6 
Colorado Experiment Station 
PART I 
THE NATURE OF PLANT DISEASES 
Every plant disease has a cause, although the cause may, in some 
cases, be rather obscure. The cause may be merely some unfavor- 
able condition of environment such as too much or too little 
water, excessive heat or cold, or the lack of some essential element 
in the soil. Most of the important plant diseases, however, are caused 
by very small microscopic plants which live upon and draw all 
their food from the diseased plant. These small microscopic plants 
Mistletoe Growing on a Western Yellow Pine 
live on the large crop plants just as the cro]) plants live on the soil 
in which we plant them. They live on the larger crop plants and 
draw their food from them just as the well knoAvn inistletoe lives 
on and draws its food from the trees on which it lives. They also 
produce ‘hseed” or spores, similar to the seed of higher plants, 
which, when they fall upon the })roper plant, under the ]) roper con- 
ditions, will germinate and penetrate the leaf, stem or root of the 
plant just as the seed of a higher plant penetrates the soil. 
