62 Forest Club Annual 



logs, coppice, and apparent cankers on living trees, both oak and 

 chestnut, gave the same fungus. 



Shortly after the isolation of the "western" fungus it was 

 noticed that its development in culture was considerably different 

 from that of D. parasitica. If the conidial "spore horns" are 

 streaked on a potato agar slant, the true blight fungus produces 

 an orange colored streak within four days. This orange streak 

 broadens keeping pace with the growth of the fungus until the 

 entire surface of the slant is covered with a deep orange growth. 

 On the other hand, no orange color is noticeable on the streak 

 from the conidia of the "western" fungus even after a period of 

 ten days. On potato agar cultures from mycelial transfers a 

 fan-shaped or irregular wavy growth is noticeable at the edge 

 of the advancing mycelium in the case of D. parasitica, while 

 the "Connellsville" fungus has an even unbroken edge. There is 

 a marked contrast in the amount of aerial mycelium developed ; 

 the true blight fungus developing scarcely any while the "west- 

 ern" fungus has a fluffy appearance due to a white mycelial 

 growth above the surface of the agar. The contrast in color be- 

 tween the growth on potato agar is especially evident in cultures 

 about three weeks old. The true blight fungus develops an 

 orange brown color while the "western" fungus has at first a 

 sulphur yellow which deepens as the culture grows older. On 

 sterile twigs in test tubes the "western" fungus develops a 

 fluffy orange mycelial growth which almost completely fills the 

 tube. This growth is at first white but turns to the orange color 

 within a few days after its development. D. parasitica does not 

 develop the heavy aerial mycelium, but only a short, white, web- 

 like growth over the surface of the twig with heavier bunches of 

 mycelium, which later become orange colored where the pycnidia 

 are to be formed. These tests were checked when possible by 

 ascospore measurements and often by inoculation on living 

 trees. 



The final test of the pathogenicity of a fungus is its ability to 

 produce the disease in its typical form when inoculated into the 

 host under normal conditions. The importance of making a 

 large number of inoculations controlled by proper checks was 

 realized when the pathological work was first started in the field. 

 When mycelium either from the tissue or from culture was used 

 a slit was made under the bark and a piece of the tissue or a 

 portion of the agar with the mycelium growing on it was intro- 

 duced into this slit. When conidia or ascospores are used 

 these are shaken up in a quantity of water and introduced in this 

 way or the dry "spore horns" may be used. The point of a heavy 



