1882.] FUNGI INJURIOUS TO VEGETATION. 169 



The Black Knot — (^Sphoeria morhosa.) 



The conspicuous excrescence often found upon plum, and wild 

 and cultivated cherry trees, and well named the black knot, is an 

 old enemy to the fruit-grower. From its destructive nature it 

 early received the attention of scientific men, and much has been 

 written and said upon this subject, in fact, much more than was 

 really known. Some have claimed that the knot was caused by 

 insect stings, a form of gall, in nature like those on the oak, wil- 

 low, etc. The fact that the knots — especially the old ones — fre- 

 quently contain insects and their eggs, remains, etc., was accepted 

 as evidence that such was the case. Though many investigators 

 have found a fungus present in the knot, it having been first de- 

 scribed by Schweinitz. in 1838, yet, until the recent and thorough 

 investigation made by Professor Farlow of Harvard University, 

 all doubt of fungus origin was not cleared away. The following 

 are his reasons for believing that the disease is not caused by in- 

 sects: "First, the knots do not resemble the galls made by any 

 known insects. Secondly, although insects, or remains of insects, are 

 generally found in old knots, in most cases no insects at all are 

 found in them when young. Thirdly, the insects that have been 

 found by entomologists in the knots are not all of one species, but 

 of several different species, which are also found on trees that are 

 never affected by the knot. On the other hand, we never have the 

 black knot without the Sphoeria morhosa^ and the mycelium of that 

 fungus is found in the slightly swollen stem, long before anything 

 that could be called a knot has made its appearance upon the 

 branch; and furthermore, is not known to occur anywhere except 

 in connection with the knots." 



The black knot is most conspicuous in the winter season, when 

 the trees are free from foliage, and they range in size, from 

 half an inch, to a foot or so in length. The excresence does not 

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

 causes the branch to bend away from that side, or twist it into an 

 irregular shape. When the knot is a large one, it usually kills the 

 branch, or continues to spread up and down from the old knot, 

 until death comes to the branch. In the spring the affected part 

 increases rapidly in size — one might almost say puffs out — and the 

 whole, growing larger, becomes thick and soft; a rupture of the 

 bark soon follows, and the soft substance coming to the surface 

 expands rapidly, and is soon coated with a characteristic greenish 



