332 Journal of Agricultural Research voi. xiii, no. e 



branch is entirely girdled and killed. The fungus often enters the dead 

 tips of twigs or small branches. It then gradually kills the invaded 

 branches back to their juncture with a larger branch or with the trunk. 

 The fungus then develops on the large branch or the trunk a more or less 

 circular canker around the base of the dead twig. Reddish fruiting 

 pustules may appear on the dead areas near the edge of the canker; or 

 in smaller branches they may appear over the entire surface killed by the 

 fungus. The inner bark of the diseased areas gradually turns black 

 and often gives off a foul, salty odor. The sap wood, especially the 

 medullary rays, is also diseased and is stained a watery, reddish brown 

 (PI. 27, A), and the heartwood is sometimes discolored. Trees 3 to 6 

 inches in diameter which are severely attacked have but a few, sickly 

 looking leaves (PI. 27, B), and such trees usually die in two or three 

 years. Trees which have been rapidly girdled by this disease often 

 develop sprouts from the roots (PI. 27, C) in a manner similar to that 

 of the chestnut when attacked by the chestnut-blight fungus (Endothia 

 parasitica). These suckers are usually ultimately killed by the invasion 

 of the fungus from the old diseased parent stem. 



Canker often attacks the old trunks and large branches of various 

 Species of poplar. In such cases the fungus causes but little, if any, 

 perceptible change in the outward appearance of the bark. The attacked 

 area is ultimately killed, and the typical dark-red spore horns of the 

 fungus which causes this disease are developed in the fissures of the bark. 



The silver-leaf poplar (Populus alba) when attacked by this disease 

 dies branch by branch, since the disease usually enters the tips of the 

 branches in the top and gradually works downward. The cankers found 

 on the branches of this species usually extend from one to several feet 

 farther on the under side of the branch than on the upper side. When 

 the canker reaches the trunk of the silver-leaf poplar, it usually travels 

 more rapidly longitudinally than transversely. This results in long, 

 narrow, dead areas extending often for several feet down the tree. 

 Finally the upper portion of the crown is killed, just as the individual 

 branches were. When a tree is slowly killed from the top downward, 

 few, if any, suckers are developed from the roots. 



The dead areas produced on trees by this disease finally develop 

 characteristic spore horns consisting of irregularly twisted threads (PI. 

 27, D), which are often flattened, ranging in color from grenadine red* 

 to English red. The spore horns when first formed are soft and sticky, 

 but they dry rapidly and become hard and brittle. 



On the young branches and small trees of the aspen (Poptdus tremu- 

 loides) the typical lesions or cankers of this disease occur, but on the 

 large, old aspens another type of canker is common in the mountains of 

 Arizona and New Mexico. These cankers are perennial and of much 



• RiDGWAY, Robert, color standards and coi<OR nomenclature. 43 p., s3 col. pi. Washington, 

 D. C, 1912- 



