ECONOMIC ASPECTS. 



93 



CONDITIONS FAVORING THE SPREAD OF THE DISEASE. 



The conditions favoring the spread of diseases may be wholly telluric, such as 

 high temperature, unusual drought, cold weather, fogs, heavy dews, and excessive 



or continuous rainfall. These diseases may be favored by 

 lack of natural drainage, or may be brought on by a 

 variety of causes which are largely within the control of 

 the grower, such as selection of improper varieties, i. e., 

 very susceptible ones, overcultivation, storage at too high 

 temperatures (in case of cabbage and root crops), the use 

 of infected soils, or manures, or seeds, or plants, and, 

 especially in hot-houses, by the mismanagement of water 

 and heat, and by the neglect to destroy the first diseased 

 plants that appear and such transmitters of disease as 

 insects and slugs, which frequently abound in hot-houses. 



Fig. 77.* 



METHODS OF PREVENTION. 



In case of certain diseases, copper fungicides have been found useful, e. g., in 

 walnut bacteriosis and some of the leaf spots, but in general we know as yet very 

 little about bactericidal treatments. In the 

 early stages of an outbreak some of these 

 diseases may be controlled by extirpation of 

 the affected parts, or by the removal of whole 

 plants as soon as they show signs. Also, if 

 possible, the common carriers of infection 

 should be eliminated. Finally, one should not 

 forget that the substitution of resistant vari- 

 eties for susceptible varieties is one of the 

 most hopeful methods for disposing of certain 

 of these vexatious diseases. Whenever any- 

 thing specially noteworthy has been discov- 

 ered in the way of treatment it will be mentioned under each particular disease. 



Fig. 78.f 



*Fic. 77. Bacterium campestre from the cavity shown in fig. 76, illustrating water-pore infec- 

 tion of the cabbage. X 2,000. 



tFic. 78. Bacterium campestre occupying a spiral vessel in a cabbage leaf near a group of 

 infected water-pores. The tissues to the right and left of this vessel, and also above and below it 

 (slide 223 33, 18.6 by 9.7), are entirely free from bacteria. The body of the leaf and all its inner 

 tissues up to within a few millimeters of the leaf-tooth, and also the outer surface of the leaf up to 

 the water-pores, are sound. On the contrary, an unbroken bacterial occupation can be traced from 

 this vessel outward to the water-pore region. The bacteria in this vessel are also less abundant 

 than in those nearer to the group of water-pores, '. e., its occupation is of more recent date. Even 

 if there were no other evidence of infection by way of the hydatodes than that afforded by this 

 vessel, the presence of the bacteria in it under the circumstances mentioned points conclusively to 

 marginal (water-pore) infection as their only possible source. The position of this vessel is in a 

 small vein a little below and at the left of X in fig. 76. Its distance from the left margin of the 

 bacterial cavity is one field of the 16 mm. Zeiss objective with the 12 comp. ocular. Its distance 

 from the sound leaf margin is two-thirds the diameter of such a field. A nucleus is shown at n. 



