BLACK ROT OF CRUCIFERS 
This is a very destructive bacterial disease of cabbage and cauliflower. 
It also affects kale, radishes, rape, turnips and mustard. 
SYMPTOMS 
This disease is detected chiefly by the lesions on the leaves, by the 
effects on the plant as a whole and on the vascular system of the mid- 
ribs and the stalk. 
On the leaves. Examine the cabbage or cauliflower leaves provided 
and OBSERVE :— 
1. The large irregular dead or dying areas in the blade. The 
yellowish or light-brown color of the lesions. 
2. Their location in the leaf. What does this indicate with 
respect to the point of initial invasion? 
3. That when held to the light the veins of the affected area 
are black. (See in N. Y. (Geneva) Bul. 232, pl. I and II; or photograph 1.) 
4. In fresh material, if available, the difference in texture of the 
diseased and healthy parts of the leaf. 
DRAW to show the character of the leaf-lesions. 
On the plant in general. Study the diseased heads provided. os- 
SERVE -—_ 
5. The long bare stalk and loose dwarfed head often reduced 
toa mere tuft of leaves (photograph 2). 
6. The stubs or leaf-scars on the lower part of the stem where 
the leaves have died and dropped away. 
7. That in some heads the leaves have dropped away from one 
side only. 
8. That diseased seedlings are dwarfed and fail to head. They 
often wilt and die early. (See N. Y. (Geneva) Bul. 251, pl. I.) 
In Smith’s Bacteria m Relation to Plant Diseases 2, pl. 18, is shown a 
peculiar effect due to slow killing of the margins of the leaves. Note 
carefully. 
Make DRAWINGS to show these characteristic effects on the plant as a 
whole. 
In the vascular system. Examine the leaf-scars on the diseased stalks. 
OBSERVE :— 
9. The black points,—the ends of diseased vascular bundles. 
This is a typical vascular disease. Break a leaf from the diseased stem, 
cut across the petiole and NoTE:— 
10. That here some of the bundles are also black. Split the 
petiole and midrib lengthwise. How far does the blackening extend? 
Make transverse and longitudinal cuts through the diseased stalks. 
OBSERVE :-— 
11. In the vascular cylinder, the blackened strands which here 
and there curve out to the margin. Explain. 
Make a DRAWING to show the stem with blackened strands both in 
cross- and longisection. 
ETIOLOGY 
The causal organism of black rot is Bacterium campestre (Pammel) 
Erwin F. Smith. It is related to the bean blight pathogene, Bacterium 
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