Here's some information I found from this site (Google "jecarpentryservices.co.uk/ which may help")
Why does it occur?
Wood begins to rot when the layers of paint or varnish (which protect the wood) wear away over time leaving the wood exposed to moisture.
How to detect wood rot!
Wood rot is commonly found at the bottom of door and window frames/sills where rain water collects.
Look for areas where the paintwork is peeling or flaking
Rotten wood is usually dark in colour and spongy in texture
How we treat it?
Chisel out mild patches of wood rot and insert new pieces of hardwood (more durable than soft-wood) and make good using 2 part wood filler.
Replace entire structure (.i.e. door frame) if wood rot is severe.
How we prevent it?
Ensure sure that all wooden surfaces (especially door and window frame/sills) are painted once a year.
Ensure that all new wood is primed and undercoated before being painted with finishing coats (i.e. gloss or eggshell).
Make sure external structures are constructed from hardwood rather than softwood as hardwoods are more durable and less prone to rotting than softwoods.
Here are some of the chemical treatment commonly used for treating this kind of problem.
Disodium octaborate tetrahydrate or sodium borate with a brand names of Bora-Care, Guardian, Jecta, Shell-Guard, Tim-bor and Impel rods are all made to protect and treat wood and wood-foam c structural components against decay fungi and wood destroying insects. For example, remedial control of organisms attacking wood, apply a 15 percent or two applications of 10 percent aqueous solution of Tim-bor 98 percent or Bora-Care diluted 1:1 or 2:1 with water. This can be applied by brush or spray make sure the surface is wet thoroughly. Or through drilling, and then injecting the solution in the infested area. Inject a sufficient amount of solution to cause runoff from exit holes drilled into the infested wood.