Fungus Diseases- 37 



sapping its life, — true parasites; others again are in some 

 stages of de\'elopment parasitic, and in other stages only 

 saprophytic. 



There are fungi living on leaves, others living on the roots, 

 others on the bark, and again others living on the wood of 

 the tree, producing its decomposition, or what is commonly 

 kno^^'n as rot. 



The young {)lants or seedlings are exposed to a large num- 

 ber of fungus enemies, which do not trouble them in later life, 

 but we shall here only discuss those occurring in older trees. 



Of the many interesting phenomena in the life history of 

 the fungi one of the most important is that many not only 

 pass, like the insects, through different phases or forms of 

 development, but in these different phases live on different 

 "hosts," as the plants are called on which they feed. 



The well-known rust of wheat lives in one phase of its 

 development in the ground, as many others do, and in an- 

 other on the barberry bush; some fungi, active on the juniper 

 tree, are found in different stages of development on apple 

 and other trees. A rust which attacks the needles of pines 

 is dependent for its development on ragweed or groundsel 

 {Senecio). Lately a new pest attacking our white pine has 

 been introduced from Germany, and this passes one stage 

 of its development on currant bushes. 



The presence of a certain plant may, therefore, be a cause 

 of the spread of the disease on another, and the removal of 

 the host plant as well as of the infected trees or parts of 

 them is often the only means of getting rid of the trouble. 

 In so far as specilic enemies exist on given species, if an 

 epidemic has set in, it may become practically necessary to 

 replace the species by another in order to avoid the spread 

 or renewal of the trouble. 



The fungi which attack the foliage usually destroy only 



