36 FOREST TREE DISEASES. 



spring rains and finally unite with neighboring cush- 

 ions to form rather conspicuous, slimy, and light 

 orange-colored masses, which later dry up to a thin 

 film and finally drop off. By this time the sprays have 

 turned quite yellow. Later they die. 



This form of the disease must not be confounded 

 with the work of a beetle which girdles the twigs, 

 causing them suddenly to turn yellowish-white and 

 hang down as though broken. Sprays killed by a rust 

 never have this appearance and are yellower than those 

 killed by the beetle. The small cushions on the under- 

 side of the leaves are composed of the winter spores of 

 the fungus. These germinate during the warm spring 

 rains, and produce a second kind of very small spores. 

 The fungus leaves the incense cedar for the time being 

 and chooses another host (western service-berry), upon 

 the young leaves of which the secondary spores germi- 

 nate. After a short time very small, orange-colored, 

 cup-shaped fruiting bodies appear on the leaves, and 

 the spores produced in these are then again able to 

 infect incense cedar. 



Incense cedar witches' brooms, which are very com- 

 mon, are also caused by the rust fungus. Only the 

 smaller sprays, apparently, are killed by the mycelium. 

 If these alone are attacked, no deformation of the 

 host tree is noticeable. When the mycelium enters an 

 older twig, however, it locally changes the branch- 

 ing system. The resulting witches' brooms (p. 21) are 

 numerous and conspicuous. The cushions of winter 

 spores develop on small sprays at the base of these, and 



