Ganoderma applanatum:

(Source: Midwest Mycology)

Ganoderma applanatum is a white rot fungus which once proliferated in a tree can inhibit both its health and structure. Although predominantly saprophytic (feeding on deadwood) it is considered virulent due to its association with extensive internal decay which predisposes a tree to failure.

White rot degrades the lignin in wood, leaving the cellulose behind, the result of which is that the wood becomes moist, soft and spongy (with a distinct white / yellowish colour)

The progression of white rot. From A to D showing the gradual delignification of the cell wall. The small dots in the interior cell wall represent the fungal hyphae whose excretions oxidise lignin. 
(Source: Springer; Fungal Strategies of Wood Decay in Trees).