276 



Minnesota Plant Diseases. 



are later uncovered by the splitting off of scales of the bark. 

 Jack pine in Minnesota is very commonly affected and the 

 white and red pines are also though not so frequently attacked. 

 Scotch pines in nurseries and experimental forests are some- 

 times very seriously affected. 



FIG. 136. Pine knot (a species of 1'eridermium), on Scotch pine. 1. The branch on the left 

 is seen with a good-sized knot which is covered with the conspicuous clusted-cup spores. 

 The branchlets of this branch are seen to be thicker than the normal branches; the 

 leaves are shorter and not as green and die early. 2. Section through a knot. 3. 

 Section through the same branch as shown in 2, but below the knot, showing that a 

 very great growth has been caused by the parasitic fungus in 2. Original. 



In view of our lack of knowledge of the life history of these 

 forms no preventive methods of combating this disease are 

 known. All diseased branches should be cut off and burned 

 as soon as they are discovered. 



Leaf rust of pines (Species of Peridermium). This is a close 

 relative of the stem rust of pines. It is likewise a rust fungus ; 



