INJURIOUS INSECTS, FUNGOUS DISEASES, ETC. 195 



Up to the present time no means has been discovered of 

 successfully combating this pest. 



LEAF DISEASES 



Shade-trees sometimes suffer from diseases which affect 

 the foliage. The assimulating area of the leaves is thus 

 reduced, and the growth of the tree is diminished in propor- 

 tion to the leaf area destroyed. While ordinarily they check 

 the growth of the trees for one season only, if the disease 

 appears several years in succession it may seriously weaken 

 and even kill the tree. 



Diseases of the foliage can be controlled by timely spray- 

 ing. The germination of fungi may be prevented by the use 

 of mineral salts. Those of copper are more commonly used, 

 especially the sulfate, usually in the form of Bordeaux 

 mixture (see page 207). Just as long as leaves or other 

 tissues of plants are covered with a thin coating of Bordeaux 

 mixture, no fungi can develop upon them. Thus a fungi- 

 cide is a preventive, and its application should begin long 

 before the disease has advanced far enough to manifest 

 itself to any extent. 



Rusts. — This term has been applied in more or less of an 

 arbitrary manner to a large group of fungi that produce 

 masses of spores on the surface of the leaves, generally yel- 

 lowish or brownish in color. The linden rust (Cercospora 

 microsora Sacc.) may be taken as an example. Leaves 

 attacked by this fungus become badly spotted, and the mar- 

 gins decayed and tattered. 



Spraying the branches thoroughly with copper sulfate, 

 one pound to twenty-five gallons, or strong Bordeaux mix- 

 ture, before the leaves unfold, will destroy all of the earlier 

 spores upon them. Then if the trees are where the Bor- 



