We have an 18-20 year old wood privacy fence in a large yard, suburban setting. Wood was treated, professionally installed. We've observed the usual graying and shrinkage. Recently, I observed damage. We have no idea as to the source. Pest? Something fungal? Whatever it is, appears at various yellowish stages, on different sections of the fence. One board split down the entire center. That board's pics are labeled Top and Middle. We need to know if we're looking at replacement of sections or pressure washing or ??? Was this preventable? Thank you for your time. I appreciate any advice or direction you can provide.