Ecological Studies with Soil Fungi.
Rhzoctonia solani, a plant pathogen of wide distribution has been shown by soil-tube experiments to be a member of the “soil inhabitant” group of fungi. Such are described as unspecialised, with a wide host range and soil distribution. Their parasitism is incidental to a saprophytic existence which they lead as members of the general soil micro-flora. These fungi are distinct from those grouped as “soil invaders” whose presence in soil is closely dependent on hostplant
tissue. In the continued absence of the latter, fungi of this grouping disappear from the soil population owing to inability to compete with “soil inhabitants” for available nutrients.
Rhizoctonia solani nevertheless appears to be susceptible to competition in the presence of other soil inhabitants. Soils differ in their capacity to support R. solani growth. In particular, soils rich in certain forms of organic matter possess characteristics unfavourable to growth of this organism.
The buried glass-slide technique has demonstrated the depressed activity of R. solani in the presence of soil supplements of a cellulosic nature.
The significance of this growth check has been examined in relation to a number of factors in the soil microbiological complex.
Microbial antagonism and competition. Elimination of the micro-population by soil sterilization enabled R. solani to grow in the organic matter series (1% addition of straw or grass meal) as vigorously as in soil to which no organic matter of this kind had been added. Conversely, re-introduction of typical soil fungi subsequent to sterilization (by steam) prevented R. solani growth.
The suppression of this fungus following additions of cellulose-rich materials seems to be associated with competitive or antagonistic influences arising from the presence of other organisms better adapted to live and develop in such a soil habitat.
Relative cellulose decomposing capacity. From growth measurements of R. solani and other soil fungi on cellulose agar and on sterile filter paper it has been concluded that the former is a weak cellulose decomposer. Following organic matter additions, there is a demonstrable quick development of active cellulose decomposing organisms. In the presence of these, R. solani, unable to adapt itself readily to a cellulose decomposing function, suffers the growth check due to failure in the competition for available nutrients.
Competition for available N and P. The hypothesis was tested that the active cellulose decomposers such as Penicillium, Trichoderma, Fusarium use up so much of the available nitrogen and phosphorus that less active fungi are suppressed through being deprived of their essential nutrient requirements.
It was found that inorganic N and P salts significantly increased R. solani growth in soils previously supplemented with cellulosic organic matter. None of the N or P treatments were able in the organic matter series entirely to overcome the growth check of the inoculated R. solani. Furthermore, these inorganic materials had no effect on the test organism growing in soil not supplied with extra organic matter. Only in the presence of additions of straw and grass meal were any stimulating effects of N and P apparent. It is concluded that R. solani is unable to secure its full requirement of N and P where the concentration of other micro-organisms is encouraged by organic supplements.
Carbon dioxide concentration. CO2 in gaseous state or solution may be present in a soil micro-climate in local concentrations inimical to normal growth of some fungi. Experiments have been carried out designed to remove any CO2 arising from decomposition of the extra cellulosic materials. Elimination of CO2 was achieved by maintaining an air current through the soil system; by alkali absorption of CO2 and by growing the fungus under study in alkaline or CO2-acceptor conditions. The results demonstrated that poor growth of R. solani where extra cellulose occurs can be partially accounted for by a CO2 sensitivity of the fungus.
No single group of these features of soil micro-climate can account in full for restricted growth of R. solani in organic-rich soils, but the cumulative influences of them all appear to demonstrate some of the mechanisms of biological antagonism among some soil fungi. A sensitivity to cellulose supplements (straw and grass) has also been observed in field experiments with the fungus, Cercosporella herpotricoides, an increasingly serious pathogen of wheat. Laboratory experiments using the same techniques as in the R. solani studies show that relative cellulose-decomposing capacity, other antagonistic micro-organisms, restricted nutrient supply, CO2 concentration, are micro-climate attributes also contributing to reduced activity of Cercenporella when the soil population is in a state of change during organic-matter decomposition.