DISEASES OF CULTIVATED PLANTS 341 
a serious decrease in the return from the crop on the special type of 
soil. While for general field culture avoidance of conditions may be 
successful, this is by no meansa simple matter. Rotation is often 
absolutely necessary, but this same rotation will not rid the soil of 
the onion smut fungus, nor of some other parasites such as in the 
case of the club-root fungus of cabbage and related plants. In these 
cases some soil treatment must be applied to field areas. In the case 
of the onion smut it is sufficient to apply a formalin. drip which will 
fall with the seed and disinfect the soil layer in proximity to the seed. 
This is effective because the smut fungus can penetrate the germ- 
inating onion plant only in the earlier stages of growth. On the 
other hand with cabbage club-root, where plants are transplanted 
from the beds in which they are grown, some general method of soil 
treatment which involves the soil mass is more effective. In this 
case it is the application of stone lime or caustic lime in liberal quant- 
ities. These examples are only illustrative of general conditions to be 
met with. In the case of potato scab, itis found necessary both to 
disinfect the seed where scab is present, even toa limited extent, 
and to plant upon new soil not infested with the scab organism. 
Potato rosette is certainly an acid loving parasite. 
| ee 
Fig. 21. Lower portion of two celery plants Showing effects of root rot, RAzzoctonia, It will be 
observed that nearly all the roots of the plants have rotted off in consequence of the attack. (From Cir- 
cular No. 72). 
