Watermark disease of tree willows

The watermark disease is caused by the bacterium Erwinia salicis. It affects six species of tree willow and of these the white willow (Salix alba) and certain of its varieties are particularly susceptible. Unusually for a bacterial disease, the pathogen only colonises the xylem tissues, which become discoloured as a result. Watermark causes severe losses in S. alba var. caerulea, the cricket bat willow, and in several Dutch clones of S. alba which have been widely planted in that country. Although numerous studies have been made of its epidemiology since the disease was first reported in 1924, the infection process remains clusive. Recent research, much of it unpublished, points to the widespread occurrence of symptomless infection, and the possible role of this in the transmission of disease through the propagating material.