A careful analysis of the relation of particle 
size to protection by dichlone by Burchfield 
and McNew (1) revealed that there probably is 
an optimum particle size for a protectant 
fungicide. A minimum amount of particle sur- 
face should be exposed to obtain the desired 
protection of vulnerable surface of leaf and 
fruit. If too much surface is exposed by grind- 
ing particles too fine, the deposit is suscepti- 
ble to erosion, volatility, photodecomposition, 
and other debilitating forces. If the particles 
are too large, an inordinate amount is required 
to cover the surface adequately. Contrary to 
the prevalent idea that the particles should be 
made as small as possible, these studies in- 
dicate that size of particle should be adjusted 
to the best intermediate level by balancing off 
several factors of fungicidal efficiency, phys- 
ical properties of the molecules, nature of 
surfaces to be protected, environmental con- 
ditions under which the material would be 
used, and persistence of chemical desired. It 
follows that every crystalline material prob- 
ably would have a different optimum size of 
particle, and under some conditions of great 
lability the particles should be of several sizes 
so each would be available to control the fungus 
at different periods of time. 
In addition to particle size, many factors 
should be taken into account in designing the 
perfect protective spray formulation, Surfac- 
tants must be used to obtain spreading and 
adhesion to the surfaces. Unfortunately the 
nature of the chemical being used, the hard- 
ness of water to be used, and the nature of the 
plant surface to be protected all must be taken 
into account in deciding on whether a cationic, 
anionic, or nonpolar surfactant should be used. 
Sometimes deposit builders or solvents to 
speed up penetration or permit emulsification 
are used, 
The formulating materials for use in soil 
disinfestation are equally important, It is pos- 
sible to disperse materials in cosolvents that 
will speed up penetration of the soil and the 
micro-organisms in them and to assure ready 
dispersion in the soil. Water solubility and 
volatility can be controlled in acidic materials 
by converting them into salts or esters or 
otherwise formulating them so they would be 
adsorbed on clay particles, They could, there- 
fore, be made to persist in the top few inches 
of soil until they could achieve partial sterili- 
46 
zation in this zone most trequently inhabited 
by the soil microflora. 
POSSIBLE NEW APPROACHES TO 
USE OF CHEMICALS 
There is no room for complacency as long 
as the farms are sustaining such large losses 
of crops and the present materials have to be 
applied repeatedly and in relatively heavy dos- 
ages to protect crops. The present system of 
screening compounds to locate new types of 
activity will continue to yield better, new 
compounds just as it is now doing. However, 
we need materials that could operate in more 
subtle ways, be applied less frequently or in 
smaller dosages, or could exploit advanta- 
geously the peculiarities of the pathogen, host, 
or environment to bring about the desired 
effect. 
There are no really satisfactory control 
measures for virus diseases except sanitation 
and breeding of resistant varieties. Once a 
crop becomes infected it is lost regardless of 
whether it is an annual or in amature orchard 
or grove of perennials. Many species of fungi 
escape contact or are resistant to the chemi- 
cals now being used. 
Design of Unstable Molecules 
Much progress has been made in using un- 
stable sulfur compounds. The outstanding suc- 
cess has been with nabam, which is essentially 
inactive against fungi. The inert deposit is 
converted to alkyl isothiocyanate, probably by 
way of ethylenethiuram disulfide and ethylene- 
thiuram monosulfide (15), The active fungicide 
is very labile and never accumulates in amounts 
sufficient to be phytotoxic. This is an ideal 
situation, because the toxicant is ephemeral 
and the system that generates it is exhausted 
in 5 or 6 days, so there can be no residue to 
harm either the crop or the consumer. Some- 
thing of a similar nature probably occurs when 
Vapam and Mylone are used, since they seem 
to be converted to isothiocyanates, 
So far no one has succeeded in creating an 
inert molecule that is rendered fungitoxic only 
after penetrating the fungus body. One of the 
very distinct possibilities would be to seek 
