174 MANUAL OP TREE DISEASES 



while in the latter species dense irregular white patches are 

 formed on the under sides of the leaves, and the black fruiting- 

 bodies are large in comparison with those of the former species. 

 The life histories and methods of control of powdery mildew 

 fungi are discussed on page 37. 



Witches'-Broom 



Caused by a gall-mite and Sphoerotheca phytoptophila KeUerman and 



Swingle 



The hackberry is affected, in central United States, by an 

 important witches'-broom disease. Although mainly impor- 

 tant because of "the unsightly appearance of affected ornar 

 mentals, some damage to the tree must result from the loss of 

 energy spent in the development of the brooms. Also due to 

 the death of the branches or their breakage, wounds are formed 

 which allow wood-rot fungi to enter. The lower branches are 

 most commonly affected, although at times brooms are found 

 throughout the crown. Hundreds of brooms are sometimes 

 found on a single tree, causing serious deformation. 



Symptoms. 



Two general types of brooms are formed. The open type 

 consists of irregular swellings or knots at the base of a branch 

 from which many short stubby twigs arise. The leader remains 

 healthy, however, and grows to its normal length and other 

 knots with diseased laterals are formed on it at intervals. A 

 closed type of broom results when the leader is diseased and 

 fails to develop normally. For several years, after the first 

 knot with its diseased laterals is formed, the new laterals from 

 the base of those of the previous year cause a compact broom 

 of many deformed and dwarfed branches all arising from a 

 large irregular mass of gall-tissue. Smaller galls may be de- 

 veloped also further out on the diseased laterals. 



