CHAPTER V 

 ALDER DISEASES 



Several native species of alder (Alnus) are common forest- 

 trees in the Northwest and Rocky Momitain region. They 

 grow in river-bottom lands and on mountain sides. In eastern 

 United States, the Em-opean alder is used as an ornamental and 

 in some localities has become naturalized. 



The alder is particularly subject to wood-rot diseases. The 

 common white and the brown checked wood-rots often cause 

 death by destroying the sapwood. The leaf-blisters, deforma- 

 tion of the catkins and catkin powdery mildew attract attention 

 when they occur, but they do only slight damage to the tree. 

 The alder also is one of the non-leguminous plants on which 

 the nitrogen-fixing bacteria form root-tubercles. The several 

 shrubby species of alder are subject to the same diseases as the 

 larger trees. 



PowDEKT ^Mildew of Catkixs 



Caused by Erysiphe aggregata (Peek) Farlow 



In northeastern United States, the female catkins of alder are 

 often covered with a powdery mildew. A similar, if not identi- 

 cal, fungus attacks the twigs of alder in Europe. The catkins 

 are covered with a white or yellowish coating of myceliiun which 

 later may be dotted with clusters of small black fruiting-bodies. 

 The life history and methods of control of the powdery mildew 

 fungi are discussed on page 37. Two other species of the pow- 

 dery mildews occiu: on alder leaves. 



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