some unidentified soil factors (82) or to the 
combined effect of antibiotics and enzymes 
produced by lytic organisms (46, 47). 
In a recent paper (61), the carbon-to- 
nitrogen balance of amendments appeared to be 
of great importance in determining the amount 
of lysis. Considerable mycelial lysis of three 
root-infecting fungi was observed by oat straw 
of carbon/nitrogen ratios 30, 50, and 85. The 
amount of lysis increased with increase of 
carbon /nitrogen ratio of amendments; and this 
increase was more pronounced with R. solani 
and Verticillium albo-atrum than with F. 
solani f. phaseoli. The least destruction of 
mycelia was caused by unamended soil, by 
soil supplemented with nitrogen only, and by 
oat straw of carbon/nitrogen ratio 10, 
Fungistasis.--Specific toxic metabolites and 
lytic agents may not be the only mechanisms 
responsible for antibiosis and biological con- 
trol. Antibiosis may also assume the form of 
fungistasis, a form of natural toxicity to fungi 
in soils manifested by nonspecific inhibition 
of spore germination. In 1953, Dobbs and 
Hinson (24) demonstrated that germination 
of inactive fungal propagules in soil and in 
plant rhizosphere is not generally limited by 
lack of nutrients but by the almost universal 
presence, in natural soils, of inhibitory and 
fungistatic factors. Spore germination of fungi 
is stimulated by counteracting the inhibitory 
effect of soil fungistasis by plant excretions 
and various organic amendments (48). 
Fungistasis was found in a variety of natural 
soils (24, 48), but is not exhibited in auto- 
claved soils. Fungistasis may be due to dif- 
fusible toxic metabolites in natural soils (48). 
Some investigators (48) were able to reinstate 
fungistasis by infesting autoclaved soils with 
common soil saprophytes now known to pro- 
duce antibiotics in culture. 
Despite extensive studies on fungistasis 
believed to be of biological origin, there still 
remains the question of the specific origin 
and nature of the widespread soil fungistat. 
Nevertheless, plant pathologists and micro- 
biologists have not failed to understand the 
extraordinary microecological significance of 
this process. Let us visualize some fungal 
propagules lying in soil matrix or in plant 
rhizosphere. Addition of organic amendments 
will overcome fungistasis and start the time 
clock for a series of unpredictable and pre- 
90 
dictable events. These will involve germination 
of propagules of soil saprophytes, soil antag- 
onists, competitors, predators, and root- 
infecting fungi. Some propagules will germinate 
and germ tubes will lyse (13), Others will 
germinate normally and grow through soil 
matrix, organic debris, or roots. Others will, 
to say the least, grow and produce toxic 
metabolites. Still others will germinate and 
precociously sporulate (3), The overcoming of 
fungistasis then will result in immense, un- 
predictable, and explosive disturbances of the 
soil microbiological equilibrium. These dis- 
turbances may result in direct or indirect 
alterations of inoculum potential of a given 
pathogen. It is up to the research man to 
discover the course of events that would lead 
to the reduction of the disease potential of a 
specific plant pathogen. 
Competition 
In addition to antibiosis, competition is 
another important mechanism of antagonism. 
Competition is considered here in its strict 
sense as an associative effect in which one 
organism adversely affects another in the 
struggle for some limiting factor of the en- 
vironment. If competition, which may be 
exerted whether or not a competing organism 
produces an antibiotic, takes a role in bio- 
logical control, this must apply almost ex- 
clusively to soilborne diseases andnematodes. 
Marshall and Alexander (52) suggested that 
"competition for nutrients is a significant 
means of ecological control among members 
of soil microflora and, together with com- 
petitive interactions for space and oxygen, 
may be the major factors governing the 
biological control of soil-borne fungi." 
Several cases of competition for energy- 
yielding materials are known (14), Garrett (30) 
ascribed the depressing effect of decomposing 
ryegrass meal in soil on O. graminis to the 
fact that numbers of micro-organisms are so 
greatly increased inside and around the added 
amendments that they consume all available 
nutrients, especially nitrogen, and thus create 
starvation conditions for the Ophiobolus myce- 
lium. Skinner (82), with the ingenious use of 
bentonite, was ‘able to distinguish in sterile 
sand cultures between the inhibition of F. 
culmorum by Streptomyces albido flavus 
caused by antibiosis and that caused by 


