a marked increase in competitive saprophytic 
activity of the pathogen and an increase in the 
longevity of active fungus mycelium in colonized 
tissues. Loss of viability, on the other hand, 
was hastened by supplementing the substrate 
with energy material that raised the carbon/ 
nitrogen ratios. 
That the saprophytic behavior and survival 
of Rhizoctonia may be affected by the 
carbon-to-nitrogen balance of amendments 
was shown in more detail in our experiments 
in which cellulose powder, oat straw, or 
soybean hay, and a nitrogen source were 
combined to produce a wide range of carbon/ 
nitrogen ratios of the amendments before 
their incorporation in Rhizoctonia-infested 
soil (20). Saprophytic activity and survival of 
Rhizoctonia were slightly suppressed by cel- 
lulose and greatly enhanced by nitrogen. By the 
cellulose-nitrogen combinations activity was 
most significantly suppressed within the 
carbon/nitrogen ratio of 40 to 80 one week 
after incorporation, 40 to 100 after 3 weeks, 
and 40 to 200 after 5 weeks, By the oat straw- 
nitrogen combination saprophytic activity was 
significantly suppressed almost within the 
same carbon/nitrogen ratio, By the soybean 
hay-nitrogen combination, however, sapro- 
phytic activity was suppressed at a carbon/ 
nitrogen ratio range lower than that of cellulose 
and oat straw. 
The foregoing results on Rhizoctonia help us 
to understand why so many conflicting reports 
have been published on the efficacy of amend- 
ments for biological control of plant diseases. 
It is now realized that the value of organic 
matter for purposes of biological control may 
be directly related at leastto its carbon /nitro- 
gen ratio which, among others, determines the 
extent of availability for decomposition. The 
carbon/nitrogen ratio may be more important 
than the absolute amount of carbon and nitrogen 
added to soil in affecting behavior of root- 
infecting fungi and nematodes. Effect of the 
carbon-to-nitrogen balance on the rate and 
degree of amendment effectiveness, to sup- 
press saprophytic activity, may be the result 
of a "flare-up" of particular groups of micro- 
organisms stimulated within a range of carbon/ 
nitrogen ratios predetermined by inherent 
microbial characteristics and microenviron- 
mental factors. 
86 
Manipulation of Environment Through 
Cultural Practices 
Cultural practices such as crop sequence or 
crop rotation and crop management constitute 
the best known, the most widely adopted, and 
perhaps the oldest method of biological control 
of plant diseases, In practice today one group 
of pathogens is kept under a fair degree of 
control by use of crop rotation and soil man- 
agement practices that retard the build-up of 
the pathogen. This method of biological control 
is applied with some pathogens that have 
narrow host ranges and relatively short per- 
sistences in the absence of their specific 
hosts. One of the most successful cases of 
biological control of a soilborne disease by 
crop rotation is that of the ''take-all'' disease 
of wheat in Great Britain (32). 
Rotations with nonhost of poor host crops 
have long been used for reducing harmful 
nematode populations (15). In general, rota- 
tions have been most effective when a single 
species of a relatively host-specific nematode 
is involved in crop losses. Rotations are not 
considered to be effective when several species 
are present, because one of them may be in- 
creased by a rotation that decreases another. 
The most complete investigations of the effects 
of rotations on nematode populations come 
from studies of control of Heterodera spp. 
Jones (37) showed that populations of H. 
schachtii decline at the rate of 40to50 percent 
each year inthe absence of host plants. Depend- 
ing on the initial population level, upto 8 years 
may be required for the reduction of popula- 
tions of this nematode to acceptable levels. 
Biological control through crop sequence has 
long been ascribed to antagonistic effects of 
natural soil microfloras associated with crop 
residues and root excretions, accumulation 
or depletion of toxic ions, increasing host 
resistance, decoying activities of root excre- 
tions and organic supplements, effects of 
nutrients, and prevention of contiguity in 
time and space of susceptible plants and 
virulent pathogens (15, 84). Crop sequence 
control measures have on occasion been dra- 
matically effective, but they are often very 
erratic and reveal great voids in our knowledge 
of scientific principles and in the advancement 
of the arts of crop management. 
