American chestnut trees. Up until about 10 years ago the trees were free from the blight because of their isolation. But now they are being infected with Cryphonectria parasitica . Cryphonectria parasitica has devastated the chestnut in North America. It was probably brought over from Asia in a load of trees that were brought to the USA in hopes of producing larger nuts.
Asian trees are resistant to the blight, but the American chestnut is highly susceptible.
Cryphonectria parasitica is a fungus that invades the tree through wounds of cracks in the tree's bark. Once the tree has been invaded, the fungus grows through the vascular cambium of the tree, eventually girdling the tree and killing it. Infected saplings will die within a year, but large mature trees can take several years to succumb to the disease. Like many other fungal pathogens, introduction of a fungus into an area where it has no co-evolved with its host makes for a more virulent and devastating pathogen. To the left is a stroma that contains several perithecia, which bear asci with ascospores inside them.