80 ILLINOIS NATURAL HISTORY SURVEY CIRCULAR 46 



Control of this disease requires first that all affected twigs 

 should be removed and burned. Additional protection may be 

 obtained by sprays of organic mercury or copper sulfate and hy- 

 drated lime (Table 1). Three applications of spray are recom- 

 mended. The first spray should be applied as soon as new shoot 



Fig. 78. — Drooping and dying of shoot tips and browning of needles of 

 spruce are typical of Diplodia tip blight. 



growth appears, the second when new shoot growth is about half 

 grown, and the third about 2 weeks later. Trees stunted by the 

 disease should be given tree food to stimulate vigorous growth, 

 as described under "Feeding." 



POPLAR 



Some species of poplar, such as Lombardy and Simon, are 

 frequently attacked by canker diseases and occasionally by rust 

 and crown gall. 



Rust. — Larch rust, a leaf disease caused by the fungus 

 Melampsora medusae, has poplar and larch as alternate hosts. 

 It occurs occasionally in Illinois. On poplar, it produces yellow- 

 ish-orange, powdery pustules on the lower surfaces of leaves dur- 

 ing the early summer. These pustules change to dark brown or 

 black during late summer and autumn. They may be scattered, 

 or they may be so closely crowded that the entire surface of an 

 affected leaf appears powdery. Leaf rust usually causes insuffi- 

 cient damage on poplar to warrant treatment. 



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

 Cytospora chrysosperma, often kills Lombardy and Simon 

 poplars in Illinois. On young trees, cankers first appear as 

 slightly sunken areas in the smooth bark of branches and trunks. 

 These cankers are circular to oval or irregular in shape (Fig. 

 79). They gradually enlarge until, frequently, affected stems are 

 girdled and killed. As the cankers enlarge, the diseased bark be- 



