184 NATURE STUDY AND AGRICULTURE 
can be prevented by methods that are now in general use; 
and many millions of dollars are thus saved every year. For 
other diseases, as grain rust, no such effective means of pre- 
vention has as yet been found. 
Treatment with Chemicals. — For the preventable dis- 
eases two general methods of procedure may be briefly men- 
tioned. 
(1) The first method is followed when the germs of the 
disease, that is, the spores of the fungus, are found upon the 
seed that is planted. These spores germinate in the ground, 
and as soon as the host plant is well started, the young fungus 
enters its tissues and continues its growth there. To pre- 
vent this, we treat the seed before planting by wetting it 
in some liquid that 
will kill the germs 
upon it. Wheat 
and other grains are 
treated in this way 
for smut, and seed 
potatoes for potato scab. The favorite substance used for 
the purpose is formaldehyde. 
(2) The second method is employed in the case of diseases 
that enter the plant by way of the leaves and tender stems. 
The spores are borne upon the wind and by insects, and 
when they fall upon the foliage they sprout, and soon the 
young fungus enters the tissues of the host plant and be- 
gins its ravages. To prevent this from taking place, we 
spray the plant with some chemical that will not injure the 
foliage but will kill the spores upon its surface. After the 
fungus has sent its filaments into the body of the plant it is out 
of reach of any remedy. The chemical must therefore be 
applied when the plant is still perfectly well. It is too late 
A CueEap Spray Pump 
