CULTIVATED PLUMS AND CHERKIES. 225 



germinate by sending out new mycelial threads, wHcli can 

 enter into the stems upon which they have fallen. The object, 

 then, should be to cut off the knots before the spores are ripe. 

 By cutting in summer we can prevent the maturing of the 

 winter spores ; by cutting early in the spring we can prevent 

 the ripening of the conidial spores. It is not enough, however, 

 simply to cut off the diseased branches. If the winter spores 

 have begun to form, they go on and ripen, even if the knots 

 are cut from the tree, notwithstanding they may be exposed 

 to a great degree of cold. Knowing this, we can infer that it 

 is safer to burn all knots wliich are removed. 



The black knot is unknown in Europe, although the Euro- 

 pean cultivated plums and cherries are botanically the same 

 as ours. How does it happen, then, that our trees have a 

 disease unknown in Europe ? The reason is this : the fun- 

 gus which causes the disease is a native of America, and grows 

 on our wild plums and cherries. In Massachusetts it is found 

 on the choke-cherry (^Prunus Virgmiana), the bird-cherry 

 (^Prunus Pennsylvanica)^ and the beach-plum (^Prunus mari- 

 tima). Farther west, it is also found on the wild plum 

 (Prunus Americana) and on Prunus Chieasa. Being a 

 native of America, when plums and cherries were introduced 

 from Europe, the fungus grew upon them as well as upon our 

 own wild species. Its injurious effects are better known on 

 the cultivated plums and cherries, because, being cultivated 

 for their fruit, they are more generally observed than the com- 

 paratively worthless wild species. All our wild cherries are 

 not attacked by the fungus, as, for example, the rum-cherry 

 (^Prunus serotina') ; and there are a number of cultivated 

 varieties of cherry which are not subject to the disease. In 

 attempting to check the disease, one should not forget to 

 remove the knots from the wild cherries growing near or- 

 chards as well as from the cultivated cherries. 



Probably but few of the tumors on trees and shrubs can 

 be said with certainty to have been caused by fungi ; yet no 

 tumor of any size is probably free from them. The number 

 of species of fungi is enormous, and not a small proportion 

 inhabit dead wood and bark ; and the rough surface of any 

 old tumor forms a suitable place of growth for a great many 

 species. They are, however, not the cause of the knots, but 

 an after-growth, and are recognized as such by those Avho 



29 



