CARTER: ILLINOIS TREES: THEIR DISEASES 91 



rays and frequently penetrates to the center of an affected stem. 

 Stems girdled by cankers die beyond the diseased areas. 



Special care must be exercised when pruning is done to con- 

 trol this disease, since the fungus can be transmitted by pruning 

 tools, wound dressings, and any other agencies or operations that 

 make contact with the fungus on diseased trees and then with 

 wounds on healthy trees. Pruning should be done in winter when 

 the fungus is least active. All pruning tools should be carefully 

 disinfected before being used on healthy trees. Denatured alco- 

 hol, 70 per cent, is a satisfactory disinfectant. Climbing ropes 

 contaminated with the fungus can be disinfected by exposure to 

 formaldehyde vapors for 3 hours (one-fourth pound formalde- 

 hyde in a 10-gallon closed container). Since the fungus can be 

 carried in the ordinary types of wound dressings, a gilsonite- 

 varnish type of paint which has 0.2 per cent phenylmercury 

 nitrate mixed in it should be used on wounds of trees being 

 treated for this disease. 



WILLOW 



Willow is susceptible to leaf and stem diseases. Trees may 

 be killed by the stem diseases but not by the leaf diseases. 



Tar Spot. — This leaf disease is caused by the fungus Rhij- 

 tisma salicinum, which produces jet black, thick, tarlike, raised 

 spots on the upper surfaces of leaves. The spots caused by this 

 fungus on willow are usually much smaller than the spots caused 

 by the tar spot fungus on maple (Fig. 61). In Illinois, willow is 

 usually not severely damaged by tar spot. The disease can be 

 controlled by sprays of organic mercury or copper sulfate and 

 hydrated lime (Table 1). Two applications are recommended, the 

 first application when the buds are opening and the second 3 

 weeks after the first. Raking and burning of leaves affected with 

 tar spot will destroy the overwintering stage of the fungus. 



Leaf Rust. — In Illinois, willow is affected occasionally by 

 two fungi that cause leaf rust. One fungus, Melampsora bige- 

 lowii, has larch and the other, M. abieti-capraearum, has fir as an 

 alternate host. Leaf rust caused by either fungus produces golden 

 yellow to orange, powdery pustules on the lower surfaces of 

 leaves during the summer. By late summer and autumn, these 

 pustules become dark brown or black. They may be scattered or 

 so closely crowded that the entire lower surface of an affected 

 leaf appears powdery. Leaf rust on willow usually causes insuffi- 

 cient damage to warrant treatment for its control. 



