Diseases of Plants 205 
attacks its host. Most measures of control aim to pre- 
vent the appearance of a disease, or at least to check its 
spread after it has appeared. In general, the various 
methods which are employed and which have been noted 
above may be grouped in the following classes : 
(t) Sanitary measures. The remains of diseased 
plants often contain countless numbers of the spores of 
the fungus or of the bacterium causing the disease. To 
leave such plant refuse scattered about the garden or in 
piles about the border often assists the fungus or bac- 
terium to live over winter, and thus invites a reappear- 
ance of the disease. On this account the garden should 
be kept clear of plant refuse, and the remains of plants 
known to be infected should never be used in a humus 
pile. Hotbeds, cold frames, and flats should be thor- 
oughly cleaned out at the end of the season; and they 
should be sprayed or sprinkled with weak solutions of 
formaldehyde. Sanitary measures are as desirable for 
the garden as for the household. 
(2) Crop rotation. Through rotation in the planting 
of crops it is often possible to kill out the fungus by not 
planting a crop that serves as a host for it. This is 
especially the case when a parasitic fungus lives, in some 
form or other, for a considerable time in the soil where 
the best sanitary measures are of little avail. In larger 
gardens and on a farm, various crops can be shifted to 
different locations from year to year. Ina small garden 
there is less chance to practice crop rotation. But even 
here it can be practiced to some extent. For example, 
late cabbage should not be planted in soil on which an 
early crop showed infection with the clubroot disease. 
