Notes on a pycnidial fungus associated with a dying-back of apple branches

Abstract

A die-back of the branches of Lord Derby apple trees in Cambridgeshire is described. A pycnidial fungus is constantly associated with this disease. There is some uncertainty about the identity of the fungus, but it is provisionally named Cytosporella fructorum, a species which Él. and Ém. Marchai have described as occurring on fruits of Pyrus in Belgium. The methods of germination of the pycnospores are described. The commonest mode of germination is peculiar in that secondary spores are budded off from one end of the pycnospore. Under cultural conditions many pycnidia give rise to small rod-like spores which have not been found in nature. Inoculation experiments with the fungus were unsuccessful in reproducing the disease. It is likely that the fungus behaves as a parasite only when the trees are already weakened from some other cause.