ASCOMYCETES 249 



the central West are practically free from this disease. In spite of 

 the interchange of plants between Europe and America, the black 

 knot has not yet been reported from either England or from the 

 continent, and so far as can be ascertained, it is not known to 

 occur in other countries. 



Host plants. Very few of the native species of plum or cherry 

 are free from this fungus. Among those which suffer particularly 

 from the disease may be mentioned the Chickasaw plum (Primus 

 aiignstifolia\ the yellow plum (Primus americana), the wild black 

 and red cherries (Primus serotina and Primus pennsylvanicd), 

 chokecherry (Primus virginiana\ the bird cherry (Primus avium), 

 and the morello varieties (Primus Cerasus). It is said to occur 

 upon other species, but definite records are not at hand. On ac- 

 count of the fact that this fungus attacks wild plums as well as 

 cultivated, it is a constant source of danger to plum growing wher- 

 ever the native plums abound in neglected places. In certain 

 seasons the knot will be found in abundance upon some species 

 only, while it will almost entirely omit other hosts. This has 

 prompted the opinion that there may be several species or forms 

 of the fungus affecting the different hosts. This may be true, but 

 it would appear to be almost as well explained by admitting the 

 very evident fact that certain hosts are in general more susceptible 

 and by assuming that during some seasons particular species may 

 be rendered peculiarly susceptible. 



Symptoms. The black knot is a most unsightly disease, con- 

 sisting of wart-like hypertrophies or excrescences which may cover 

 a considerable area on the twigs and limbs. It is confined entirely 

 to the woody parts. The term black knot applied to this disease 

 is a very fortunate one, since the deformities take the form of 

 elongated blackened knots, usually extending a distance of from 

 one or two to four or five inches upon the affected branches. 

 For the .most part the injury is confined to one side of the 

 branch, or at least it does not generally form a complete ring, 

 which would effectually cut off the nutriment from the tip portion. 

 The first appearance of the knot is usually noted in the spring, 

 although it has also been observed to make its appearance in the 

 fall (Humphrey). At first it consists of a slight swelling of the 

 branch, originating upon any portion whatever, but generally on 



