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THE ROLE OF CHEMICALS IN CONTROLLING PLANT DISEASES 
George L. McNew, Managing Director, Boyce Thompson Institute for Plant Research, Inc., 
Yonkers, N.Y. 
Tremendous strides have been made incon- 
trolling plant diseases by chemical means in 
the past three decades. The fungicidal activity 
of the alkyl dithiocarbamates was reported in 
1934, and within 6 years the quinone fungicides 
were discovered and introduced into commer- 
cial use. 
These discoveries pointed the way to breath- 
taking possibilities in plant disease control. No 
longer were agriculturists to be restricted to 
inorganic ions or molecules of copper, zinc, 
sulfur, and mercury, where about the only 
possibilities for improvement lay in changing 
solubility. With the advent of organic fungi- 
cides it became possible to design molecules 
to the exact specifications desired. There 
were almost limitless nucleophilic groups that 
could enter into and immobilize or retard bio- 
chemical processes of the cell. By proper 
modification of such active components, or 
addition of substituent groups, it should be 
possible to modify the physical-chemical at- 
tributes of a class of fungicides so it would 
become more, or less, selective in its action, 
capable of penetration into the living cells, 
safe for use on specific crops, persistent on 
foliage and fruit, resistant to erosion by wind 
and rain, photolabile, etc. 
Three decades and 200,000 compounds later 
we are sadder but wiser men, The dream did not 
41 
fail because we have marvelous materials, 
such as the alkylene bis-dithiocarbamates, 
quinones, imidazolines, s-triazines, captan, 
dodine, and an assortment of other hetero- 
cyclic nitrogen and sulfur compounds to choose 
from. However, the harsh facts are that prob- 
ably no more than 40 million pounds of these 
new chemicals are being used on the 358 mil- 
lion acres of cultivated land in the United 
States. Very few of our 8,000 fungus diseases 
are controlled satisfactorily and scores of 
them not at all. We produce more and better 
crops with less damage than heretofore, but 
we still suffer staggering losses, 
Wherein have we failed to live up to our 
responsibilities and opportunities? We need 
to know why we have not attained the brave 
new world of our dreams of 1940. To this 
end we would like to look at the magnitude of 
our practical failure, the useful new knowledge 
gained and how it can be used, and to briefly 
explore some new approaches that could be 
taken in facing up to the future. 
CURRENT FAILURE TO MEET 
DISEASE PROBLEMS OF CROPS 
No one knows exactly how much damage is 
done by any one disease in any one locality, 
much less what the aggregate losses may be 
