OAK DISEASES 241 



leaf at the infected point to become much thicker. The ex- 

 pansion of the affected area laterally, due to the increased 

 number and size of the cells, causes it to bulge and thus the 

 blister is formed. All the cells of the mycelium then increase 

 greatly in size and push the cuticle off. Within each mycelial 

 cell which is now an ascus, the spores are formed. 



Control. 



Apparently no spraying experiments of value have ever been 

 made for the control of leaf-blister of oaks. The method used 

 to control peach leaf-curl should be tried. It is, therefore, 

 suggested that the trees be sprayed with bordeaux mixture 

 4^—50 or lime-sulfur 1-8 at any time after the leaves fall 

 and before the buds swell. The spraying should be thorough, 

 since the solution must coat every twig and bud to accomplish 

 the desired results. Peach leaf-curl is easily controlled by a 

 single application of any good fungicide in this way. The 

 spores must, therefore,, in some way be present on the outside 

 of the twig or bud scales and the spray mixtiu-e kills them. 

 (See Hesler, L. R., and Whetzel, H. H. Manual of fruit dis- 

 eases, pp. 277-283. 1917.) 



Repebence 



Wilcox, E. M. A leaf-ciirl disease of oaks. Alabama Agr. Exp. 

 Sta. Bui. 126:171-187, pi. 1, flgs. 1-3. 1903. (Bibliography 

 given.) 



Powdery Mildews 



Caused by Microsphara alni (Wallr.) Salmon, M. alni var. extensa 

 (Cooke and Peek) Salmon, Phyllactinia corylea (Pers.) Karst. 

 and Erysiphe trina Harkness 



Four species of powdery mildew fungi (besides the brown 

 mildew, see page 243) are known to attack the leaves of oaks 

 in the United States. The first and third species mentioned 

 above occur commonly throughout the country on the leaves 



