From the Sci-Tech section over at CBS News comes this AP article about beech bark disease (BBD), a complex attack on beech trees that is threatening forests across America. First, non-native beech scale insects (Cryptococcus fagisuga) descend on American beech trees (Fagus grandifolia), covering the bark with a white waxy substance. Then, beech scale infestation predisposes beech trees to infection by fungi in the genus Nectria. Studies have repeatedly shown that there is a strong association between the scale and the fungus, though the exact mechanism allowing the fungus to attack remains unknown. To complicate things even further, it turns out that there are at least two different species of Nectria that take advantage of the beech scale attacks, one native to America (Nectria galligena) and one not (Nectria coccinea var. faginata).
Once Nectria fungus has infected a tree, dead spots occur along the trunk - sometimes appearing as oozing sores - and cause callus tissue to form (wound response), disfiguring the tree as it attempts to isolate and fight off the fungus. Under ideal conditions, a beech tree can live for years with a Nectria infection. However, infected trees are much more susceptible to environmental extremes such as drought and deep freezes, and sustain wind damage more frequently than healthy trees. Those open wounds are also ripe for infection with other fungi, and are often an "in" for damaging insects as well.
Scientists expect that as infected American beech trees die out, wildlife dependent on this forest species for food and shelter will suffer. At the very least, forest composition will be changing as BBD continues to spread. Interested readers may want to view the Proceedings of the Beech Bark Disease Symposium (large pdf) that was held back in 2004, or this article about BBD research currently underway at the University of Maine. Also, Dave Houston gave a good talk about BBD for the New England Botanical Club back in 2003 (pdf).