154 MANUAL OF TREE DISEASES 



similar. The first two species mentioned above occur on both 

 sides of the leaf, while the last usually affects only the under 

 side. The spots are usually not distinct and the mycelium may 

 be only slightly noticeable (Fig. 22). The black fruiting- 

 bodies are formed in patches or scattered over the leaf in all three 

 species but require microscopic examination to determine their 

 specific characters. The life histories and methods of control 

 of the powdery mildew fungi are discussed on page 37. 



Brown Wood-Rot 



Caused by Pleurotus itlmarius Bull. 



The white elm is often affected by this brown wood-rot. The 

 toadstool fruiting-bodies of the causal fungus are commonly 

 seen in the autumn projecting from pruning woimds and crotches 

 between limbs. The heartwood is first affected and later the 

 decay extends into the sapwood. The wood becomes brown 

 and is easily separated into" its respective annual rings. The 

 cell-walls of many of the fibers of the wood are destroyed or 

 partially delignified. 



The sporophores of the fungus are large, fleshy annual struc- 

 tures attached to the wood of the tree by a long and more or 

 less eccentric stalk. The top is convex, smooth, and varies 

 from white to yellow or brown. On the under surface are 

 many radiating gills or pendent plates which are notched at 

 the point of attachment to the stalk. It is on these gills that 

 the spores are formed. 



Reference 



Learn, C. D. Studies on Pleurotus ostreatus Jaequ. and Pleurotus 

 ulmarius Bull. Annales Mycol. 10 : 542-^556, pis. 16-18. 1912. 



