5 CIRCULAR 85 3, U. S. DEPARTMENT OF AGRICULTURE 



tion. Clitocybe root rot appeared on some of these plants. So destruc- 

 tive was the disease and so rapid was its spread that culture of the 

 plants was discontinued within 5 years after the first apperance of the 

 disease {26). In this case the unusually rapid spread and destructive- 

 ness of root rot was attributed to maintaining a high moisture content 

 of the soil by daily use of an overhead sprinkling system to keep the 

 moss layers wet, 



SYMPTOMS 



The symptoms of Clitocybe root rot vary considerably with the kind 

 and size of the plants attacked and the rapidity with which girdling 

 occurs. Root -rot symptoms are not apparent until the fungus has 

 invaded the root system or root crown, or both, sufficiently to interfere 

 with the life processes of the host plant. The dying of small trees or 

 shrubs may follow rapidly, while that of large ones is likely to proceed 

 much more slowly, with large trees often showing symptoms of decline 

 for a few years before death. 



On conifers, such as arborvitae and pines, a slight yellowing of 

 the foliage may occur occasionally, but as a rule, at least on small trees, 

 the foliage on the first limbs to be affected by the progress of the 

 invading fungus simply turns brown and dies and the decline of the 

 tree progresses more or less rapidly. 



Small broadleaf trees and shrubs frequently are killed so rapidly 

 that they may not develop any particular symptoms until they begin 

 to wither and die. Large trees and shrubs usually show more or less 

 yellowing and defoliation, accompanied by the rapid wilting and 

 dying of individual limbs or trunk divisions. Finally, the whole 

 plant dies. In other cases the wilting develops more or less simul- 

 taneously through the whole crown. When large trees or shrubs are 

 not killed during the year in which they become partially girdled, the 

 foliage developed the following year tends to show reduction in size, 

 sparseness, and marked chlorosis, which usually increases in intensity 

 toward the top. 



In the case of Australian pine trees, the first symptom is a slight 

 yellowing of the foliage branches, 4 which generally develops first on 

 the lower limbs of the crown on the side where the roots are first 

 attacked. This yellowing soon becomes more conspicuous in extent 

 and is accompanied by shedding of the affected foliage branches. As 

 the disease progresses the crown acquires a thin, sickly appearance 

 (fig. 2) and an unusually heavy litter of shed foliage branches accu- 

 mulates under the tree. Attacked trees generally become about three- 

 fourths defoliated, and often completely so on the terminal parts of 

 the crowns, before dying. However, in very young trees, where gird- 

 ling proceeds quite rapidly, the foliage branches over the entire crown 

 may turn brown and shrivel within a brief period without any prelimi- 

 nary yellowing. and shedding. In older trees, where girdling may 



4 In these trees the leaves are reduced to whorls of mere scalelike teeth at inter- 

 v.-ils around the dehiscent foliage branches, which may appear to the layman 

 as needlelike leaves. 



