die out if the saprophytes were encouraged to 
grow. It has been well established that some 
of the purified antibiotics are potent fungi- 
cides, but they are not suited for use in soil 
either because they adsorb on clay and are 
rendered inactive, are decomposed by soil 
micro-organisms, or are too phytotoxic or 
too expensive to be used at the dosages re- 
quired. The genesis of such materials in the 
soils by antagonistic soil micro-organisms 
is a much more promising solution to the 
problem. 
Several moves have been made in this de- 
sirable direction. One of these is to plow 
under crop refuse, green manure, or livestock 
manure to stimulate growth of saprophytic 
bacteria. Such measures have proved effective 
in reducing the severity of strawberry root 
rot. Texas root rot, and other root-infecting 
fungi. It was only one step from this observa- 
tion to the addition of sugar for the strawberry 
root rot, since glucose- and sucrose-requiring 
bacteria inhibit the fungi responsible for the 
trouble. 4 
A second possibility exists in observations 
made on changes in the microflora following 
soil disinfection. As a general rule, the patho- 
genic fungi that are not regular soil inhabitants 
but manage to survive in crop refuse are de- 
stroyed rather readily, and the more aggres- 
sive pathogens require heavier dosages or 
more active disinfectants. In keeping with this 
scheme of things, the soil saprophytes that 
are indigenous to the soils often escape the 
treatments designed to destroy the pathogen 
(13). Among these are the Actinomycetes and 
Streptomyces spp. that have exceptional abil- 
ity to produce antibiotic substances effective 
against a wide assortment of fungi and bacteria. 
Even more resistant is Trichoderma viride, 
the fungus that produces glyotoxin, which is 
strongly inhibitory to fungi. 
There are two good reasons why these stud- 
ies on shifting the character of soil populations 
by chemical treatments should be investigated 
much more thoroughly. There is always the 
possibility that standard fungicides would do 
irreparable harm to the soil by injuring desir- 
able micro-organisms, such as the nitrogen- 
fixing bacteria, the legume nodule bacteria, or 
the antagonists of plant pathogens. The extent 
to which these changes are going to be induced 
should be explored very carefully in different 

48 
types of soils. In addition, a search should be 
made for selective organic chemicals that 
would stimulate antagonists for plant pathogens 
either nutritionally or by releasing them from 
competition with their coinhabitants of the 
soil. The screening methods now employed for 
locating soil disinfectants are not designed to 
exploit the possibilities of soil antibiosis, 
since they are aimed solely at locating fungi- 
cidal action. However, there are distinct pos- 
sibilities in trying to harness this force to 
kill out less well-adapted pathogens rather 
than to attempt their destruction by massive 
application of fungitoxicants. 
Regulation of Plant Resistance 
Regulating plant resistance is a distinct pos- 
sibility for future research, Already there is 
good evidence that wilt diseases, such as the 
Dutch elm wilt (Ceratocystis ulmi), can be 
alleviated by use of chemicals that change the 
nature of tracheal walls. It is rather clear 
that this fungus is destructive only early inthe 
season when new tracheal tubes are being 
proliferated, and once these become lignified, 
the plant may recover sufficiently to survive 
the season and years ahead with walled-off 
infection. 
A more interesting possibility for future 
use of chemicals for changing hereditary pat- 
terns of susceptibility in crops is coming over 
the horizon of science, The new knowledge on 
the chemistry of hereditary control over cell 
activities reveals that the deoxynucleic acids 
(DNA) control inheritance from the nucleus, 
and the ribonucleic acid (RNA) carries the 
DNA code to the sites of protein synthesis, 
which are essential for formation of the en- 
zymes that direct biochemical processes. The 
methods of synthesizing polynucleotide com- 
positions on a sugar-phosphate chain are 
available, and methods of regulating the se- 
quence of the four nucleotides on that chain 
are at hand. 
If one is allowed to postulate that plant 
viruses operate by having their RNA enter 
competitively with normal messenger RNA 
into combination with ribosomes to form a 
new polysomal complex, then it is possible to 
visualize methods of interfering or destroying 
virus activity by introduction of new types of 

