542 INSECTS AFFECTING VEGETATION 



branch freely in the stem and in cases where the disease gains 

 a foothold only on one side of the plant the growth on that side 

 is retarded so as to produce a marked curvature. The lower leaves 

 turn brown and drop off, but when the plant succeeds in forming 

 a head the upper leaves are held in place and often turn black and 

 decay, thus destroying the commercial value of the head. 



The extreme variation in the activity of this disease in different 

 years depends largely upon weather conditions. A combination of 

 abundant moisture with high temperature during August and 

 September is favorable for an epidemic. 



Caused by Bacteria. The blackening of the fibre-vascular bun- 

 dles and the accompanying decrease in the water flow is due to the 

 growth in the tissue of a bacterium known as Pseudomonas cam- 

 pestris, Smith. (N. Y. [Ithaca] E. S. B. 232.) 



Prevention of Black Rot. No practical treatment for black 

 rot has yet been discovered. It has been shown that the leaf -pulling 

 treatment instead of being beneficial is positively harmful. Rotation 

 of crops affords little if any protection against the disease. Placing 

 the seed bed on soil which has never grown cabbage or related plants 

 is a good practice, but it remains yet to be proven that it is of any real 

 value as a preventive of black rot. Spraying with resin-bordeaux 

 mixture is, perhaps, worthy of trial, but can not be relied upon to 

 control the disease. 



The virulence of the disease depends largely upon weather con- 

 ditions, and it is unfortunate that the conditions most favorable 

 to the growth of cabbage are also the most favorable to the disease. 

 Rapidly growing plants are especially liable to be attacked. 



It appears to the writers that before much progress can be made 

 toward the control of the disease it will be necessary to determine 

 more definitely how the germs spread from plant to plant and field 

 to field ; also, to what extent they live over winter in the soil, to what 

 extent root infection occurs and whether the disease is transmitted 

 through the seed. (N. Y. [Ithaca] E. S. B. 232.) 



Cabbage Club Root. When a field is badly infected with dis- 

 ease, it may appear at the seedling stage as a dwarfing of the young 

 plants, but the fields are not usually badly enough infected the first 

 season to manifest the disease until the cabbage is half grown. In 

 districts where the disease has just begun to get a foothold, the 

 grower is not likely to notice any trouble, therefore, until after the 

 first of July. At about that time it will be noticed that the plants 

 which are being infected show r a tendency to wilt on bright sunny 

 days, although at night they recover, and do not wilt on the following 

 day unless it is again bright and warm. Such plants may succeed in 

 making enough growth to produce a salable cabbage, although it 

 is somewhat undersized and slightly loose. The earlier the plant is 

 infected with the disease, the smaller is the head produced. If the 

 diseased plant is pulled up, one finds that it has a swollen and con- 

 torted root, in place of the fine fibrous roots of the normal plant. 

 This explains the wilting, when one remembers that the soil and its 

 dissolved food substances are taken up through the fibrous roots. 



