54 



ILLINOIS NATURAL HISTORY SURVEY CIRCULAR 46 



Witches'-Broom. — This disease, which produces broomlike 

 growths on branches (Fig. 25, 51), is common on hackberry in 

 Illinois. The cause of witches'-broom is not definitely known. 

 However, a powdery mildew fungus, Sphaerotheca phytop- 

 tophyla, and a gall mite in the genus Eriophyes are usually asso- 

 ciated with the disease on hackberry. In a hackberry affected 

 with witches'-broom, the buds swell and open wider than is nor- 

 mal for them. Frequently the bud scales become distorted and 



Fig. 51. — On hackberry, witches'-brooms, composed of numerous twigs 

 arising from localized areas on branches, are conspicuous when the trees 

 are without foliage in the dormant season. 



