Diseases of Plants 305 



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 : 



(1) 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. In a 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. 



