92 ILLINOIS NATURAL HISTORY SURVEY CIRCULAR 46 



Cytospora Canker. — This stem disease, caused by the fungus 

 Cijtospo)-a chnjsoHpernia, produces cankers on branches and 

 trunks of willow similar to the cankers described on poplar (Fig. 

 79). The treatment given for the control of this disease on pop- 

 lar is effective on willow. Species of willow resistant to this dis- 

 ease should be used when available. Black willow and peach wil- 

 low are reported as resistant to Cytospora canker. 



Crown Gall. — This stem disease, caused by the bacterium 

 Agrobactermm tiimefaciens, produces swellings or warty, tumor- 

 like galls on many species of trees and shrubs (Fig. 23). On most 

 trees the galls are confined mainly to the bases of the trunks or 

 the roots. However, on willow and poplar the galls may appear 

 on the branches also. Severely affected trees are retarded in 

 growth, and affected roots and branches may die. Crown gall 

 usually causes very little noticeable damage on large trees, but it 

 has been known to kill young trees. 



Young trees affected with galls should be destroyed. Affected 

 branches on large trees should be removed. In nursery plantings 

 where crown gall occurs, all diseased stock should be destroyed. 

 Wounding of stems and roots of healthy plants should be avoided, 

 since infection occurs through wounds. Susceptible stock should 

 not be planted in soil infested with the crown gall organism. 



TREES RELATIVELY FREE OF DISEASES 



Many native and naturalized trees in Illinois are relatively 

 free of diseases. Some of these trees grow large and are used for 

 shade as well as for ornamental purposes. Others grow as large 

 shrubs or small trees and are used mostly for ornamental pur- 

 poses. The group of large trees includes ailanthus, bald cypress, 

 beech, black locust, ginkgo, hackberry, hickory, honey locust, 

 Kentucky coffee tree, larch, magnolia, mulberry, Osage orange, 

 sweet gum, tulip tree, tupelo, and walnut. The group of small 

 trees includes dogwood, hop hornbeam (ironwood), hornbeam 

 (blue beech), pawpaw, persimmon, plum, sassafras, shadbush 

 (serviceberry), and water elm. Some of these trees, especially 

 walnut, are subject to frequent attacks by insects, and some have 

 habits of grovd;h or other characteristics that make them unde- 

 sirable in some situations. 



A supplementary list of trees that are relatively free of dis- 

 eases in Illinois is given below. Many of these trees are used only 

 occasionally in shade and ornamental plantings, a fact that may 

 account for the general absence of diseases. The large trees in- 



