Minnesota Plant Diseases. 



359 



protuberances, each of which has an opening at its apex. These 

 openings communicate with the pear-shaped cavities of the cap- 

 sules, which contain the numerous spore-sacs, each bearing eight 

 spores. These are the winter spores and are capable of causing 

 infection during the year following their formation. The fungus 



Fig. 191. — Black knot of wild cherry, showing various stages in the development of the 



knots. Original. 



mycelium may also live over the winter in the tissues of the host 

 and grows from year to year. The ultimate effect of the black 

 knot on a branch is to kill off the entire branch above the knot. 

 When a knot works downward to another branch the latter will 

 also soon be killed. 



