several soilborne diseases and nematodes may 
also be difficult, unprofitable, or inadvisable 
to control with pesticides. 
The difficulties may arise from lack of 
effective pesticides, need of relatively large 
amounts of expensive pesticides, andinactiva- 
tion, decomposition, or accumulation of pesti- 
cides in soils or in other habitats. They may 
also arise from problems of diffusion and 
contact between the pesticide and the parasite 
(53), Therefore, since encouraging and profit- 
able results may not always be in prospect 
in the chemical treatment of these diseases 
and nematodes, biological or "natural" control 
may be the best possible approach. 
DEFINITION OF BIOLOGICAL 
CONTROL 
Biological control, which is as ancient as 
recorded agricultural history, includes direct 
or indirect use of biological factors by chang- 
ing the microbiological equilibrium in or 
around the infection court so as to eliminate 
or suppress the pathogen or nematode. Suc- 
cessful biological control implies encourage- 
ment of the beneficial organisms that share 
the same niche with the parasite and creation 
or augmentation of deleterious effects on the 
development, multiplication, survival, and 
virulence of the parasite. Biological control 
may be broadly defined to include control 
effected indirectly by roots of higher plants, 
as for example the use of decoy crops to in- 
duce germination of resting spores or sclerotia 
of fungi and the cysts of nematodes (31). 
Biological control may also be broadly defined 
to include control effected indirectly by chang- 
ing the metabolism of the plant, through 
exogenous supply of specific nonfungicidal 
substances that may bring about changes in 
host physiology and increase resistance to 
attack. 
In this paper we are not concerned with 
introducing biological control as a modern 
method of pest control, We are concerned 
with understanding the complexity of the 
problem that stems from numerous micro- 
ecological, microbiological, physiological, 
and biochemical processes in the plant 
environment. We are concerned with inter- 
actions of micro-organisms mediating sur- 
83 
vival of plant parasites and with the basic 
principles that underlie interactions of specific 
pathogens and associated antagonistic micro- 
organisms, We are concerned with collecting, 
analyzing, and interpreting fundamental in- 
formation of this sort in order to devise 
practices needed for a widespread utilization 
of biological control. 
METHODS OF BIOLOGICAL 
CONTROL 
Direct Introduction of Antagonistic 
Micro-organisms to Soil or Plant 
Tissues 
Many workers have attempted to control 
plant parasitic bacteria, fungi, and nematodes 
by use of the activities of antagonistic micro- 
organisms and predators (5, 16, 26, 30, 31, 
39, 91). One of the approaches employed is 
the direct inoculation of infested soil or plant 
tissues with cultures of antagonists or preda- 
tors (26, 30). 
Direct Introduction of Antagonists or Preda- 
tors to Soil.--Several investigators thought 
that direct introduction of antagonistic or 
predacious micro-organisms into soils, in 
the form of dried spores, appeared the most 
feasible method for large-scale application 
and utilization of such micro-organisms for 
biological control, Direct introduction of antag- 
onists to soil to control biologically such 
economically important and widespread fungal 
pathogens as Fusarium, Helminthosporium, 
Pythium, Rhizoctonia, Sclerotium, Verticil- 
lium, and others, exerted a slight and tem- 
porary biological control (16, 30). 
Several unsuccessful attempts have been 
made to introduce nematode-trapping fungi 
into soil to control plant parasitic nematodes 
(27, 28, 50). Efforts by Mankau (51), however, 
to isolate the nematode-trapping fungi intro- 
duced into field soils were also unsuccessful, 
although the fungi rapidly and readily colonized 
sterile soils. In these studies, and in those 
with introduced antagonists to control fungal 
and bacterial pathogens, significant biological 
control has been achieved only when sterile 
soil or synthetic media were inoculated with 
the parasite and the antagonist or predator. 
The addition of antagonists or predators to 

