Measuring resistance to chestnut blight (Cryphonectria parasitica) and American chestnut (Castanea dentata) morphology of backcrossed hybrids in Lesesne State Forest

The American Chestnut (Castane dentata) was a pivotal species in Eastern hardwood forests before populations declined to near extinction across their entire range following the introduction of Cryphonectria parasitica, the fungus responsible for the chestnut blight. Backcross breeding is one mechanism used to introduce blight resistance to C. dentata specimens following hybridization with resistant Chinese chestnut (C. mollissima) specimens. This study focused on measuring blight resistance and C. dentata morphology of backcrossed C. dentata specimens in Lesesne State Forest, Virginia (LSF). Observations of blight resistance and C. dentata morphology were recorded for a subset of trees in LSF from May 2022 to October 2022 to calculate a phenotypic blight resistance index and C. dentata morphology index in the field. Analysis compared the phenotypic blight resistance index and expression of C. dentata morphology to genetic-based indices and genotyping-by-sequencing data to identify correlation between field and lab-calculated indices used in selecting specimens for further backcrossing. Results showed a strong positive correlation between estimated C. mollissima genotypic content and both phenotypic (R = 0.88, p = 0.0044) and genetic-based blight resistance indices (R = 0.95, p = 2.2𝑒-16). Strong positive correlation was found between phenotypic and genetic-based blight resistance indices (R= 0.66, p = 2.2𝑒-16). Moderate negative correlation was found between the phenotypic blight resistance index and the C. dentata morphological index (R = -0.41, p = 2.9𝑒-16). Weak negative correlation was found between the C. dentata morphological index and the genetic-based blight resistance index (R = -0.34, p = 1.4𝑒-11). The C. dentata morphological index was not found to correlate significantly with C. dentata genotypic content (p = 0.25) in a small sample. These results identify strengths and weaknesses in relying on field-based indices to make selections for backcross breeding, which will have implications for progress and success in restoring the American chestnut.