produced pectic acid depolymerase, but not 
pectin methylesterase. Depolymerase, con- 
stitutively produced, rapidly degraded pectin 
and reduced the viscosity of pectates without 
significant release of reducing sugars. Pectin 
and pectinic acids of increasing methyl esteri- 
fication were more resistant to attack by the 
fungal enzyme than demethylated pectins or 
pectins of low methyl content. 
MECHANISMS OF BIOLOGICAL 
CONTROL--ANTAGONISM 
Finally, lest it appear that plant pathologists 
and microbiologists exaggerate the applied 
aspects of biological control, this review may 
reasonably include some basic considerations 
in regard to the mechanisms of biological con- 
trol, Specific mechanisms of biological control 
of plant diseases and nematodes are highly 
elusive because of the complex nature of the 
problem and limitations in approaches and 
methodology. Antagonism has been perhaps the 
most widely recognized mechanism, Ina recent 
symposium in Berkeley, we (63) proposed to 
place antibiosis, competition, and myco- 
parasitism and predation under the general 
term of antagonism and to accept the term in 
its broadest meaning, i.e., as encompassing 
all possible microbial associations and inter- 
actions, in which at least one of the interacting 
microbes is harmed, This breaking down of 
antagonism was also suggested by Park (73). 
It is appropriate to say at this point, how- 
ever--lest it appear that antagonism has been 
correctly implicated in all instances as the 
mechanism of biological control--that in many 
cases conclusive evidence is still lacking as 
to whether there is any relation between the 
relative abundance of organisms that 
antagonize a given pathogen or nematode and 
the ability of that species to survive in its 
microenvironment. This difficulty usually 
stems from the fact that antagonism on agar 
plates may be absent when the situation is 
studied under natural environmental conditions 
(42) and vice versa; organisms that show no 
antagonism in cultures may be antagonistic in 
a natural environment (88). The second point 
was shown by Slykhuis (83), who found that, of 
anumber of antagonists to Fusarium culmorum, 
the most active forms did not produce anti- 
biotics in culture. 
88 
Antibiosis 
Antibiosis, a form of antagonism, was recog- 
nized as being mediated by specific or non- 
specific toxic metabolites of microbial origin, 
by lytic agents, and by fungitoxins (34, 63, 
78). It is still mainly conjectural as to whether 
antibiosis is important in biological control of 
plant pathogens and nematodes, The significant 
line of evidence is based on the fact that 
microenvironmental conditions necessary for 
biological control and the requirements es- 
sential for expression of various phases of 
antibiosis are parallel. 
In our work with Rhizoctonia we (17, 65) 
showed first that incorporation of crops resi- 
dues produced a marked increase in the 
abundance and activities of nonpathogenic 
micro-organisms and a concomitant decrease 
in the Rhizoctonia disease of beans. Green 
amendments when added to Rhizoctonia-in- 
fested soil, substantially increased total num- 
bers of soil and rhizosphere fungi and strepto- 
mycetes, as well as the total number of soil 
bacteria; and they also suppressed the 
Rhizoctonia disease (65). Decomposing green 
corn and oats stimulated the highest number 
of streptomycetes antagonistic to R. solani, 
gave the susceptible host the best and longest 
protection, and decreased the competitive 
saprophytic activity and survival of the 
pathogen in soil. Sudangrass, green snap 
beans, buckwheat, and no amendment (control) 
resulted in fewer antagonistic streptomycetes 
than corn and oats, in the highest disease, 
and in the highest saprophytic activity and 
survival of the pathogen. 
Antibiosis in the control of soilborne plant 
pathogens, mediated by rising populations of 
antagonists, does not preclude other mechan- 
isms of biological control during amendment 
decomposition. This was suggested by Garrett's 
experiments (30) on O. graminis and our own 
(67) on carbon dioxide and its effects on 
Rhizoctonia. In naturally or artificially in- 
fested soils, we observed considerable in- 
hibition of the saprophytic activity of Rhi- 
zoctonia in soils permeated by atmospheres 
containing various CO, concentrations. The 
inhibitory action of CO, on the saprophytic 
phase of Rhizoctonia was shown by a stright- 
line dosage-response curve obtained from 
the values for percentages of inhibition of 
