from all diseases of crops inthe United States, 
Reasonably good, educated guesses can be 
made from balanced judgment based on many 
observations of the gains that ensue when 
disease-control methods are practiced along- 
side untreated plots or fields for a number of 
years or when general yields are compared 
in disease-infected and healthy fields in dif- 
ferent localities or years. A recent compila- 
tion of such balanced judgment of authorities 
in the United States has been summarized by 
LeClerg (14) for the losses during 1951-60. 
The final bill totals $3.6 billion per annum, 
which would seem to be a reasonable estimate. 
A full warning must be made that if all this 
lost crop productivity, equivalent to the yield 
from 30 million acres, were dropped in the 
marketplace, it would further depress prices 
disastrously so these dollar values would not 
be realized, Probably the best way is to look 
at this as a loss of over 8 percent in efficiency 
on the farms in use of human labor, equipment, 
fertilizer, seed, and soil resources. 
The percent loss in fie¥d crops averages 
about 12 for cereals, 16-23 for sugar crops, 
11 for tobacco, pasture crops about 5, hay 
15-24, fruit crops 2-34, with the principal 
pome and stone fruits suffering 8-17, and 
vegetable losses 2-22, with principal crops 
suffering 10-20, The specific causes of this 
damage will not be published until 1965, but 
it is well known that the blights, scabs, fruit 
rots, and cankers are major causes of loss 
even though there are effective chemicals for 
them. 
The losses on field crops continue to be 
heavy, because sources of disease resistance 
in resistant varieties are being continuously 
broken down by evolution of new parasitic 
races of pathogens, and the returns per acre 
on these crops are not sufficiently large to 
justify the relatively heavy cost of repeated 
applications of protective chemicals, Stakman 
and Harrar in their book on "Plant Pathol- 
ogy" (31, p. 507) stated that black stem rust 
of wheat has been adequately controlled in no 
more than 20 years between 1904-54 because 
of the rapid succession of races, In general, 
the obligate parasites, such as the rusts and 
powdery mildews, continue to be major dis- 
ease problems, 
Probably the largest unresolved disease 
problem is the root rots caused by a variety 
42 
of fungi and bacteria aided and abetted, on 
occasions, by nematodes, These relatively 
unspecialized organisms are inadequately de- 
terred by natural resistance of their hosts 
because of the pathogen gaining access through 
wounds and other breaches in their protective 
devices of the plants, prolonged extension of 
the susceptible juvenile condition during ad- 
verse weather conditions that prevent escape, 
and the high cost of treating soils with chemi- 
cals ($50 to $400 an acre), 
There is no way to estimate what the crop 
losses would be if chemicals were not available. 
It is known that many fruits and vegetables 
simply could not be produced in the areas best 
adapted to them. Acceptable apples would not 
come from 60 percent of the present area, 
peaches could not be grown in the Southeast, 
and potato culture would be driven inland to 
the higher, dry plateaus by late blight (Phyto- 
phthora infestans), Many of the relatively suc- 
cessful blight-resistant varieties would fall if 
they could not be protected under duress of 
environmental conditions. The real issue be- 
fore scientists is not how muchcropinsurance 
is now being bought by chemicals but why we 
are still losing about $3.6 billions annually! 
Has everything possible been done to find rea- 
sonably efficient, reliable control measures by 
perfecting chemical devices, cultural practices 
to promote biological control, or breeding 
resistant varieties? Obviously since we have 
not done much more than establish a bulwark 
to keep our crops from utter destruction, 
much more skill is needed. 
PROGRESS IN DESIGNING 
MOLECULES 
A substantial volume of data has been ac- 
cumulated on the relationship of chemical 
structure to activity. There is little need to 
go into great detail, because the subject has 
been reviewed adequately by Horsfall (7, 8, 
9), McCallan (16), Sexton (29), Rich (25), 
Sisler and Cox (30), and Owens (20), who give 
citations to the original research. A few se- 
lected examples will serve to illustrate the 
major points. 
The mode of penetrating cell walls and 
membranes has not been fully explained. 
However, it is obvious that many of the major 
