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STATE BOARD OF AGRICULTURE. 



Fig. 21. — Black Knot fungus, a, mature knots on plum branches; b, magnified section 

 of a knot, showing the spore cases (perithecia); c, spore stalks (conidiophores), which 

 grow on the surface of the knots when young and give rise to summer spores (conidia), c; 

 d, section of a winter spore case (perithecium), more highly magnified, showing numer- 

 ous contained spore sacs (asci), one of which is shown very highly magnified at e; f, 

 several of the two celled winter spores germinating in water. (Original.) 



color and a velvety surface. This appearance is due to the great number of 

 minute spore-bearing stalks which grow on the surface of the knot. The spores 

 shed from the velvety knots are capable of infecting other branches during the 

 present growing season so that the knot becomes a source of contagion even in 

 its early stages. Late in the winter the knot has lost its velvety appearance 

 leaving a dead black, uneven, more or less cracked surface. Under a hand lens 

 the outside appears to consist of small, closely crowded pimples or pustules with 

 a minute depression in the center of each. A section through one of the knots, 

 when examined under the compound microscope, shows these pustules as hollow, 

 thick-walled bodies lined with a layer of club-shaped cells (asci) containing 

 spores. These are the winter spores and serve to start the fungus anew the next 

 season. Hence the knots if left on the trees over winter also serve once more as 



