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DISEASES OF FOREST AND SHADE TREES 



By. W. P. Fraser, Macdonald College. 



Forest and shade trees are subject to many kinds of disease. This 

 paper deals only with a lew of those caused by flowering plants and fun- 

 gi, more especially with the diseases caused by rusts and shell or pore 

 fungi. 



DISEASES CAUSED BY FLOWERING PLANTS 



Arceuthobium pusillum— Dwarf Mistletoe. The most serious disease 



of trees in Canada due to flowering plants is that caused by the Dwari 



Mistletoe. This parasite is generally distributed in the spruce woods of 



Eastern Canada, attaciking especially the black spruce. The diseased trees 



cam be recognized by the develoTimen of witches' brooms, close clusters of 



bushy twigs. An examination of these bushy masses will show short, 



greenish, leafless stems half an inch or less in length growing from the 



bark; these are the stems of the mistletoe. The flowers are produced in 



earh^ spring albout the time the alder is shedding its pollen. When the 



seeds are ripe they are projected from the capsule and adhere to the bark 



of neighbouring branches and there germinate. Gro\vth takes place into 



the bark, a connection is established \vith the living twig and in time 



the witches' broom is formed. The parasite spreads slowly over the tree 



and eventually kills it. The disease could doubtless be held in check by 



cutting off the v^^itches' brooms before the seeds ripen. 



DISEASES CAUSED BY FUNGI 



Powdery Mildews^ ( Erysipaceae ) . The powdery Mildews are 

 common and generally distribiited. Several species attack the leaves oi 

 trees. The diseased leaves are covered with a whitish mould which ap- 

 pears po\vdery when the summer spores are present. The poplar, willow, 

 oak, maple and birch suffer most,^ but usually the injury is slight except 

 in the case of young trees. Where these fungi attack nursery stock, dust- 

 ing with sulphur will help to check the disease. Spraying with lime sul- 

 phur w^ould probably also be effective. 



Tar Spot of Maple ( Rhytisma acerinum ) . This fungus is very 

 common on the lea'ves of m:aples, 'especially in damp situations. It causes 

 conspicuous black angular spots on the upper surfaces. Sometimes the 

 attack is so severe that the leaves fall prematurely. During the winter 

 and the following spring spores de\ielop in the black areas. These are ma- 

 ture by June and thus the fungus is carried over the \vinter and infects 

 the leaves in the spring. Gathering and burning the leaves where practi- 

 cable would help to control the fungiis. 



