June, 1921] 
STEVENS — PLANT PATHOLOGY 
congestion of crop, we find, to a large degree, explanation of the fact that 
plant diseases are more prevalent now than formerly. 
The multiplicity and diversity of plant diseases are especially striking. 
While the physician has but one species of patient and the veterinarian but 
a few species, the phytopathologist has to advise regarding many species 
of plants each of which has, to a great extent, its own large list of diseases. 
Thus, on the apple alone there are 18 major diseases; on wheat 10; on 
potatoes 12; while for each crop the number of minor diseases is more than 
ten times as great. 
I have attempted thus briefly to indicate the damage done by plant 
disease, as a background for a discussion of the part played by plant path- 
ology. The point of importance is not how great is the loss from plant dis- 
ease, but rather how much influence has the science of plant pathology had 
in lessening this loss. 
Like bacteriology the science is young, dating back barely to the middle 
of the last century. It was first taught in any American college in 1873 
(Illinois), and first as a special subject in 1875 (Harvard). The science has 
grown until today the American Phytopathological Society enrolls nearly 
500 members, the majority of whom are professional plant pathologists, 
and whereas but one paper appeared in America on the subject in 1861, 
each month now adds scores of titles and nearly a hundred papers are 
presented here this week. Large federal and state appropriations sustain 
its researches. 
What is the nature of the return that plant pathology has given? The 
achievements may be summarized briefly as falling within seven great 
categories demonstrating the value of: protective applications, sprays and 
dusts; excision; seed steeps; general sanitation leading to diminution of 
infective material; breeding for disease resistance; modifications of agricul- 
tural practice; quarantine restrictions. 
It is unnecessary to discuss these, but I wish to point out that while a 
modicum of the present benefit doubtless would have obtained from an 
empirical, rule-of-thumb procedure, the great body of our present knowledge 
of disease prevention is the direct outcome of truly scientific investigation. 
It is difficult as you journey from coast to coast today, and see spraying 
practiced everywhere, to realize that prior to 1885 no spraying was done in 
the United States. The vast sums spent for copper sulphate, lime-sulphur, 
etc., and the large factories devoted to making spraying machinery also 
attest the wonderful growth of this custom. Yet it was not the accident 
associated with the stealing of wayside grapes that was responsible for the 
discovery of the efficiency of fungicidal applications; it was the close obser- 
vation of Millardet followed by his keen analysis and exact experimentation, 
all of which would probably have failed were it not for the basic knowledge 
that Millardet had regarding fungi and parasitism. His receptivity of 
mind was doubtless dependent upon mycological studies of many decades. 
