CARTER: ILLINOIS TREES: THEIR DISEASES 83 



ing branch may die, and occasionally wilt-affected trees die. 

 Some trees may recover from this disease and not wilt in succeed- 

 ing years. Treatment of trees affected with Verticillium wilt is 

 described in the section on maple. 



SPRUCE 



The most destructive disease of spruce in Illinois is Cytos- 

 pora canker. Occasionally spruce trees are affected by Diplodia 

 tip blight. 



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

 Cijtospora kimzei, appears most frequently on Norway spruce 

 and Colorado blue spruce. Occasionally it attacks Roster's blue 

 spruce and Douglas fir. Browning of needles and dying of the 

 low branches of affected trees are usually the first symptoms 

 of Cytospora canker. As the disease progresses, it spreads to 

 higher branches. In time, affected trees become unsightly and 

 lose their value for ornamental purposes. 



The cankers produced are inconspicuous because the affected 

 bark does not noticeably change color or become depressed. Fre- 

 quently white patches of resin appear on the bark in areas where 

 cankers have formed (Fig. 81). Careful removal of a thin outer 

 layer of bark in the area that separates diseased and healthy tis- 

 sues will reveal tiny, black, pinhead-like fruiting bodies of the 

 fungus in the diseased bark. These fruiting bodies contain mi- 

 nute spores which can be spread by rain, wind, or pruning tools. 

 The development of trunk cankers may result in girdling and 

 death of affected trees. Cytospora canker, most common on trees 

 over 15 years old, is known to occur on younger trees, also. 



Control of Cytospora canker requires that all diseased 

 branches should be cut back to the nearest living laterals or to 

 the trunk of the affected tree. Wound dressing should be applied 



Fig. 81. — The occurrence of Cytospora canker on spruce is usually indi- 

 cated by conspicuous deposits of resin on the bark. This resin frequently 

 forms as a white incrustation. 



