No abstract

WHILE investigating the biology of wound parasitism as shown by Stereum purpureum--the fungus which causes Silver-leaf disease of fruit trees-some interesting facts have recently come to hand concerning the initiation of invasion by this organism. Hitherto little attention has been paid to the exact manner in which wound parasites invade their hosts, especially those like Stereum purpureum which infect woody tissues rather than the bark. Recent papers on Silver-leaf diseasee have shown that this fungus indubitably behaves as a wound parasite, i.e. that infection of a plum tree for instance only occurs when spores alight upon some wound and there germinate, forming a mycelium which passes deep into the tissues. It was formerly supposed that the spores of such a fungus germinated on the surface of the wound, producing germ tubes which in some way or other passed into the wood, although it was difficult to understand how the young hyphae were orientated into the vessels. In a further paper on Silver-leaf diseaset, which is in the press, it is shown that at certain times of the year the spores of Stereum purpureum will readily infect the freshly cut extremities of plum shoots kept in water, passing rapidly thence through the woody parts and killing the tissues as it goes. In view of the ease with which inoculation experiments of this kind can be carried out both in the laboratory and in the field, it was thought worth while to examine carefully the initiation of infection. In the first experiments, thrce-year-old shoots of Victoria plum trees, about 8 inches long, were placed in water in the laboratory, and upon their freshly cut upper extremities an emulsion of spores of S t e r m purpureum in sterile water was applied. The shoots were then left exposed to the atmosphere, and longitudinal sections were cut through the inoculated ends of the twigs at intervals of two, four, and six days after the spores had been added. Sections cut after a period of two days showed that the majority of the spores had been taken into the vessels, some to a distance of 3 mm., where they were beginning to germinate. A few spores were left on the surface of the wood but these were not Brooks, F. T. "Silver-leaf disease, I and 11,"Journ. Agric. Sn'. 1911 and 1913. Brooks, F. T.