American chestnut restoration: Progress and direction

Background/Question/Methods

Castanea dentata (American chestnut) was formerly a primary component of eastern North American forests. The chestnut blight (Cryphonectria parasitica), an accidentally introduced fungal pathogen from Asia, decimated the species. The American Chestnut Foundation (TACF) has been breeding hybrids of C. dentata and C. mollissima (Chinese chestnut) that retain most C. dentata characteristics but are resistant to blight and are expected to be released to the public by 2020. A major root pathogen of Castanea spp. is Phytophthora cinnamomi, an introduced water mold normally found in the soil. Attempts at using a hypovirus have largely failed on a population scale, but frequent treatment has allowed individual trees to survive. Stands of C. dentata planted in Wisconsin, which is outside the species’ native range, allow for studies of mature trees. Many attempts at reintroduction have failed due to the blight, but some efforts like mine reclamation have been successful without establishing a mature stand. C. dentata has a unique ability to persist in a seedling stage through resprouting that enables them to survive in the understory. Studies in Wisconsin and historical data have improved our understanding of the shade and soil preferences of C. dentata.  

Results/Conclusions

Until blight resistant chestnut hybrids are released by TACF, studies should focus on determining best establishment conditions, P. cinnamomi resistance, and soil preferences. Dry sandy soils provide the most resistance to P. cinnamomi, but C. dentata performs better on more moist soils. A better understanding of specific nutrients is needed, and studying fertilizer effects could help with this. Seedling performance on different slopes and aspects is not well known. A bacteria or other control agent for P. cinnamomi would greatly accelerate the wide establishment of blight resistant chestnut hybrids. Some susceptible species such as avocado have been maintained by chemical application, but this is not a reasonable method for large scale establishment. Since the introduction of chestnut blight, some insect pests have been introduced, and a biological control may be needed for those also. The effects on chestnut seedlings and possible treatments need to be studied. The fire tolerance of C. dentata and the possible role of fire in helping reintroduce it to forests dominated by oaks, hickories, and maples are not fully understood. Seedling selection should continue and aim for creating hybrids that are both blight and P. cinnamomi resistant.