86 



PROTECTION OF PLAXTS, 1915-16 



up and down the branch for several inches. The smaller branches and twigs 

 it generally girdles, stopping the flow of sap and leaving them often badly 

 distorted and dead. 



The disease, as a rule, appears in the spring in the form of a slight 

 yellowish swelling. This generally takes place at some junction of the 

 limb or at a notch in the branch where spores have found a convenient 

 lodging place. As the swelling increases, the bark cracks and the parts 

 affected become coated with a soft, velvety, olive-green mass — the summer 



Black Knot Fungus — a, mature knots on plum branches; b, magnified section of a knot showing the spore cases 



c, spore stalks which grow on the surface of the knots when young and give rise to summer spores; 



d, section of a winter sporecase more highly magnified, showing numerous contained spore 



sacs, one of which is shown v-ery highly magnified at e\ /, several of the two-celled 



winter spores germinating in water. (After Longyear). 



