nonsterile infested soils either had no effect 
on the parasites or the effect was slight and 
transitory. 
Direct Introduction of Antagonists to Plant 
Tissues.--The introduction of antagonists, 
with or without nutrients to control pathogens 
of aerial plant organs, has not met with any 
great success (16), Newhook (59) introduced 
spores of Cladosporium herbarum and Pencil- 
lium sp. to control Botrytis cinerea, which 
attacks tomato fruit after becoming established 
on dead petals. Biological control of B, cinerea 
was completely successful when the antag- 
onists were introduced into recently dried 
petals, but only 30 percent successful when 
introduced to petals that had dried for several 
days. Under natural conditions, however, 
inoculation was not necessary, because the 
antagonists had been isolated from dry petals 
to begin with. 
Mention of experiments to control bacterial 
plant diseases biologically by introducing 
phages in the appropriate yicinity of the 
pathogen is worthwhile despite the fact that 
this subject was covered in a recent review 
(16). Most interesting experiments were per- 
formed by Keil and Wilson (38) on the control 
of peach bacterial spot with Xanthomonas 
pruni bacteriophage. In greenhouse experi- 
ments, these investigators obtained significant 
control of the disease either by mixing the 
phage with a suspension of X. pruni inoculum 
and then spraying it onthe plants or by applying 
the phage as a spray and allowing it to dry on 
the plants before inoculation. Control with 
phage was equal or superior to that obtained 
with zinc sulfate plus hydrated lime. 
On the other hand, attempts to control plant 
diseases biologically, by inoculating plant 
seeds with antibiotic-producing saprophytes, 
have met with considerable success, Wright 
(94) in England obtained Pythium control of 
white mustard seedlings by dusting seeds with 
spores of antibiotic fungi. Of three strains of 
Trichoderma viride (a soil-inhabiting fungus) 
tested against the Pythium disease, a gliotoxin- 
producing strain was more effective than a 
viridin-producing strain; and that strain was 
more effective than an antibiotically inactive 
strain, In experiments by Gregory et al. (33), 
alfalfa seedlings were protected from damping- 
off incited by Pythium debaryanum by the 
incorporation of T. viride spores into a 



84 
pellet mixture surrounding the seed. Tveit and 
Wood (89) reported very good control of 
Fusarium blight of barley seedlings by 
inoculation of the seed or soil with Chaetomium, 
some species of which produce antibiotics 
in culture. 
Biological control methods, involving pur- 
poseful introduction of antagonists or predators 
to soils or plant tissues, are subjectto certain 
general features and limitations. Some workers 
believe, or at least hope, that once an organism 
is found to be antagonistic against a specific 
pathogen, it can be added to soil or to plant 
tissue to control a plant parasite. That alone 
seems too much to hope for, because the 
antagonist usually comes from soil or plant 
tissues to begin with. As Pramer (74) recently 
pointed out, antagonists and predators are 
normal inhabitants of soil, and their simple 
addition to soil usually has only a transient 
effect. If micro-organisms are to be used for 
biological control of plant diseases and 
nematodes, it is not their presence but their 
growth and predaceous activity that must be 
encouraged. The practical importance of 
micro-organisms as biological control agents 
would appear, then, to be dependent, to an 
extent far greater than observed with the use 
of beneficial insects, on the favorable extrinsic 
factors and on microenvironmental conditions 
expected to upset the status of existing micro- 
equilibrium. 
Specific or Nonspecific Modification 
of Microenvironment 
Effective antagonists and predators and 
conditions suitable for them are more likely 
found in environments in which a serious 
infestation has been alleviated through micro- 
environmental changes favorable to the action 
of antagonists naturally present. Conditions 
that favor the antagonists will have to be sought 
in these microenvironments and not on synthetic 
media. This opens the way to the control of 
plant diseases and nematodes by specific or 
nonspecific modification of the environment 
that is shared by the parasite and the antagonist 
or predator, 
Amendments.--The possible use of organic 
and inorganic amendments to enhance, se- 
lectively, a particular segment of the micro- 
flora to such an extent that a suppression of a 
