CHAPTER XXVI 

 OAK DISEASES 



Over fifty species of oak (Quereus) are native in the United 

 States. Many of these are important timber-trees. No region 

 is without one or more species of oak, except the northern Rocky 

 Mountains and the treeless plains. Although many kinds of 

 oak occur on the Pacific Coast and in the Southwest, the most 

 important forest-species grow in eastern and central United 

 States. Many oaks are used for shade and ornament. 



The oak is more destructively affected by wood- and root- 

 rots than any other important deciduous timber-tree. These 

 diseases are also common in oaks used for ornament. Many 

 species of fungi cause leaf-spots and powdery mildews. White 

 oak, especially in the East, is more or less seriously affected by 

 leaf-blight caused by the same fungus which occurs on syca- 

 more (see page 333). This disease often results in defoliation. 

 In the South the leaf-blister is very common and often de- 

 structive. The twig-blight and Strumella canker described 

 below cause the death of many oaks in the East. 



Leaf-Blight 



Caused by Gnomonia veneta (Sacc. and Speg.) Klebalm 



Several kinds of oaks, especially the white oak, are attacked 

 commonly by this leaf-blight. Sycamores (or plane-trees) are 

 more seriously affected by the same disease. The spots de- 

 veloped on the leaves may vary from small isolated light 

 brown areas to large coalescing spots which involve a large 

 portion of the leaf (Fig. 44). When the spots occur on the 



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