seedling blights, and wilts take a toll every 
year. Some of them are so chronically omni- 
present that losses are accepted year after 
year as almost a normaleffect, andcomplaints 
are raised only when weather conditions favor 
an extremely severe infestation. Only moderate 
progress has been made in developing chemi- 
cal aids for their control. To obtain the re- 
quired 100 p.p.m. or so of toxicant in the 
upper 6 inches of soil (2 million pounds per 
acre), approximately 200 pounds of chemical 
are required. In actual practice, 20 to 400 
pounds are used, but up to 3,000 pounds would 
be required to suppress the more resistant 
species of fungi. 
The obstacles against suppression of these 
soil inhabitants are formidable. Soilfumigants 
are lost by volatilization. There is substantial 
loss due to adsorption onto clay and humus 
particles. The amount of sorption loss will 
vary with the nature of the clay fraction and 
the compound used (5, 13), The nonpolar chem- 
icals, such as ethylene dibromide, are less 
readily sorbed on montmorillonite than on 
Kaolinite or illite because of differences inthe 
particle surface, Cations are generally ad- 
sorbed on the outer shell of clay micelles, and 
the degree of anion sorption is affected by the 
ratio of silicon to aluminum or iron, so soils 
with kaolinite or illite clays should sorb 
anionic materials most readily. Thus, every 
soil presents its own peculiar problem in 
arriving at a suitable dosage level. The fac- 
tors behind soil treatment have been sum- 
marized very well by Kreutzer (12, 13), 
Domsch (5), and Kendrick and Zentmyer (10). 
In addition to the problem of obtaining a 
suitable dosage at reasonable cost, there are 
two other interlocking problems regarding soil 
disinfestation. The first is an adverse effect 
on beneficial soil micro-organisms, The mi- 
crobiological balance may be upset to such an 
extent that it takes weeks to months to recover. 
One of the adverse effects is suppression of 
Nitrosomonas and Nitrobacter, so that nitri- 
fication is seriously impaired. There may 
be serious accumulation of ammonium in 
treated soils since ammonification is not 
seriously impaired. There is also a tendency 
to impair agglomeration of soil particles 
by micro-organisms, so texture of soil may 
be impaired slightly by some of the soil 
treatments. 
45 
The second problem is reinfestation of 
treated areas by windborne or mechanically 
transported inoculum, There is a strong tend- 
ency for the number and assortment of 
micro-organisms to be sharply curtailed by 
fumigation, so survival of pathogens or their 
reintroduction may lead to a destructive rein- 
festation. The fungi that operate as normal soil 
inhabitors seem to be particularly resistant to 
disinfection. Fortunately since some of the 
normal antibiotic-producing organisms, such 
as species of Streptomyces and Trichoderma, 
survive equally well and often build up innum- 
bers very rapidly in the posttreatment period, 
they probably serve to hold the pathogens in 
check during the period of population recovery. 
PERFECTION OF PERFORMANCE 
BY FORMULATION 
Fungicides are used predominantly as crop 
protectants, The purpose is to place them so 
they will be available in overwhelming con- 
centration when the pathogen comes in contact 
with them while seeking an infection court. 
There are several reasons why a minimum 
deposit of chemical has been sought in achiev- 
ing protection. As a simple matter of eco- 
nomics, the smallest possible dosage was 
recommended, The value of efficient appli- 
cation to avoid excessive dosages was in- 
creased when agriculturists shifted from the 
relatively inexpensive inorganics to carefully 
designed, expensive organic compounds, Fur- 
thermore, there was always danger of injuring 
the crop by excessive application, especially 
when substances of high biological activity 
were being used. The need for the least pos- 
sible residue to achieve the maximum protec- 
tion for the plant has become even more 
critical in recent years as a means of meeting 
the legal requirements for safeguarding public 
health and avoiding needless dislocation to the 
biological forces innature. Very few fungicides 
have been criticized or even been very suspect 
for damage to man, domestic animals, or wild- 
life. The only serious exceptions have been the 
organic mercurials used as seed treatments 
and to a limited extent in orchard spraying. 
However, this does not excuse use of the most 
conservative dosage commensurate with ef- 
fective crop production. 
