2,2,6 Journal of Agricultural Research voi. xiii, no. 6 



aerial mycelium after about three weeks has changed to a mouse-gray 

 color in the center of the plate, but at the margin it still retains its white 

 to light-buff color. The entire surface of the plate is now covered with 

 many elevated pustules 4 to 7 mm. in diameter. These develop pycnidia 

 later if sufficient moisture is available. These pycnidia extrude a mass 

 of pycnospores which is orange-chrome when fresh and soft, but which 

 changes to English red on drying and hardening. These spore masses 

 rarely form long spore horns, but usually remain as large orange-chrome 

 drops at the point of issuance. However, if the air in the plate is dry, 

 the typical spore horns of Cytospora chrysosperma are developed. Only a 

 few pycnidia mature and discharge their spores at a time. This gradual 

 ripening of pycnidia and the subsequent discharge of the spore masses 

 may continue for two or three months after the tube or plate is inoculated, 

 the length of time depending upon the quantity of culture medium 

 present and the rapidity v/ith which it dries. 



Pure cultures of this fungus on 2 per cent corn-meal agar, -f-0.25, are 

 very similar to those on malt agar, except that the mycelium produced, 

 both aerial and submerged, is much less in quantity, and no pycnidia, or 

 only a few, are finally developed. 



MORPHOLOGICAL CHARACTERS 



The hyphse of the aerial mycelium are hyalin, fairly uniform in size, 

 ranging from 2 to 4 a^ in diameter, and very sparingly branched, with 

 septa few and distant; the submerged mycelium is dark, neutral gray to 

 blackish slate, black when seen in mass. Often several of these hyphae 

 are joined into a long bundle from which individual hyphae put off at 

 intervals. Individual submerged hyphae are 2 to 4 ju in diameter, spar- 

 ingly branched, and distantly septate. The black submerged mycelium 

 does not penetrate deeply into the agar, but is more or less limited to the 

 substratum immediately beneath the aerial growth. 



DISSEMINATION OF THE DISEASE 



The canker, as previously stated, enters the host through wounds or 

 dead twigs and branches. Once established in the growing inner bark, 

 it is easy to see how other parts of the tree are infected. During every 

 rainy or damp spell some of these spore horns which have formed on the 

 diseased area are dissolved, and the water containing the spores runs 

 down the tree, in this manner transmitting them to wounds or dead twigs 

 present on the tree, and thus originating new lesions. 



At present only pycnospores are known, and since these are borne in 

 gelatinous threadhke horns, it is impossible to state definitely with our 

 present knowledge of this fungus how it travels from tree to tree. It is 

 probable, however, that large numbers of these spores, after once being 

 freed from the spore horns by rains, dry and are carried by the wind to 

 other trees. Many spores are washed into the soil at the base of the 



