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A plant disease is a dynamic process where a living or nonliving
entity interferes with the normal functions of a plant over a
period of time. Things that happen just once, like lawnmower blight
or lightning strikes, are not considered diseases, but rather
injuries. Plant diseases result in visible symptoms that can help
diagnose the disease or disorder. Disease epidemics can also
threaten entire plant species. Exotic pests can threaten the
continued existence of a species. Historical examples of
destructive plant disease epidemics include three American
tragedies: American chestnut blight, Dutch elm disease and
butternut canker. Along with these three diseases some other
diseases like palm lethal yellowing, oak wilt, cypress canker and
Xylella outbreak caused severe epidemic.
Wood decay is a deterioration of wood by primarily enzymatic
activities of microorganisms. For practical purposes, fungi are the
only agents of wood decay. Fungi which grow on wood are sometimes
called "lignicolous" fungi. But why develop a set of keys limited
to fungi utilizing wood as a substrate? After all, being
lignicolous does not define a taxonomic category. Lignicolous fungi
include ascomycetes and basidiomycetes and a large number of
classes and orders within each of these groups. Most of these taxa
include both lignicolous and terrestrial species. Rather than
taxonomy, the keys focus on the biological activity holding this
otherwise disparate group of fungi together: their ability to
degrade cellulose and lignin, the major components of wood. Forest
trees and valuable landscape trees can be infected and rotted by
these fungi. Knowing the species growing on a tree can help the
forester determine the likely extent of loss. Different species are
associated with different amounts of decay in the tree.
Additionally, some species are restricted to sapwood and will not
affect the merchantable volume of heartwood. Some fungi can decay
sound wood; others decay only decaying wood.
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