1891.] PUBLIC DOCUMENT — No. 33. 201 



their striking appearance has given to the disease the name 

 by which it is too well known, " the black knot," or" plum 

 wart." 



The cause of this disease has been the subject of much 

 discussion and of innumerable contributions to agricultural 

 journals, from an early date. Some of the first remarks 

 on the subject are to be found in an article by Prof. W. D. 

 Peck of Cambridge.* The appearance of the knot is here 

 attributed to the same insect which causes the falling of the 

 fruit, now called the " curculio." This insect was believed 

 to injure the branch in such a way that, as the writer says, 

 the sap is diverted to the bark, which absorbs it and swells, 

 forming the knots. The general theory here promulgated, 

 that of an insect cause, was for over fifty years very tena- 

 ciously held and earnestly supported, and indeed is still 

 held by many persons, although wholly discredited by 

 scientific study. The fact that eggs or larvae of the 

 curculio are very commonly found in the tissue of the knot 

 has been a strong argument with many in support of 

 Peck's theory ; but others, while holding to the general 

 belief in an insect cause, have believed the knots to be due 

 to some species of gall fly (Cynips), since none of the 

 beetles, including the curculio, are known to be gall- 

 producing insects. Thus argued the entomologist, B. D. 

 Walsh, in an early paper. f But the difficulty here lies in 

 the fact that no gall fly has been found in or raised from a 

 knot. 



Several writers have attributed the trouble to that con- 

 veniently indefinite cause, — a diseased condition of the 

 sap ; and Dr. Fitch, the former New York State Entomol- 

 ogist, held the disease to be of internal, " constitutional" 

 origin. Quite early, Dr. Joel Burnett of Southborough 

 discussed \ the question, showed that the curculio merely 

 lays its eggs in the juicy mass of the knot, after it is 

 formed, and attributed its formation to the attacks of a 

 fungus. This is, perhaps, the earliest statement of this 



* Mass. Agric. Repository, 1819, p. 307. 



t Proceedings Entomol. Soc. Philadelphia, Vol. Ill, p. 613 (1864). 

 J Hovey's Mag. of Hortic, Vol. IX. p. 281; and N. E. Farmer, 16 Aug., 1843, 

 p. 49. 



