82 
Colorado Experiment Station 
If a system of rotation is adopted in which the susceptible crop is 
grown on the same soil not oftener than once in three or four years 
the organism will die out or at least will not develop in sufficient 
quantities to seriously injure the crop. There are a number of dis- 
eases such as wilts and root rots that can be held in check only by 
this method. Moreover, a number of disease-producing organisms 
normally live over the winter on diseased parts of plants that are 
left scattered over the field. If the crop is planted on the same 
soil two or more years in succession this diseased rubbish and plant 
parts will be in the best possible place for starting the disease the 
second year. Crop rotation involves almost no expense other than 
a little forethought and planning. The returns in the disease free 
crops pay for this trouble many times over, not to speak of the 
added gain in soil fertility. 
Seed Selection and Seed Treatment 
Selection of clean seed and seed disinfection not only prevent 
the introduction of disease-producing organisms to new localities 
but they also act as a protection to the resulting crop. The spores of 
many parasitic fungi are carried over on the seed and infect the 
young plant when it begins to grow. Disinfection with certain solu- 
tions of proper strength will kill the fungus spores without injuring 
the seed and the plants produced from such seed are usually free 
of disease. Solutions of formaldehyde, copper sulfate or corrosive 
sublimate are commonly used for this purpose. There are other dis- 
ease-producing organisms however which infect the developing seed 
and live over inside the tissues of the seed. Ordinary methods of 
seed disinfection will not kill the organism within the tissues with- 
out injuring the seed. For these diseases, therefore, the selection 
of disease-free seed is the most effective means of control. Seed 
should be selected in the field from disease free plants because dis- 
eased seed often cannot be detected after harvest. 
Spraying 
This is one of the most common methods of plant disease control. 
The object of spraying is to keep the surface of the plant covered 
with a substance that will kill any spores that fall upon it and at 
the same time will not injure the plant. The spraying must be 
applied before the spores appear, for, if the spores once gain en- 
trance into the plant, spraying is useless. Spraying should be done 
before rains and not afterwards, because spores require water for 
germination and it is at this time that protection is needed. Sprays, 
