90 MANUAL OP TREE DISEASES 



Leaf-Blight 

 Caused by Keithia ihujina Durand 



The leaf-blight or black leaf-spot of western arbor-vitse is 

 common and destructive, especially to yomig trees, throughout 

 its range in northwestern United States. In dense stands 

 and in localities where humid conditions prevail, this disease 

 causes the death of a large percentage of the seedlings less than 

 four years old. In late summer the lower branches of older 

 trees when affected by this blight appear as if scorched by fire. 

 In some localities the foliage of the upper parts of the trees 

 also may be affected. This is, however, essentially a disease 

 of seedlings. The affected parts are those which are covered 

 by snow until late in the season. 



Symptoms. 



In spring and summer the affected leaves show from one to 

 three more or less circular brown cushions bursting through the 

 epidermis. Later these bodies turn black. The affected leaves 

 die and tm-n brown in late summer. The twigs bearing the 

 brown leaves also fall, leaving the branches bare. In autumn 

 the black bodies in the older leaves often fall out, leaving holes, 

 and these leaves turn gray. 



Caiise. 



The leaf-blight or black leaf-spot of the western arbor-vitse 

 is caused by the fungus Keithia thujina. This fungus is closely 

 related to the tar leaf-spot fungi of maple and willow and the 

 black-specked leaf-spot fungus of maple. The black fruiting 

 bodies on the leaves crack open, irregularly, and expose the 

 ascospore-bearing surface within. The ascospores are wind- 

 blown and infection usually occurs in the autumn. Moist 

 weather is necessary for the discharge of the spores. 



