CHAPTER XIX 

 HACKBERRY DISEASES 



Two species of hackberry (Celtis) occur in eastern and central 

 United States. These trees are not important forest species 

 and in many sections they are shrub-like. West of the Missis- 

 sippi River, the hackberry is commonly used for shade and 

 ornament. The most important disease of this tree is the 

 witches'-broom. Several leaf-spot fungi and powdery mil- 

 dews are common on hackberry. Although no wood- or root- 

 rots have been recorded as affecting the hackberry, doubtless 

 some of the more common wood diseases of other deciduous 

 trees may also be found in them. The small amount of atten- 

 tion that has been given to the diseases of the hackberry is 

 due to its unimportance as a timber-tree. 



Powdery Mildews 



Caused by Uncinula parvula Cooke and Peck, and Uncinula polychwta 

 (Berk, and Curt.) ex Ellis 



Besides the powdery mildew fungus which is associated with 

 the formation of the prominent knots and witches'-brooms, 

 two other species of the same group attack the leaves of the 

 hackberry. The one, Uncinula parvula, is reported through- 

 out the United States, while the other species, Uncinula 

 polychcsta, is apparently confined to the southeastern states. 

 The former species causes inconspicuous powdery growths on 

 both sides of the leaf. The black fruiting-bodies are very 

 small and usually confined to the under surface of the leaf, 



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