280 Forty-second Report on the State Museum. [138] 



The Black-knot of the Plum-tree and its Guests. 

 A piece of a limb of a plum-tree having the well-known " black 

 knot " upon it was received in July, with the inquiry of the kind of 

 insect that caused its growth, and if there was any remedy for the 

 attack. The tree from which the piece was taken was wholly free 

 from it, it was stated, in the spring. 



It is a popular belief that the black-knot, so common on plum 

 and cherry trees, and which causes annually the death of thousands 

 of these trees throughout the United States, is produced by an insect 

 attack. There is some foundation for this belief in the fact that 

 insect larvae are frequently found within the knot. These, however, 

 are not the cause of the obnoxious growth, but merely enter it for 

 food or shelter during its early formation. 



The common plum curculio, Conotrachelus nenuphar, which is so 

 destructive to the fruit of the plum-tree, has been bred by Dr. Fitch, 

 Mr. Walsh, and others, from larvae inhabiting the black-knot. Mr. 

 Walsh has also bred from it five other species of insects — two of 

 flies, viz.: Geratopogon sp. and Diplosis septemmaculata Walsh, and 

 three species of small moths, referred with doubt to the genus Hedya 

 {Practical Entomologist, i, p. 50). Larvae have on different occasions 

 been taken by me from their cocoons made upon the margin of the 

 black-knot, where it was overgrowing an excision of the preceding 

 year, and the empty pupa-cases of evidently the same moth have been 

 seen protruding from the knot. The moth, unfortunately, was not 

 obtained, but it was probably that of JEgeria pictipes Grote-Eob., 

 which is known to infest plum-trees sometimes in great numbers 

 (North American Entomologist, i, 1879, pp. 17-21, with plate). 



Although it is not many years since the origin of the black-knot 

 was in doubt, for even in 1859, Dr. Fitch pronounced it not a fungus 

 (Trans. N. Y. State Agricultural Society for 1859, xix, p. 606), it is 

 now known to be a fungus growth of a species long ago described 

 and named as Sphceria morbosa Schw. Quite recently it has been 

 transferred to the genus Plowrightia, and this later generic name will 

 probably ere long be generally accepted. 



The specimen sent is of a brown color, for it is not until late in 

 July or about the first of August that it presents its well-known black 

 appearance, caused by " numerous coal-black hemispherical plates of 

 about the size of the head of a pin, each of which is a distinct fungus." 



Professor Riley has quoted Mr. Walsh as having shown that the black- 

 knot fungus infesting the cultivated cherry " was quite distinct froir 

 that attacking the cultivated plums." He has also indicated another 

 species occurring upon the " Miner plum," which may be seen " at a 

 single glance to be essentially distinct from the common black-kn#t 



