Fomes annosus Butt-Rot of Sitka Spruce

A study was made of butt-rot caused by F. annosus in 484 Sitka spruce trees from 27 sites in Great Britain. The crops sampled ranged from 23 to 50 years old, and varied in yield class and site history. Three categories of affected wood were recognized: decay, incipient decay and stain. They were differentiated on the basis of their colour and texture, and were assumed to represent different stages in the development of butt-rot. Five different patterns of butt-rot were observed and they appeared to reflect differences in the modes of disease development or in host response to attack by F. annosus . The heights of columns of infected wood were found to vary with the size as well as the age of trees. Trends based on the mean heights of columns of incipiently-decayed wood in trees of different ages suggested that the average height of infected wood increased by 5–7cm per year. The volumes of infected wood varied with both the type of butt-rot and the size of tree but did not appear to be closely associated with the age of tree. On average, 9 per cent of the volumes of the trees were occupied by decayed and incipiently-decayed wood. No consistent soil or site-history relationships were found which could be used to explain the variation (7 per cent to 68 per cent) in the numbers of diseased trees per plot.