CARTER: ILLINOIS TREES: THEIR DISEASES 53 



Wetwood. — This vascular disease is caused by the bacterium 

 Erivinia nimipressuralis. It occurs in many genera of trees, in- 

 cluding apple, birch, elm, fir, hemlock, hickory, linden, maple, 

 mulberry, oak, pine, poplar, redbud, sycamore, tulip tree, and 

 willow. It is more widespread and causes more injury in elms, 

 especially Asiatic elms, than in any other trees. A detailed de- 

 scription of this disease is given in an Illinois Natural History 

 Survey circular, "The Wetwood Disease of Elm." 



Verticillium Wilt. — This vascular disease, caused by the 

 fungus VerticiUium albo-atrum, is described in the section on 

 maple. 



Dothiorella Wilt. — The fungus DothioreUa ulmi, which causes 

 this vascular disease, produces curling, yellowing, and wilting of 



Fig. 50. — Soon after Dothiorella cankers of elm are formed, fruiting 

 bodies of the fungus appear as black, raised pustules in the dead bark. 

 Under moist conditions, spores of the fungus ooze out of the pustules and 

 are deposited on the surface of the bark. 



leaves, followed by defoliation, development of cankers on 

 branches, and dieback of affected branches. Diseased bark turns 

 reddish brown and becomes shrunken. In time, black raised 

 pustules of the fungus appear in the diseased bark (Fig. 50). 

 Brown streaks produced in the young sapwood of wilting 

 branches may be confused with those caused by Dutch elm dis- 

 ease or Verticillium wilt. 



Dothiorella wilt can be controlled in its early stages if all 

 diseased branches are cut off a foot or more below any evidence 

 of brown streaking in the sapwood. Trees in low vigor should 

 be fed. Spraying to prevent insect injury, as by spring canker 

 worm, is important in preventing infections, especially in nursery 

 plantings, since the fungus enters through wounds. 



HACKBERRY 



Hackberry is affected by a disfiguring stem disease called 

 witches'-broom. 



