8 



THE CAUSAL ORGANISM. 



A single species of fungus is invariably associated with this 

 disease. It has repeatedly been isolated from all of the different 

 types of the disease on a variety of hosts. ^ The fungus grows 

 well in ordinary culture media, but 3% dextrose agar has been 

 used in making most of the isolations. Tissue transfers made 

 by planting small fragments of diseased bark or affected wood 

 in agar plaies give pure cultures in the majority of cases. 

 Typical isolation plates are shown in. Figs. 15 and 16. The 

 fungus produces a cottony aerial mycelium and this turns to a 

 dirty gray after one to three weeks. The under side of the 

 colony shows a dark brown or black coloration which first be- 

 comes noticeable at the center and gradually spreads towards 

 the periphery of the colony. Different strains show great varia- 

 tion in the amount and intensity of darkening (fig. 17). No 

 spore formation has been observed in any cultures. 



The fungus in question, ^trwnella coryneoidea Sacc. & Wint., 

 is not a new one, but is an old species which has not pre- 

 viously been accused of being a parasite. It was first collected 

 at Perryville, Mo., in 1883, by C. H. Demetrio and described by 

 Saccardo and Winter.^ The fungus has also been collected at 

 London, Ontario, on oak bark by J. Dearness (No. 94, June, 

 1889). The two following collections referred to this species 

 were apparently incorrectly determined : 



1. — Flora of Washington. No. 307. Collected Jan. 28, 1894, 



on dead branches of Rhus diversiloha J. and G., by W. N. 

 Suksdorf. Columbia River, W. Klickitat Co. 



2. — Flora Tudoviciana. No. 1799. Collected Mar. 30, 1889, on 



young dead twigs of Garya olivacformis by A. B. Langlois. 

 St. Martinsville P. O., La. 



A careful comparison of our specimens with those of Deme- 

 trio (Ellis and Everhart. North American Fungi, No. 1653), 

 and Dearness (No. 94^, has failed to reveal any morphological 



1. Many of the isolations were made by Mr. R. O. Walton, who was associated with 

 the writers during the earlier work on this disease. 



2. Saccardo and Winter. Rabenherstii Fungi europaei extraeuropaei. No. 2984. 

 Hedwigia, 1883: 175-176. Ellis and Everhart. North Am. Fungi, No. 1653, June 1885. 



