Diamon willow is willow distinctively shaped as the result of attack by fungus (Valsa sordida, and possibly others), which has resulted in a diamonding effect occurring in the wood of the shrub or tree as the tree forms cankers, or diamonds (elongated ovals with pointed ends), in response to the infection, according to Lutz (see reference below). Lutz is careful to indicate that this is the most likely--but not necessarily definitive--cause of diamonding in willows.