DANGEROUS INTERNATIONAL FOREST TREE DISEASES 31 



Oak Wilt 



If oak wilt should suddenly appear in other continents, some of 

 the following control measures might be worth considering. When 

 the infected tree and all other trees within 10 to 15 meters are 

 killed with sodium arsenite, local spread of the disease is stopped. 

 Sodium arsenite must be used with suitable precautions so that ani- 

 mals are not poisoned. This chemical has been effective, but many 

 others have been inadequate or worthless. Where desirable, the local 

 spread may be prevented by cutting the roots between infected and 

 healthy trees or by placing a chemical barrier between healthy and 

 diseased trees with methyl bromide or Vapam injected in the soil. A 

 girdle through at least the last three annual rings may hasten the 

 desiccation of the tree and reduce fungus mat formation. 



Poplar Cankers 



Natural and artificial poplar Irybrids are under test for resistance 

 to various diseases. Cankers caused by Hypowylon. Dothichiza. 

 SeptoHa. and Fusariwm are among the more important. In addition, 

 attention is being given to such problems as wet wood, spring de- 

 foliation, rust, and dieback. 



Some of these fungi are well known in Europe and elsewhere. 

 However, we do not yet know whether they are the same races as 

 those which occur in North America. 



White Pine Blister Rust 



In addition to the conventional control of white pine blister rust 

 by the eradication of Ribez bushes, research is underway to develop 

 resistant white pines. First generation progenies from controlled 

 crosses with six resistant selections contain a considerable percentage 

 of resistant seedlings. 



The incidence of white pine blister rust infection in Wisconsin 

 is associated with the microclimate. Silvicultural aids to control can 

 be made on the basis of forest stand microclimates. Descriptions 

 of local climatic situations have been worked out that serve as guides 

 for removal of Ribes bushes only when necessary. They serve also 

 for selecting sites for planting white pine. In some places the danger 

 from the blister rust is great; in others the danger is too small to 

 consider. Such areas may be quite close together. Scions and seed 

 from elite white pines have been sent to a number of places abroad. 



Dutch Elm Disease 



Research is underway to reduce the damage from the Dutch elm 

 disease through genetic resistance and with systemic chemicals both 

 against the fungus and against the insect vectors. 



Seed Collections 



At the request of Dr. Takahashi, seeds from various Wisconsin 

 species have been collected and forwarded to Japan. 



The research reported has been done in cooperation with the 

 Wisconsin Conservation Department, the U.S. Forest Service, the 

 Nekoosa Edwards Paper Company, and others. 



