298 Wisconsin State Hoeticultueal Society. 



the most striking disease of vegetable origin occurring on fruit 

 trees in this country. The disease takes its name from the un- 

 sightly, black, wart-like excrescences, with which every one is 

 familiar on plum trees and different kinds of wild and cultivated, 

 cherries. It is found in all parts of our country, east of the Eocky 

 mountains, and is so common and destructive that in some districts 

 one seldom sees a plum tree free from the knot; In some parts 

 of New England, particularly in Maine and along the sea-coast, 

 the raising of cherries has also been almost abandoned in conse- 

 quence of the ravages of the black knot. The disease is peculiar 

 to America, and has been the bane of fruit-growers from early 

 times ; and although much has been written in agricultural papers 

 about its injury to the fruit crop, the subject has been almost 

 entirely neglected by botanists. As a preliminary step it will be 

 well to trace the development of the knot as it occurs on a single 

 species, and for this purpose the choke cherry (Prunus Virgini- 

 ana) may be selected." The size of the knots varies greatly, be- 

 ing found on the species of Prunus under consideration all the 

 way from a few lines to several inches in length, with an average 

 of two inches in circumference. The knot does not usually 

 entirely surround the branch, but growing from one side, often 

 causes the stem to bend or twist into an irregular shape. In the 

 winter, when the branches are leafless, the knots are much more 

 noticeable, and at this eeason they are often cracked, broken, 

 worm-eaten and hollow. 



In the swollen portions of the branch, above and below the 

 knot, sections under the microscope show the vegetative portion 

 of the fungus in the form of minute threads, twisted together and 

 extending from the cambium towards the outer portion of the 

 stem, where they become separated. The fungus first reaches the 

 cambium, either by germination of spores on the surface of 

 the branch, or by mycelium proceeding from a neighboring knot. 

 The part of the cambium free from these bundles of mycelial 

 threads grows in the usual manner; and in an old branch shows 

 one more layer of wood on the sound side than on the diseased 

 side. From this it is to be concluded that the growing layer of 

 tissue of the plum or cherry branch is the place in which the 



