Coi.oKADO Plant Diseases 
79 
PART III 
PLANT DISEASE CONTROL 
General Principles of Control 
Every measure that is recommended for the control of a plant 
disease is based upon a knowledge of the factors which produce it. 
Every plant disease has a cause. The logical way to control a plant 
disease is to destroy the cause. If the disease is produced by un- 
favorable conditions of environment the environment should be 
modified to suit the needs of the plant. If it is caused by a parasitic 
organism the organism should be destroyed, or else the plant should 
be protected against its attack. 
A knowledge of the life historj^ of the organism is necessary in 
order that the measure for destroying the organism or protecting 
the plant may be applied at the proper time. The conditions which 
favor the development and spread of the organism should also be 
known in order that these conditions may be avoided. 
In addition to the above, one should be able to recognize a dis- 
ease in all its stages. It is especially important to be able to detect 
a disease in its early stages of development in order that control 
measures may be applied in time to be effective. Many diseases are 
localized in their distribution but they are constantly spreading to 
new areas. Too often diseases invade new territory and are not 
discovered until they become so strongly established that control 
measures are difficult. One should be able to recognize not only the 
diseases that are already present, but also those which are liable 
to be introduced at any time. 
All plant disease control measures may be divided into four 
classes; (I) those based upon the exclusion of a disease from a 
given area, (2) those which depend upon the eradication of a dis- 
ease producing organisms in a given area, (3) those involving the 
protection of plants against a disease producing agent and (4) those 
based upon the use of disease resistant plants. 
Exclusion 
Many of the most injurious plant diseases occur only in restricted 
areas. There are diseases common in Europe which do not occur in 
this country, and the diseases common in this country do not occur 
abroad. With the development of commerce and travel, however. 
