OAK DISEASES 243 



Brown Mildew 



Caused by Sphmrotheca lanestris Harkness 



The leaves and twigs of several species of oak are often at- 

 tacked by this powdery mildew fungus in southern, central 

 western and extreme western United States. The mycelium 

 grows externally on the under sides of the leaf, the spots at 

 first being white and mealy but later appearing as a dark brown 

 felt due to a color change in the mycelium. The entire under 

 surface of the leaf may be covered as well as the growing twigs. 

 When infection occurs early in the season, the brown felt may 

 completely cover the young leaves and twigs, causing the leaves 

 to cease growth and remain dwarfed. The black fruiting- 

 bodies are buried in the mycelium on the under sides of affected 

 leaves. For a fuller discussion of the life history and control 

 of the powdery mildews,- see page 37. 



Largk Leaf-Spot 



Caused by Monochcetia Desmazierii Sacc. 



The leaves of red oak are affected by the large leaf-spot in 

 the southern Appalachians. The same disease affects chestnut 

 leaves in that region. The spots when fully developed are 

 very large, often being from one to two inches or more in di- 

 ameter. Two or three such spots cause the death of most of 

 the leaf -tissue. The center of the spot is pale green or yellow 

 and is surrounded by concentric bands of red and brown. 

 The colored bands are less distinct on the under surface of the 

 leaf. Small black fruiting-bodies, arranged in clusters, dot 

 the affected area. The spores from these fruiting-bodies cause 

 the infection of other leaves. Fuller details concerning this 

 disease are given under chestnut diseases, page 139. Many 

 other leaf-spots occiu* on oak. A general discussion of the 

 leaf-spots will be found on page 27. 



