140 



DISEASES OF ECONOMIC PLANTS 



When the disease extends completely around the twig, 

 the supply of nourishment to more distant parts is seriously 



interfered with. If 



less than the whole of 

 the circmnference be 

 involved, the damage 

 is not so great, but it 

 is even then sufficient 

 to very seriously im- 

 pair the fruiting of 

 the tree. Black knot 

 may either kill the 

 tree in a year or two, 

 or simply destroy its 

 value. All varieties 

 of plums and nearly 

 all cherries are sub- 

 ject to the disease, 

 which is therefore 

 very widely distrib- 

 uted. 



It is probable that 

 infection can occur 

 only through injured 

 bark. One step to- 

 wards lessening the 

 disease is therefore 

 to exercise all care 

 not to bruise the tree. Effective measures, however, must 

 consist of pruning out and burning all black knots. This 

 course, if followed persistently and thoroughly, will remove 



Fig. 61. — Portion of a plum tree badly infested 

 with black knot. Original. 



