Decay is one of the most dangerous tree defects within the tree. The structural strength of a tree can be compromised by the degree of decay that is present within the tree. Fungi enter a tree by various ways but one of the most common is through some type of injury. These fungi break down wood which reduces its strength. This process happens over time and can vary with the type of wood decaying fungi present. Wood degradation fungi break down the wood by secreting various enzymes. Depending on the fungi and the tree host these wood decaying fungi attack different parts of the wood. This reduces it's strength, stiffness or both. These rots are classified as soft rot, brown rot, and white rot.
Wood decay can easily be hidden from view because of the inability to see inside of a tree. Many trees that fail or are wind thrown looked perfectly healthy from the perspective of visual appearance. This can be very deceptive. There are visual signs of possible wood decay within a tree and a certified arborist trained to see these potential indicators of decay would be your best choice if you have any concerns about your tree(s). I want to emphasize that someone who cuts (fells) trees for a living does not qualify them to determine tree structure problems or tree risk assessment evaluations. I will explain it like someone that mows grass for a living. This does not qualify them as an horticulturalist. They should not be given the keys to perform any task needed on your landscape. Many awful horticultural dyslexic (backwards and wrong) practices occur everyday by well meaning people that have no business; as an example; pruning shrubs or planting trees. The same goes with arboriculture. Just because someone cuts trees for a living; dosen't qualify them to make tree structure assessments. I've seen many damaging practices to trees that can not be replaced or regrown within our life time. Sad... be aware; it happens every day.
To find a certified arborist in your area: go to http://www.isa-arbor.com/ and click on the right hand side of the page under: I am a Tree owner - find a tree care service.