by Zentmyer, andit should lead to fungitoxicity, 
provided there are no rapid detoxication proc- 
esses nor other toxicant mechanisms of reac- 
tion by the quinolinol compounds in the cell 
environment, Other potential chelating agents, 
such as the dithiocarbamates, enter into other 
reactions that are known to supersede any re- 
moval of metal catalysts (20). 
Several of the new fungicides probably op- 
erate as alkylating agents, Among these are 
the halogenated compounds that react readily 
with amines, thiols, and nucleophilic com- 
pounds by substitution reactions, Dichlone, for 
example, will react with functional sulfhydryl 
or amino groups to inhibit many enzymes, 
such as dehydrogenases, carboxylases, and 
coenzyme A, The very thorough study on the 
kinetics of the alkylating reaction was made 
by Burchfield and Storrs (2, 4) on 2,4-dichloro- 
6-(o-chloroanilino)-s-triazine (Dyrene) and 
1-fluoro-2,4-dinitrobenzene, Rates of reaction 
with a number of cell metabolites at different 
pH values give a clear interpretation of how 
these materials would compete for receptor 
sites in the normal cell environment. 
Interference with the electron transport 
system of the cell probably is more wide- 
spread than is given credit. Elemental sulfur 
has long been known to be reduced to hydrogen 
sulfide when brought into contact with living 
surfaces, probably by enzymatic action. This 
probably occurs by a single electron transfer 
from donor inthe cell to the sulfur as described 
by Owens (20), when the eight-membered sulfur 
molecule undergoes ring cleavage into a poly- 
sulfide chain, The reduction of sulfur leads to 
the release of a molecule of carbon dioxide 
from the fungus spore for every molecule of 
H2S produced (17), 
The decarboxylation processes that lead to 
carbon dioxide evolution could better be ex- 
plained as a result of the fungitoxic reaction 
rather than as the primary cause of fungi- 
toxicity if electrons are being diverted from 
the regular electron transport system. This 
would reduce the amount of ATP formed and 
lead to rapid starvation of the spores, ac- 
cording to Tweedy (33), who found only 7 per- 
cent of normal incorporation of 32P into RNA 
and DNA in spores of Monilinia fructicola 
when sulfur was being reduced to H2S, His 
data show that amino acids are deaminated, 
ammonia Collects in the spore, and respiration 
44 
is accelerated, thereby suggesting that the 
amino acids are being catabolized via the 
Krebs cycle. The most logical point for sus- 
pecting diversion of electrons from the trans- 
port system would be between cytochromes 
b and c. If his theory stands up, we will have 
at long last a valid understanding on the me- 
chanism of action of sulfur--the oldest of all 
fungicides, 
The growing volume of information on the 
design of molecules, which will penetrate the 
fungus body or be sorbed to it and enter into 
specific reactions fatal to it, has served to 
orient the thinking of chemists and plant pa- 
thologists toward the design of more effective 
molecules with greater selective action. How- 
ever, much of the search for better compounds 
must still be done by empirical testing. There 
are too many imponderables both in the living 
cells and in the nature of new molecules to 
predict exactly what will happen before they 
are tested, The gain in efficiency of research 
from the fundamental knowledge obtained is 
more than offset by the increase in standards 
of performance expected of new materials. 
It is relatively easy to find a compound with 
a reasonable degree of fungitoxicity, but it is 
very difficult to find one that can be used for 
crop protection as they are now used. The 
necessity of obtaining low phytotoxicity, low 
mammalian toxicity, reasonable retention in 
face of erosion from environmental forces, 
and securing reasonable protective films with- 
out excessive residues at harvest cause a 
high mortality of new compounds. 
In spite of this, new compounds, such as 
dodine, are being discovered and developed to 
fill the unmet needs on the farms. The most 
recent of these is tetrachloroisophthalonitrile. 
Turner et al. (32) report exceptional fungi- 
toxicity in this preferred member of a series 
and performance equal or superior to that of 
a large number of successful products now 
being used commercially on a wide assortment 
of fungi. 
PROBLEM OF SOIL-INHABITING 
PARASITES 
None of the major crops escape damage 
from soil-inhabiting fungi, bacteria, andnema- 
todes, The root rots, collar rots, damping-off, 
