PLUM DISEASES 
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ing the black outer shell. On plums the interior of an old 
knot is honey-combed. J'inal stages in the 
development of the knot show the exuda- 
tion of gum and the growth on the surface 
of a pink mold. 
An interesting response is shown in the 
case of twigs affected near their tips. When 
the knot so develops, the twig becomes bent 
so that a right angle will be made from the 
knotted side. Sometimes, in the case of 
affected branches which are not killed, a 
swelling is produced just above and below 
the knot. ‘ 
Cause. 
The black-knot disease is caused by the 
fungus Plowrightia morbosa. Its action 
stimulates the tissues of the twigs and limbs 
to form the characteristic galls or knots. 
The fungus begins its work by disseminat- 
ing its spores in the spring. This process 
is promiscuous, but some of the spores find 
lodgment upon the plum or cherry. At 
various points, already enumerated, the 
knots have their beginning. With the 
growth of the spore there develops a sys- 
tem of vegetative threads which pervade 
the bark and which very soon attack the 
growing tissue (the cambium) between the 
bark and the wood. The fungus irritates 
this region and the tree responds by form- 
ing not only an excessive amount, but also 
an irregular arrangement of the bark tissues 
(Figs. 103 and 104). This irregularity of 
development in the affected region pro- 
Fie. 101. — Black- 
knot; one-year-old 
knot with a brownish 
velvety surface. 
