FUNGI ASSOCIATED WITH “DIE BACK” IN STONE FRUIT TREES. I.1

Abstract

Summary. Diaporthe perniciosa, first isolated by Marchal from rots in stored fruits, has proved to be parasitic and one of the causes of “die back” in stone fruit trees. The various strains of D. perniciosa isolated from “die back” on peach and different varieties of plums have proved to be parasitic, morphologically identical, and cause definite wilt and “die back” on peach, plum, apricot and nectarine. Cross inoculations of the same organisms on two year old wood of Prince of Wales plum have so far not brought about typical symptoms of “die back,” but considerable disturbance, accompanied by gumming, has occurred at the points of inoculation. A strain obtained from apricot, which shows morphological differences, has also been proved to be capable of causing “die back” on peach and apricot. All the controls remained healthy. The progress of D. perniciosa in the host tissue is slow; infection occurs a considerable time, in some cases years, before any external symptoms appear. The mycelium permeates the internal tissues more especially the medullary rays, cortical tissues, phloem cambium, and vessels. D. perniciosa has a pycnidial or Phomopsis stage with “a” and “b” spores, and a perithecial or Valsa stage with numerous eight‐spored asci. The pycnospores emerge as whitish tendrils or globules through numerous lenticel‐like slits in the bark; the perithecial necks are long and protrude mostly through the same slits as the tendrils, and the ascospores are given off either through the ostiolum at the tip of the neck of the perithecium, or may burst through any part of the neck. The external symptoms are sunken areas and longitudinal slits in the bark; elongated strands of numerous lenticel‐like transverse slits through which tendrils and perithecial necks emerge; rapid wilt and browning of the trees in summer, or premature yellowing and fall of the leaves in the autumn. The internal tissues are discoloured and secondary thickening arrested. The pycnidial stage occurs on living or more or less moribund bark, the perithecial stage on the parts which are dead. D. perniciosa shows a certain degree of polymorphism on the host plant; on artificial media this polymorphism is much more marked. The Phomopsis stage of D. perniciosa grows and sporulates well on artificial media, but perithecia develop sparingly in cultures from pycnospores. Cultures of mass infections of ascospores of D. perniciosa have given rise in some cases to the Phomopsis stage with “a” and “b” spores; in others no “b” spores could be found. Mono‐ascospore cultures develop the pycnidial stage very sparingly, and no “b” spores have been observed in any of the cultures. Large numbers of perithecia develop in mono‐ascospore cultures on various media.