(^OLOUAi)O Plant DiseAvSes 
19 
Unfavorable climatic conditions may often seriously injure plants. 
Very strong sunlight produces a condition known as sunscald. Too 
miieh shade interferes with the manufacture of starch by the leaves. 
Frost injury and freezing are common among plants. In addition 
to the direct injury from frost or freezing such a condition often 
makes a plant more susceptible to certain parasitic diseases. For 
example, the bacterial stem blight of alfalfa develops extensively 
only after the alfalfa plants have been injured by frost. The bacteria 
gain entrance to the plant tissues through the cracks produced as a 
result of freezing. 
The weight of ice and snow often breaks off the limbs of trees 
and thus opens the way for parasitic fungi. The fall of hail stones 
Frost injury on apples. 
is a source of much damage to all forms of crops. Strong winds 
often are the cause of much damage. Limbs and buds are broken 
from trees; corn and small grain are sometimes blown down when 
the wind is accompanied by heavy rain. Heavy rains often injure 
the blossoms and prevent the pollination of fruit trees. Lightning 
injury is common although not of very great importance. 
Smoke and gases from smelter and manufacturing plants have 
