154 THE CULTIVATED EVERGREENS 



eighteen inches from the butt all round) uncovered for an 

 indefinite period when replacing the soil. When a number of 

 trees are growing in close proximity, diseased individuals 

 should be isolated by digging a trench about two feet deep all 

 around them just beyond the limits of their root systems. 

 No roots or fungous strands should be allowed to cross this 

 trench. 



Trees in which the disease is well advanced should be 

 sacrificed immediately and burnt on the spot. The soil should 

 then be turned over and treated with a generous dressing of 

 quicklime. This area should remain unplanted for at least 

 three years, during which period the soil should be repeatedly 

 turned over and exposed to the sun. 



Rusts. 



Of the fungous diseases to which coniferous trees are sus- 

 ceptible, it is probable that the rusts are of the greatest 

 importance from an ornamental standpoint. The leaves, twigs, 

 and branches or trunk may be affected with varying degrees 

 of injury, resulting in a serious depreciation in the ornamental 

 value of the tree or even in its death. The important rusts 

 occurring on conifers may be divided into two groups — the 

 "blister-rusts " * which are particularly encountered on pines, 

 and the "Gymnosporangium rusts" which attack junipers 

 and cedars. The fungi causing these diseases are extremely 

 interesting on account of their complicated life history, for 

 the completion of which two different hosts are required. 

 This will be brought out in connection with the discussion of 

 the typical examples below. 



White pine blister-rust (Fig. 20) is caused by the fungus 



*Only a few of the many forms can be mentioned. For a more complete list the 

 following publication should be consulted: Rhoads, A. S. et al.— Host relationships of 

 the North American rusts, other than Gymnosporangium, which attack conifers. — 

 Phytopathology 8 : 309-352. 1918. 



