Plant Diseases 
317 
those can hope to cure diseases in plants who under- 
stand the plants thoroughly. 
The first step in such training is to obtain a com- 
plete knowledge of those plants which are to be cured 
of disease, or, better still, kept free from it. The diseases 
themselves must then be thoroughly understood, how 
they act, and what will destroy them without injuring 
the plants. Of course this, to be of any use, must be 
done economically. 
Now the reader already knows something of the plant 
itself. We may therefore go on to study the diseases. 
There are a great many of these. They are of various 
kinds, and act in different ways. Not only this, but 
some, at different stages of their development, appear 
to be quite different, act in a different kind of way, and 
produce different results. Thus it will be seen that the 
study of plant diseases is a difficult matter. When 
practical operations commence it is found to be more 
difficult still, for during one stage of a disease it will 
not injure a particular plant, while at another stage it 
will do considerable damage. Even if we root out and 
destroy all diseased plants, fresh ones may be infected 
by a disease of an entirely different kind, and by what 
appears to be a distinctly different species; but later it 
is found to be only another stage of the same disease. 
Not all diseases, however, have such complicated life- 
histories. Many are straightforward from beginning to 
end. 
