Which fungi can usually be brushed or washed off from wood surfaces?

Mold is the type of fungi that can typically be brushed or washed off from wood surfaces because it primarily grows on the surface of the material rather than within it. Mold develops in damp and humid environments and can create a superficial layer that does not penetrate the wood deeply. This characteristic makes it relatively easy to remove with physical cleaning methods or even mild detergents.

In contrast, other fungi like sapstaining fungi and brown rot fungi can be more ingrained into the wood structure. Brown rot fungi, in particular, not only stain the wood but also cause significant degradation and weakening of the wood's structural integrity. Sapstaining fungi produce dark stains in the wood; while they can be surface-level as well, they might require more intensive methods to fully address their presence compared to mold. Blue stain, another type of sapstaining fungi, also tends to be more persistent once established.

Thus, mold stands out as the answer since it is generally the least invasive and most easily removable form of fungi on wood surfaces.