15 



Any leaves on which the spots may appear should be promptly cut 

 off and burned. 



When the autumn is long and mild, plants which have lost their 

 leaves from Black-Spot during the summer often put out fresh 

 shoots from the terminal buds of their branches. This process 

 exhausts the plant and lessens its ability to withstand the winter, and 

 should be prevented by clipping off the terminal buds, leaving those 

 lower down to make the next season's growth. There is no advan- 

 tage in spraying the already affected plants in summer and fall, but 

 the " spotted" leaves should be collected and burned, as they drop, 

 to prevent further mischief as far as possible. 



THE BLACK-KNOT OF THE PLUM. 



Ploiorightia morbosa 8 ace. 



This wide-spread and fatal disease, so common on cultivated plums 

 and cheriies and on some species of wild cherries, is peculiar to 

 America, being, as yet, unknown in Europe. Jts characteristic 

 elongated, black, knot-like excrescences are too well known in 

 Massachusetts to require detailed description, since its attacks have 

 practically put an end to the culture of plums in many parts of the 

 state. 



The disease is caused by a fungus, Sphaeria or Ploiorightia morbosa, 

 which attacks the branches of the trees and whose mycelium lives in 

 the swollen tissues of the knots. One of these may often extend 

 nearly or quite around the branch, girdling it and causing the death 

 of all above the knot. When this is not the case, the tree is greatly 

 weakened and soon ceasts to produce fruit, while the knots increase 

 rapidly and finally kill it. 



Besides reproducing itself by spores, the fungus spreads within 

 the branch by the growth of its mycelium and the consequent gradual 

 extension of the knot. Thus it is common to find, in the spring, a 

 new knot immediately adjoining the remains of that of the preceding- 

 year. 



The fungus produces two chief forms of spores. In spring and 

 early summer the surface of the young knot becomes covered by a 

 "bloom," composed of short threads which bear what we may call 

 the summer spores. These germinate promptly and can probably 

 produce fresh infections at once, though our suppositions on this 

 point are based rather on analogy than on direct evidence. Later in 



