BULLETIN 
OF THE 
Ohio Agricultural Experiment Station 
NoumBer 214 ‘Marcu, 1910. 
A BRIEF HANDBOOK OF THE DISEASES OF 
' CULTIVATED PLANTS IN OHIO 
By A. D. SELBY 
INTRODUCTION 
The idea of disease is not a simple one, though it may seem so 
before trying to define it. In reality the term “‘disease’’ as applied 
to plants, means any change in that plant toward reduced vigor, etc., 
from the ordinary or average behavior. To put it another way, a 
plant is said to be ‘‘diseased’”’ when it shows any deviation from the 
ordinary or average behavior of that plant in respect to appearance, 
growth, color of bark, foliage, fruitfulness, time of dropping leaves 
or length of life; in short, when the plant fails to conform to those 
averages which we have established by extended observation for 
the species and variety in question, we say it is diseased. Under 
such a general definition, variegated or purple hued sports would 
be included, although potentially rather than actually in diminished 
-vigor. Variegated sports succumb easily to parasitic attack and, as 
later investigations show, are really suffering from enzymatic 
troubles. 
The more usual symptoms of disease are marked by evident 
differences in the plant. The leaves become spotted, curled or 
discolored, or may even drop prematurely; the fruit may develop 
unevenly or be marked by decayed spots, or the twigs may blight, 
wilt or die. In all such cases we have a manifest loss of vigor and 
reduced profit. Yet we may not attribute all these to parasitic 
fungi or to parasitic insects; purely physical or chemical agencies 
may be at the bottom of certain troubles. Plants may be as- 
phyxiated by too much water which excludes the air supply; they 
may likewise, be strangled by escaping gases, especially in the case 
of city shade trees, or their protoplasm may be attacked by chemi- 
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vy 
