The Disease Triangle of Chestnut: A review of host, pathogen, and environmental interactions of chestnuts cultivated in the eastern United States.

Chestnuts, the edible seeds of the genus Castanea, are a perennial food crop closely tied to the global migration of humans throughout history and have recently been gaining popularity in agriculture and forest restoration in eastern North America. Cultivation of chestnuts yields nutritionally balanced food while fostering economic development, food security, and environmental health. However, diseases and insect pests threaten successful ecological restoration and food production. In this review we explore conditions affecting chestnuts in the eastern United States through the lens of the disease triangle. A "host" in the disease triangle is not merely a single tree, but a tree including its constituent population of fungal endophytes. Chestnut trees are rich with microbial life, and the sustainability of chestnuts in forest and cultivated settings may lie in understanding and manipulating microbial communities to improve plant health and control destructive diseases. To benefit from the ecological and economic potential of chestnuts on the landscape, it may be necessary to select locally adapted chestnut trees, regardless of pedigree, that are resilient against cosmopolitan pathogens. With transport of plants and pathogens throughout the globe, and with landscape level environmental changes over the last century, chestnut trees in the eastern United States (U.S.) are in a unique disease landscape compared to their ancestors. Diseases of economic concern from fungi and fungal-like organisms include chestnut blight () and ink disease () on American and European chestnuts, oak wilt (Bretziella fagacearum) on all chestnut species, and the emerging diseases of brown rot () and chestnut anthracnose (). The eastern U.S. has experienced profound environmental changes over the 20th century and into the early 21st century. These changes happen to coincide with the demise of the American chestnut in the eastern forest, efforts to re-establish chestnut as a forest species, and the rise in cultivation of multiple chestnut species and hybrids as a culinary crop. Chestnut trees growing in the early 21st century face different environmental circumstances than the American chestnuts of pre-colonial times, including changes in forest composition, rainfall changes and acidification, industrialized agriculture's increased chemical inputs, rising global temperatures, and increased levels of carbon dioxide in the atmosphere. We conclude that chestnut tree species for both forestry and agriculture should be considered based on current fitness, adaptability, and economic and ecological value considering continued dynamics in host and pathogens distributions and anthropogenically driven climatic and edaphic conditions.