Minnesota Plant Diseases. 



275 



appear on the mycelium as very minute dots which are at first 

 yellowish and finally become black. Each capsule, when seen 

 under the microscope, is surrounded by a dense circle of thread 

 appendages which are hooked at the end in a manner similar 

 to those of the powdery mildew of willows. Each capsule con- 

 tains numerous sacs and each sac contains two very large 

 spores. It is very possible that this fungus causes a considera- 

 ble amount of damage to young elms in forests. The summer 

 spores are formed earlier and cause the starchy appearance of 

 the young mycelium. The fungus does not seem to be widely 

 enough distributed to be a serious menace at present though 

 the vigor with which it attacks is an indication that it may at 

 some future time become a dangerous pest. 



Pine stem rust (Species of Peridennium). The branches and 

 stems of pine trees are attacked by this fungus. The result- 

 ing disease is commonly known as pine knot. The infections 

 are more or less localized and do not extend through the whole 

 plant. The infected portion is stimulated to the production of 

 boil-like swellings which may continue to grow for years. The 

 swelling is often accompanied by an abundant formation of 

 resin and turpentine which sometimes exudes from the canker. 

 The fungus attacks the growing zone in the stem and may 

 finally completely encircle the latter, but this usually happens 

 only after a struggle lasting through several years. The con- 

 duction of the water through the stem is seriously interfered 

 with and a drying up of the upper part of the tree may result. 



The fungus causing this disease is a rust fungus and is a 

 true parasite. The complete life histories of the American 

 pine-stem rusts have not yet been unraveled ; but from a com- 

 parison with better known European forms it seems very prob- 

 able that our species pass a part of their life on another host, be- 

 side the pine. On these so-called alternate hosts the winter 

 spores are to be looked for ; these spores are therefore at pres- 

 ent unknown. The swellings of the pine stem described above 

 always bear the cluster cup spores. The latter are found in 

 cluster cups or over flat areas on the surface of the canker. 

 They are light yellow or yellowish-orange in color. When the 

 cluster cups open in the spring, large areas on the surface of the 

 canker are covered with a bright yellow coat of spores which 



