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, z. ¢., 
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 
To mm _ plants that appear and such transmitters of disease as 
Fig. 77.* insects and slugs, which frequently abound in hot-houses. 
METHODS OF PREVENTION. 
In case of certain diseases, copper fungicides have been found useful, ¢. 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 On 
plants as soon as they show signs. Also, if 
possible, the common carriers of infection 
should be eliminated. Finally, one should not il | | ( | | 
forget that the substitution of resistant vari- | (( wl i | 
eties for susceptible varieties is one of the (ll 
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.+ 
*Fic. 77—Bacterium campestre from the cavity shown in fig. 76, illustrating water-pore infec- 
tion of the cabbage. xX 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 a3, 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, i. ¢., 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. 
