Classification of Diseases. 349 
(b) Attacks of insects which suck their food, 
and which are, therefore, destroyed by caustic 
applications which injure the bodies of the 
pests. All the tribes of plant-lice and scale 
insects belong here, and for these the kerosene 
emulsion, resin washes, and the like, are the 
specifies. 
2. Parasitic fungous diseases, such as the apple- 
scab, black-rot and mildew of the grape, leaf-blight 
of the plum and pear, black-knot, and the like. 
These diseases are characterized by definite spots, 
discolorations or exerescences, which are more or less 
scattered over the surface of the leaf, fruit or branch. 
As a rule, the leaves and fruits which are attacked 
have a tendeney to drop from the tree. The gen- 
eral treatment for these diseases is to spray with 
some fungicidal mixture, like the Bordeaux mixture 
or the ammoniacal carbonate of copper. The treat- 
ment is useful in proportion as it is applied early 
and thoroughly. After the fungus once gets into 
the tissues of the host-plant, it is difficult, if not 
impossible, to kill it. If, however, the fungicide is 
upon the plant before the fungus is, the parasite may 
not be able to obtain a foothold. Even after it does 
obtain a foothold, it is probable, however, that the 
spray will check its spread by preventing the devel- 
opment of its external parts. 
3. The physiological and bacterial diseases, or 
those which are termed constitutional troubles. In 
these cases, there are rarely any definite spots, as 
in the attacks of parasitic fungi, but the entire 
