Coi.oHADO Plant Diseases 
57 
No ortVctive control iiieasures have yet been worked out. Drain 
from infested fields should never be used for seed. A rotation keep- 
ing any cereals off the land for four or five years should be practiced. 
Any suspicious cases should be immediately reported to the State 
Agricultural College. 
CHERRY 
Black Knot (Plowrightia morbosa) (21) — Black knot is common 
on cherry and plum trees throughout the state. The disease is con- 
fined entirely to the woody part of the 
tree, causing black swollen wart-like knots 
extending for several inches along twigs 
and branches. Affected twigs are often 
bent and twisted or otherwise deformed. 
The knots are at first soft and cushion like 
with a velvety surface but turns black, 
hard and rough with age. 
The knots are due to a fungus which 
invades the tissues of the branches caus- 
ing the abnormal growth. The knots are 
made up partly of fungus growth and 
partly of deformed tissues of the twig. In 
the early stages when the knot is soft and 
cushion like many small spores (conidia) 
are borne over the surface giving it a 
velvety appearance. Later Avhen the knot 
become hard, spore cases are formed im- 
bedded in the surface. These spore cases 
or perithecia contain spore sacks or asci, 
each ascus containing eight small asco- 
Biack knot of plum. (Original spores. The ascospores protected by the 
in Bulletin 212, by Robbins.) , 
spore cases may easily live over the winter 
to start the disease again next spring. 
Black knot may be controlled by pruning out and destroying all 
knots as soon as they appear. Wild plums and cherries are also 
''affected and the disease may spread from these. Where it is not 
practical to eradicate these, spraying with Bordeaux mixture is ad- 
vised. One application should be made during late winter and 
another when the buds begin to swell. Other applications may be 
made if found necessary. 
