2 68 Minnesota Plant Diseases. 



like a smooth-surfaced palisade fungus. The whole fruiting body 

 seldom attains a length of more than three-eighths of an inch 

 and a width of one-fourth inch. The sacs, with eight spores 

 each, line its upper surface. The whole cup is colored similar- 

 ly to the mycelium. This rot is particularly abundant in the 

 northern part of the state, though on account of its slow 

 growth and its preference for the smaller branches it does com- 

 paratively little damage. 



Stem canker of balsam fir [Dasyscypha resinaria {Cook 

 and Phil.) Rehm.]. The cause of this disease on the common 

 balsam-fir is a very small cup fungus. It causes canker swell- 

 ings on the stems and branches. These cankers usually par- 

 tially and sometimes entirely encircle the stem. In the latter 

 case the tree trunk is killed and the balsam dies. In the canker 

 an abundance of resin is formed. The disease is very similar 

 to the European larch canker, which forms on the common 

 larch of Europe a similar resinous canker. The fungus fruiting 

 cups are produced only on the cankers and are formed within 

 a year of the death of the infected branch or tree. The fruiting 

 bodies are thus produced during the saprophytic life of the fun- 

 gus. The earlier life of the fungus is probably parasitic, though 

 no infection experiments have been carried out to prove this 

 point. 



The fungus cups are very small — about one-fifth of an inch 

 in length and of about the same width. They are provided 

 with a very short stalk and are covered with very fine hairs. 

 The disc or inside of the cup where the spore sacs are formed is 

 orange-colored. The attacked portion of a branch shows a 

 thickened inner bark and the wood rings are also increased in 

 thickness. No remedial measures have been worked out. 



Tar spot of maple [Rhytisma acerinum (P.) Fr.]. One fre- 

 quently meets with black, tar-like spots on leaves of maples in 

 late summer or early fall. These tar spots are caused by a fun- 

 gus of the cup-fungus group and the spots are in the nature of 

 a storage mass of threads which persist through the winter. 

 In the early summer the spots are yellowish ; they then produce 

 small pear-shaped depressions, containing very small spores, 

 which flow out of the depressions onto the surface of the spot. 

 In the fall the spot turns black and then resembles a drop of 



