388 Diseases of Economic Plants 



it spreads in all directions until the diseased parts meet on 

 the opposite side of the branch, thus girdling the twig. Dead, 

 discolored, sunken patches with numerous yellow, orange, 

 or reddish-brown pustules are produced ; spores are extruded 

 in greenish or yellow horns. Cankers in midsummer may en- 

 large at the rate of a half-inch in diameter each week. The 

 appearance of the fungus upon the trunk indicates speedy 

 death, but if small branches are first diseased, the tree may 

 continue to live for a few years. When once established, no 

 means of checking this blight 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 treatment. Every general 

 measure that has been tried has been abandoned, and at 

 present the outlook indicates that the disease will eventually 

 exterminate the American chestnut. To the present time 

 $165,000 has been used by Federal appropriation and 

 $282,500 by states, $275,000 by Pennsylvania alone, in com- 

 bating this disease. 



Anthracnose, leaf-spot {Marssonina ochroleuca B. & C). — 

 The characters of this disease are small bleached areas bear- 

 ing spore pustules. It has been quite injurious to nursery 

 varieties. 



Bordeaux mixture is advised. 



Large leaf-spot (Monochetia desmazierii Sacc). — This 

 leaf-spot is 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 

 feasible in forests, but isolated trees can be protected by 

 spraying. 



ELM 



Leaf-spot ''^^ {Gnomonia ulmea (Schw.) Thm.). — The leaf 

 spots are 1-3 mm. in diameter, and each spot is seen to con- 

 tain a cluster of small, black, rounded elevations, the cluster 



