44 
IOWA ACADEMY OF SCIENCE 
TWO KINDS OF DISEASES, 
Diseases of plants may be grouped under the following main heads; (1) so- 
called physiological diseases, due to the action of the non-living environment, 
such as the soil, atmosphere, temperature and light, (2) those produced by 
living plants and animals. 
FACTORS OF ENVIRONMENT IN PLANT DISEASE. 
THE PLANT AN ORGANISM RESPONSIVE TO OUTSIDE CONDITIONS. 
A new era is approaching in vegetable pathology, in which the plant is the 
center. The lamented H. Marshall Ward^^ has well put it in the following 
words, “Until comparatively recent times it was the habit of farmers, foresters, 
planters and gardeners, all the world over, to look upon the plant as a mere 
item or as a mystery of Importance in their calculations, and to regard the soil 
as the ‘chief factor in their studies. Now all is changing, and the world is 
gradually awmkening to the recognition of the truth that the soil and the 
clouds and the atmosphere are merely reservoirs of more or less inert materials, 
from which the living plant draws its supplies, and works them up by means 
of energy focussed from the sun, into new plant substance.” 
The life processes of all plants are essentially similar. The green plant 
constructs its material from carbon dioxide and water, but carbon dioxide is 
not food for higher plants any more than it is for the fungus. The higher 
plant as well as the fungus feeds on the carbohydrates, made by the green 
plastids called the chlorophyll grains. The plant then is a living machine 
making food out of raw material, breaking up the carbohydrates and making 
the living protoplasm, a highly complex substance, thus utilizing the energy 
of the sun, and storing the material made for future use. This stored ma- 
terial is used to build and construct new protoplasm, cells, and to repair waste. 
The activities of the plant will depend on conditions from without. Every 
living plant is therefore influenced by certain factors; these may be grouped 
under soil, heat, light, moisture, air and living organisms. 
In order therefore to protect plant life from the invasion of fungus diseases, 
we must have accurate knowledge both of the diseases and the conditions 
which induce them. 
Soil and Air. The soil in which the root hairs of the plant occur, consists 
not merely of dead organic matter but of living organisms as well, which play 
an important part in the activities of the plant. These organisms not only 
require organic materials, but oxygen, to carry on their work. Near the sur- 
face the decomposition of organic matter goes on much faster than in the 
subsoil, because there is more oxygen present, and the conditions for con- 
structive work are more favorable. A raw" prairie soil seems to be less favor-* 
able for decomposition of organic matter than a thoroughly tilled soil. This 
consideration is intimately connected with the diseases of plants, as will be 
shown under the heads of rust. 
Soil and Moisture. Another factor in connection with soils is the relation 
they bear to the solutions they yield when in contact with water. Some salts 
are held, others pass. Some soils hold water more than others. This is a 
factor of utmost importance to the plant in health and disease. 
Light. Too much moisture may lead to disease, especially when there is 
not sufficient light, for it may make the tissues so delicate that the plant is 
^iDisease in Plants. 1. 
