RNA into the cell. Such RNA could be of syn- 
thetic origin. Furthermore, the obligately 
parasitic fungi of the Uredinales, Erysiphales, 
and Peronosporaceae, which number among 
their members many of our most destructive 
and least effectively controlled plant parasites, 
may be dependent on host RNA for their meta- 
bolic activities. If so, they could be very 
sensitive to treatment with synthetic RNA once 
the proper code could be developed to affect 
them but not the host plant. 
It is obvious that the type of molecules re- 
quired will be very complex in nature, and it 
will be difficult to obtain their permeation of 
living cells without destroying cytoplasmic 
functions. However, since there is a wealth 
of information on inoculation procedures with 
plant viruses and since the use of foreign 
nucleotide chains to change the carbohydrate 
metabolism of bacteria has been achieved 
recently, the idea is not absolutely impossible. 
It is already known that certain forms of viral 
RNA can be used to render a plant immune to 
infection by other viruses closely related to 
them, and resistance to fungi can be enhanced 
by viral infections, such as virus Y increasing 
resistance of potato to the late blight fungus 
Phytophthora infestans. Furthermore, it is 
known that some plant viruses multiply regu- 
larly in plants without causing visible symp- 
toms or reducing growth (potato virus X, ring 
spot of tobacco secondary infection). 
Still farther over the horizon lies the possi- 
bility of using synthetic DNA as an artificial 
gene to change heredity of both crops and 
pests. The potential danger of such experi- 
mentation is obvious, but the advantages are 
equally impelling. Many plant pathogens pro- 
duce new races regularly, and it is only a 
matter of time until some of them circumvent 
the known genes for resistance in the crop 
species. By intensive inbreeding and hybridi- 
zation of crops, the plant breeder has been 
discarding genetic versatility in our domestic 
species of plants, which render them suscep- 
tible to large deviations in climate or attack 
by pests. The time may very well come when 
such genetic diversity will have to be re- 
established by chemical means. 
Of course, the unraveling of the nucleotide 
code behind gene action will require a tre- 
mendous effort, because up to 6,000 nucleotide 
groups may be involved in each gene. Experi- 
49 
mentation would obviously have to start with 
simpler molecules unless partial biological 
control can be used in the synthesis of the 
proper nucleotide sequence. This might be 
achieved to a large measure by the marriage 
or matching technique of RNA and DNA ona 
cellulose column. 
POSSIBLE ROLE OF BIOLOGICAL 
CONTROL 
In the foregoing discussion on chemical aids 
to plant disease control, emphasis has been 
placed on the use of materials under man's 
control to meet crises in crop production. 
These techniques, of course, are to be applied 
in conjunction with the best possible horti- 
cultural practices to escape, suppress, or 
circumvent the effects of parasitic attacks. 
It is doubtful whether any chemical control 
measure could be fully effective under the 
most adverse environmental conditions with a 
maximum inoculum potential of the pathogens 
operating in the habitat. Certainly control 
under such conditions would require such 
massive dosages at repeated intervals that the 
treatment would be economically unsound and 
potentially hazardous to an already weakened 
crop and the desirable biological forces of the 
environment. 
The question may be asked fairly as to why 
all plant diseases should not be controlled by 
biological means, It is well to point out that 
plant pathologists probably spent about 95 
percent of their effort between 1866 and 1940 
trying to work out the biological factors be- 
hind the causal nature of disease, the capacity 
of the host to resist or recover from parasitic 
attack, and to exploit this knowledge in plant 
disease control. The tremendous volume of 
knowledge obtained is being put to use every 
day in the United States in choosing as nearly 
disease-free localities as possible, rotating 
crops to suppress pathogens, use of disease- 
free seed, proper fertilizer practice to pro- 
mote disease-escaping and recovery proc- 
esses, stimulation of soil antibiosis by proper 
soil amendments, choice of proper time for 
seeding, sanitary practices in propagation and 
harvesting, use of disease-escaping and re- 
sistant varieties, etc. These essential devices 
often have the great advantage of being the 
