_Characteristics._--The woody or spongy excrescences which appear on the
vines, and which are known as black-knots, are really only a wart-like
growth, the origin of which is entirely unknown. It is supposed that an
insufficient outlet for the sap in the spring caused by too close
pruning is the chief cause. Certainly closely pruned vines are more
subject to the black-knot than long pruned vines, but on the other hand
neglected vines which have had no cultivation, and which accordingly
could hardly have had too rapid a flow of sap, suffer more than any
others. The woody warts appear quite frequently on the ends of the spurs
of the old wood, or on places of last year’s growth which have been
wounded or injured in some way, but never on the green wood. They vary
in size from that of a pea to that of lumps weighing several pounds.
When present in small quantities, the warts cause no injury, but when
they become larger the vines may even die. These black-knots always die
with the year, and never survive to the next season. At the end of the
season, they burst open and then often display black spores of fungi,
which, however, are only parasitical growths on the already decayed
wood, and not the cause of the disease. As I said, it is generally
supposed that the flow of sap is during spring time so great that it
ruptures the cells of the vine and causes the warts to form. Under the
microscope, however, there are no such ruptured cells visible. It is
more natural to suppose, that through the accumulation of sap an
irritating poison is originated, which causes the warty growth to form
in a manner similar to the formation of galls. On sandy soil the
black-knot is the most common, probably on account of the earliness and
the natural warmth of this kind of soil.
_Remedies._--So far no decidedly successful remedy has been found. Some
growers advise leaving plenty of spurs on the vine, so as to give a
sufficient outlet to the sap, but it remains to be seen if this will
mitigate the evil. If the black-knot should be very destructive, a
cutting out of the same in summer time while they are forming would be
beneficial. This could best be done in June and July. Mixtures of
coal-oil and lime, etc., have been used during the winter after the
vines were already pruned, but, as the black-knot is then already dead,
no advantages can result from this remedy.