut quality and competitive ability in forest-type plantings.

In the chart above, we are seeking the most resistant trees with the best American characteristics - the path of most resistance.  The Chinese chestnut tree was selected as the original source of resistance because of its high degree of blight resistance and its cold hardiness.  Although it does not have a good timber quality, it is crossed with American trees in an effort to retain those blight resistant genes.  American Chestnut pollen and nuts are obtained from stump sprouts which still grow from old root systems.

On average the first generation (F-1) is a half Chinese and American. Additional crosses ( or backcrosses) are done to increase the American characteristics while retaining the resistant Chinese gene.  Each generation is inoculated with blight and selected for resistance and also for American traits.  A generation takes approximately 5 to 8 years to produce nuts for the next cross.  It's a long term process taking 30 to 50 years.

During the next two intercross generations we cross the selected trees of the fourth generation( BC-3) to boost resistance to the blight.  By the time we reach the sixth generation, we may have a tree to test for resistance and American characteristics. We then certify and distribute the trees for forest planting.  It appears that this will take 20 years or more from today.  The good news thus far is that the blight resistance percentages and American characteristics levels are following the predictions of Dr. Burnham.