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Rust Fungi. The rusts constitute a large group of fungi which are 

 parasitic on flo^vering plants and ferns. Their structure consists of an 

 inconspicious mycelium in the tissues of the host plant and more or less 

 conspicuous spores that usually break through the epidermis and aptpear 

 as powdery masses or crusts. Spores of five kinds may be produced 

 thoug;h they are not all present in every species. These spores always 

 follow each other in a definite order; first the spermatia, which as far 

 as is known have no function and take no part whatever in the develop- 

 ment or spread of the fungus. Aecidiospores or spring spores soon follow 

 and later uredospores or summer spores are producedi. These are followed 

 by teleutospores or winter spores whose function is usually to carry 

 the fungus over the winter, though in some species this is not the use of 

 the teleutospore. The teleutospores under favourable conditions which 

 vary in the different genera and species send out a tube and on this spores 

 called sporidia or basidiospores are prodkiced which cause infection. One 

 or more of the spore forms may be absent in certain genera or species 

 except the teleutospores, but the spore succession is always the same. 



Many rusts pass part of their life cycle on one plant and part on 

 another species, and usually these plants are not closely related. The for- 

 iner rusts are called heteroecious and the latter autoecious. In the hetero- 

 ecious rusts the spermaitia and aecidiospores are developed on one host 

 and the uredospores and teleutospores on the second host. Most of the 

 rusts that attack trees are heteroecious or two host rusts. The follow^ing 

 are the most common and injurious species attacking trees in Canada. 



( a ) The Rust of Poplar and Hemlock (Melampsora medusae). 



The aecidiospores or spring spores of this species are common 

 on the cones, young leaves and twigsi of the hemlock ( Tsuga canadensis ) . 

 The parts attacked turn yellow, the twigs curl up, the leaves fall and 

 the cones do not mature. These spores are produced in June and they are 

 carried by the wind to the young leaves of poplars and yellow dusty 

 spores, the udespores or summer spores soon appear on leaves. La- 

 ter the w^inter spores are form.ed on the leaves in crust-like masses. The 

 infected) leaves soon turn black and fall from the trees. These teleuto- 

 spores or winter spores carry the fungus over the Avinter and infect the 

 hemlock in the following spring. In the western United States this spec- 

 ies of rust has its spring spores on the larch instead of the hemlock. 



( b ) The Rust of Larch and Willow (Melampsora Bigelowii). 



This rust m.uch resembles the last, but attacks the larch ( Larix 

 laricina ) and produces the spring spores on its leaves. The summer and 

 winter spores are found on the willow ( Salix species ) . 



( c ) The Rust of Willow and Balsam Fir. (Melampsora arctica). 



Spores of this rust are produced on the fir, Abies balsamea. The stages 

 on the willow are so much like the last species named that it is difficult 

 to distinguish them. 



