Colorado Plant Diseases 
7 
A plant living upon another in this manner is known as a parasite. 
The plant on which the parasite lives is said to be diseased.- A 
disease caused by such a parasite is known as a parasitic disease. 
Diseases, then, can be divided into three general classes on a 
basis of their cause. Those diseases caused by one organism living 
upon another are put in one group and called parasitic diseases. 
Those caused by unfavorable conditions of environment such as 
heat, moisture and food are placed in a second group and called 
physiological or non-parasitic diseases. As mentioned above there 
are a few diseases the causes of which are not understood. These 
constitute a third group, which may be called diseases of uncer- 
tain cause. 
PARASITIC DISEASES 
The parasites which live upon the crop plants are very numerous 
and of many kinds. There are almost as many kinds of parasites 
as there are plants on which they live. For convenience in study, 
those parasites which cause plant diseases are divided into five 
groups, those which are very much alike being placed in the same 
group. These groups have been named as follows : 1. Bacteria, 
2. Slime molds, 3. Fungi, 4. Flowering plants, 5. Insects and other 
animals. 
Bacteria — Bacteria are sometimes spoken of as germs or mi- 
crobes. They are, how^ever, very, small plants, extremely simple in 
form and structure. They are indeed the smallest and simplest 
plants known. In size they range from 1/50,000 to 1/10,000 of an 
inch in diameter. In shape they may be spherical (ball shaped), 
cylindrical (rod shaped) or spiral (shaped like a corkscrew). Those 
that cause plant diseases are mostly rod shaped. The other kind 
live on dead organic matter or as parasites on animals. 
Some common forms of bacteria, greatly magnified. From Transeaus 
Science of Plant Life. Copyright 1919, by World Book Co., Yonkers- 
on-Hudson, New York 
