colonization by increasing CO, concentrations. 
The parasitic phase of Rhizoctonia was more 
sensitive to CQO, than its active saprophytic 
phase; and the latter was more sensitive to 
CO, than its saprophytic surviving ability with- 
in precolonized substrate segments (62, 71). 
Antibiotics.--Although there is as yet far 
too little available information, the accumulated 
evidence suggests that specific harmful micro- 
bial metabolites or antibiotics may be involved 
in antibiosis. Two questions concerning anti- 
biotic substances in soil have repeatedly been 
asked and extensively debated: Are antibiotics 
produced by micro-organisms in their natural 
microhabitats? If so, do they really mediate 
ecological relationships that result in the 
suppression of plant parasites? The existing 
evidence now suggests that in an least some 
cases the well-known antibiosis between sapro- 
phytes and plant pathogens is actually 
associated with the production of antibiotics 
by the saprophytes (6). In specific soil, or in 
such plant-surface microhabitats as segments 
of decomposing organic debris, plant residues, 
seedcoats, and soil-root interfaces, a local 
concentration of antibiotic substances may be 
maintained at a rate sufficiently high to exert 
a profound effect on biological control, even 
though the overall concentration is too low to 
be detected by conventional extraction pro- 
cedures. For instance, the adpressed growth 
habit of T. viride around hyphae of R. solani 
would insure not only direct parasitism but 
also a local concentration of gliotoxin and 
viridin toxic to the pathogen. A concentration 
sufficiently high to be toxic to R. solani would 
not be attained if the two organisms were 
growing far enough apart. 
Whether antibiotics are produced in natural 
habitats is perhaps only of academic interest. 
If however, antibiotics are produced innatural 
amended habitats, or innatural organic micro- 
habitats, this would be of extraordinary prac- 
tical significance, because production after 
amendment is a result of upsetting dynamic 
soil equilibria by a treatment that can be 
applied by any farmer. Wright (92, 93) showed 
that energy-yielding materials in soils are 
critical in the production of antibiotics, and 
that local concentrations of antibiotics may 
be produced in the vicinity of suitable carbon 
sources, Since soil micro-organisms tend, by 
necessity or ecological preference, to be 
89 
concentrated in such sites, the antibiotics 
formed may attain sufficiently high concen- 
trations to exert significant ecological effects. 
Lysis.--Antibiosis as a mechanism of an- 
tagonism mediated by specific antibiotic sub- 
stances may not be the sole mechanism 
responsible for biological control. Other 
mechanisms, such as lysis of fungal mycelium, 
are also important. Considerable evidence is 
now available that lysis, a form of antibiosis, 
is an important mechanism of biological con- 
trol. Lysis may be enhanced by saprophytes, 
associated with decomposing amendments and 
other agricultural practices. Recent work by 
Sequeira (81) on lysis of Fusarium oxysporum 
f. cubense in the presence of sucrose and 
other amendments, and work by Mitchell and 
Alexander (55, 57) on chitin, substantially 
Support the contention that lysis affected by 
amendments is an important mechanism of 
biological control, Not only mycelia of plant 
pathogenic fungi may be subject to lysis or 
disintegration and destruction (46, 47) but also 
resting fungal propagules (72, 86). 
Lysis of fungi has been reported (11, 12) 
and certain plant diseases have been con- 
trolled experimentally with mycolytic micro- 
organisms (9, 13, 22, 40). For instance, 
Mitchell and Alexander (55, 56, 57) found a 
correlation between the amount of disintegra- 
tion of mycelium or other fungal parts and 
reduction of Fusarium diseases. Chinn and 
Ledingham (13) reported lysis of Helmin- 
thosporium sativum germ tubes after spores 
were stimulated to germinate by soybean 
meal, molasses, and vitamin C. A similar 
biological control mechanism of Phyma- 
totrichum root rot of cotton was reported by 
Mitchell et al. (58). 
Despite all the recent progress on lysis, very 
little is known of the exact mechanism and 
the precise role taken by antagonistic 
microbes. Lysis may be mediated by ex- 
posure of fungal mycelium to enzymes pro- 
duced either by the micro-organisms that 
participate in the decomposition of special 
amendments (55, 57) or by an increase in 
staling as a result of saprophytic growth of 
associated organisms (73); or it may be 
brought about by nutrient deficiencies, internal 
metabolic processes, and toxins (7). Some 
investigators have shown that lysis may be 
due to the combined effects of antibiotics and 

