Bi,ack-Knot of Plums and Cherries- 641 



Black knots are now known to be brought on by a fungus, 

 Plowrightia morbosa. It was long suspected that such is the 

 case, for the fungus was imperfectly described by Schweinitz 

 as early as 1821.* But the first published statement that the 

 black-knot is caused by a fungus appears to have been made by 

 Dr. Joel Burnett, of Southboro, Mass. , although no proof of the 

 proposition was given. f It was based upon the fact that the 

 curculio, which is a regular inhabitant of the knots, did not lay 

 its egg in them until the latter had formed, and thus it could not 

 be held as the cause of the first appearance of the swellings. It 

 was not until 1876 that the relation between the knots and the fun- 

 gus was clearly shown. J Dr. W- G. Farlow then removed all 

 doubt as to the cause of the disease, and although the entire life 

 history of the fungus is not described, no further question can be 

 raised as to the origin of the trouble. The following plants also 

 seem to suffer from this disease : Chickasaw plum {Prumcs angusti- 

 folia), beach plum {P. viaritima), native red or yellow plum {^P. 

 Americana), choke cherry (/*. V irginiana') , wild black cherry 

 {/*. seroti?ia), wild red cherry {P. Pennsylvanica) . The sweet 

 cherries {P. Avhim) are said to suffer occasionally, but no authen- 

 ticated case seems to be on record. § 



The life history of the fungus as now understood is in brief as 

 follows :|| The first external appearance of the disease is an en- 

 largement which may be visible in autumn but is most marked in 

 early spring. This swelling is caused by the irritation of the fun- 

 gus which has entered the cambium layer of the branch. This 

 layeris theone from which the inner bark and the wood are formed, 

 and the presence of the fungus causes an abnormal growth to take 

 place (see page 638). The first crop of spores are matured in 

 May and June, and it is these and the threads bearing them which 

 give to the knots the velvet coating they possess at this time of 

 the year. The spores mature quickly and they are then scattered 



* Synopsis Fungorum Carolinae superioris, No. 134. 



\ New England Farmer, August 16, 1843. 



X Harvard University Bulletin, Bussey Institution, Part V., 1876, p. 440. 



\ I once collected knots from a tree of /"rwiM^^zz/ww in southern Delaware. 

 The tree, vrhich was of the mazzard or wilding type, was a foot or more in 

 diameter of trunk, and the small limbs bore very numerous well developed 

 knots. — L. H. Batley. 



II A list of the most important papers relating to the black knot disease 

 may be found on page 636. 



