Editing Chestnut blight (section)

==Overview==
''Cryphonectria parasitica'' is a parasitic [[fungus]] of [[chestnut]] trees. This disease came to be known as '''chestnut blight'''. Naturally found in South East Asia, accidental introductions led to invasive populations of ''C. parasitica'' in North America and Europe. In the first half of the 20th century, the fungal disease had a devastating economic and social impact on communities in the eastern United States.  It killed an estimated four billion trees;<ref name="Roane-et-al-1986"/> or, by another count, 3.5 billion trees through 2013.<ref name="Fisher-et-all 2012"/> Less severe impacts have occurred in Europe due to widespread [[Hypovirus|CHV1]]-induced ''[[hypovirulence]]''.<ref name="ACF-biocontrol" /> CHV1 is one of at least two [[Mycovirus|viral pathogens that weaken the fungus]] through hypovirulence and helps trees survive a blight infection.<ref name=Milgroom/><ref name=Hypovirulence/>

The [[American chestnut]] (''Castanea dentata'') and [[Castanea pumila|American chinquapin]] (''Castanea pumila'') are highly susceptible to chestnut blight. The [[Castanea sativa|European chestnut]] (''Castanea sativa'') is also susceptible, but due to widespread CHV1 hypovirulence, blight-induced tree death is less common.<ref name=Robin/> The fungus can infect other tree species such as [[oak]]s, [[red maple]]s, [[staghorn sumac]]s, and [[shagbark hickory|shagbark hickories]].<ref name=Missouri-Botanical-Garden/> Once infected, these trees' bark also exhibit orange cankers but may not die. The pathogen can persist in these trees, producing [[spore]]s that may infect other trees.

Fungal strains spread by wind-borne [[ascospores]] and, over a shorter distance, [[conidia]] distributed by rain-splash action.<ref name=Forest-Pathology/> Infection can be local in range, so some isolated American chestnuts survive where there is no other infected tree within {{cvt|10|km}}. Soil organisms at the root collar and root system of the chestnut tree are antagonistic to the fungus. Chestnut tree roots are resistant to blight infections.  Consequently, a large number of small American chestnut trees still exist as [[Shoot (botany)|shoot]]s growing from existing root bases. However, these regrown shoots seldom reach the sexually reproductive stage before above ground growth is again girdled by the fungus.<ref name=Appalachian-Woods/>
Fungal strains originally infected the [[Castanea mollissima|Chinese chestnut]] (''Ca. mollissima'') and the Japanese chestnut (''Ca. crenata''). These two species have co-evolved with the pathogen, making them most variably resistant to its ill effects.<ref name=Rigling/>