PLUM DISEASES 



359 



ing the black outer shell. On plums the interior of an old 

 knot is honey-combed. Final stages in the 

 development of the knot show the exuda- 

 tion of gum and the growth on the surface 

 of a pink mold. 



An interesting response is shown in the 

 case of twigs affected near their tips. When 

 the knot so develops, the twig becomes bent 

 so that a right angle will be made from the 

 knotted side. Sometimes, in the case of 

 affected branches which are not killed, a 

 swelling is produced just above and below 

 the knot. 



Cause. 



The black-knot disease is caused by the 

 fungus Plowrightia morbosa. Its action 

 stimulates the tissues of the twigs and limbs 

 to form the characteristic galls or knots. 

 The fungus begins its work by disseminat- 

 ing its spores in the spring. This process 

 is promiscuous, but some of the spores find 

 lodgment upon the plum or cherry. At 

 various points, already enumerated, the 

 knots have their beginning. With the 

 growth of the spore there develops a sys- 

 tem of vegetative threads which pervade 

 the bark and which very soon attack the 

 growing tissue (the cambium) between the 

 bark and the wood. The fungus irritates 

 this region and the tree responds by form- 

 ing not only an excessive amount, but also 

 an irregular arrangement of the bark tissues 

 (Figs. 103 and 104). This irregularity of 

 development in the affected region pro- 



FIG. 101. Black- 

 knot ; one-year-old 

 knot with a brownish 

 velvety surface. 



