BLACK KNOT OF PLUMS AND CHERRIES 
This is one of the most striking diseases of stone-fruits. It is an indi- 
genous disease peculiar to our native wild species of plums and cherries 
and is frequently destructive to our cultivated forms, especially certain 
varieties of plums and sour cherries. The disease is not known to occur 
outside of North America. 
SYMPTOMS 
The disease affects only the woody parts of the host, usually only the 
twigs, though it may extend from spurs to the.larger limbs or body of the 
tree. (See illustration specimen.) Two seasons are required for the 
full development of the knot-. : 
Study the material provided showing the character of the knots in the 
spring of the first season. OBSERVE :— ; 
1. The relation of the knot to the twig. Is the entire circum- 
ference involved? 
2. The shape and color of the gall. 
3. The relation of the swollen tissues to the epidermis and cork- 
layer of the healthy part. 
4. The character of the surface of the knot. The olivaceous 
lumps or patches scattered over the brown woody surface. These are 
the conidial stromata. These stromata bear mature conidia in the spring. 
Compare: several specimens on the above points. SKETCH a typical 
knot. 
The galls are probably initiated at buds or short spurs. Study the 
very young galls provided and oBSERVE:— 
5. The unruptured swellings on the side of the spur or in the 
twig nearby. What does this indicate as to the infection-court? DRAW. 
Study some of the specimens showing the character of the galls the 
second season. OBSERVE:— 
me 6. The darker color and more hard and woody character of the 
galls. 
7. The continuous black perithecial stroma covering the exposed 
surface of the gall. 
8. The minute pimple-like perithecia arising from, and crowded 
over the surface of the stroma. 
. 9. Secondary galls arising just above or below some of the 
primary ones; known by their lighter color and conidial stromata. 
DRAW to show the character of secondary galls and secondary gall- 
formation. 
10. That affected twigs may be bent at right angles to the 
knotted side. pRaw. 
ETIOLOGY 
The black knot is caused by the ascomycetous fungus, Plowrightia 
morbosa (Schweinitz) Saccardo. 
Life-history. This pathogene presents in its life-history one striking 
variation from the usual type; it requires two years to complete a life-cycle. 
The Primary Cycles are initiated in the early spring by inoculum 
produced on two-year-old knots. 
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