438 DISEASES OF ECONOMIC PLANTS 



Infection is supposed to be spread through nursery stock, 

 and when once established, no means of stopping it is known. 

 Only preventive measures can be taken, such as destruction 

 of diseased trees by fire, careful inspection of all nursery 

 stock, and excision in the case of isolated trees which are 

 considered valuable enough to justify this mode of treat- 

 ment. 



Septoriose {Septoria ochroleuca B. & C.)- — Small dead 

 spots 3 mm. in diameter are produced upon the leaves, 

 and premature defoliation results, injuring the beauty of 

 the tree. 



Anthracnose, leaf spot (Marsonia ochroleuca B. & C). — 

 The characters of this disease are small bleached areas with 

 spore pustules in them. It has been quite injurious to 

 nursery varieties. 



Bordeaux mixture is advised. 



Monochetiose (Monochetia pacyspora Bubak). — This leaf 

 spot is very abundant in forests, and causes much loss 

 of vigor to the tree. It is recognized as large, 1-5 cm. 

 circular spots with irregular concentric marking. No 

 treatment is possible in forests, but isolated trees can be 

 protected by spraying. 



COTTONWOOD 



Rust (Melampsora). — In general this resembles the 

 willow rust. See p. 450. 



ELM 



Leaf spot, dothidellose (Dothidella ulmea (Schw.) Ell. & 

 Ev.). — The leaf spots are 1-3 mm. in diameter, and in 

 each spot is a cluster of small, black, rounded, slightly ele- 



