30 
NOTES ON PLUM CULTURE. 
BLACK KNOT. (Ploivrightia morbosa • (Schw.) Sacc.) 
The fungus causing Black Knot has proved destructive 
to plums and cherries in many of the eastern states. It has 
not, so far as my information goes, attacked cultivated 
plums in Colorado, but from its presence as a common dis¬ 
ease of the wild plums of the foothills, it seems likely that 
it may at any time appear in orchards. 
Black Knot has been known as a disease of plums for a 
long time, but the cause was for many years a mystery. The 
larvae of insects being commonly found in old knots, led 
many to believe that the trouble was due to them, but ento¬ 
mologists proved that the larvae found were only using the 
abnormally developed tissue as food and had nothing to do 
with its production. The fungus was named as early as 
1821, but discussion regarding the true cause continued un¬ 
til Dr. Farlow, * of Cambridge, worked out the life-history 
of the fungus and established beyond controversy that it was 
the cause of the trouble. 
The presence of the disease is first seen in swellings on 
twigs; these are due to an abnormal growth induced by 
some irritative action of the fungus threads. As develop¬ 
ment proceeds the bark is ruptured, the exposed inner sur¬ 
face becomes covered with spore bearing threads, and as¬ 
sumes a greenish-brown color. These spores are carried by 
winds and insects and serve to infect other branches or 
trees. The knot continues to enlarge, becomes hard and 
changes to a brown and finally black color. Later in the 
fall cavities form in the tissue of the knot and in these are 
produced a second form of spores which may escape in 
spring to further disseminate the fungus. Two other spore 
forms have been found in connection with the fungus, but 
further mention of them is not necessary here. The threads 
of the fungus are perennial within the tissues of the plant, 
and when once started, growth will continue until the tree 
dies. While spraying at the proper time may be of use in 
preventing spreading to other trees, the only effective rem¬ 
edy for trees attacked, is to cut and burn the knots as soon 
as discovered. One of the characteristic knots is shown in 
Fig. 1, Plate 4. 
* Bulletin Bussey Institution Part V pp. 440-453 (1876). 
