Strawberry plants, variety Premier, were grown in naturally-infected root rot soil in which consecutive "crops" of several agricultural plants had been turned under, and in other lots of the same soil that had been steam sterilized or fertilized with barnyard manure. On examination of their roots it was found that the incidence and severity of root rot were closely correlated with soil treatment. Plants grown in sterilized soil remained free from disease as did those of the soybean series until the third season when they were slightly affected. Plants in the manure, corn, red clover, timothy, and untreated soil series all became diseased, the severity of attack increasing in the respective series in the order named.Although roots of the various cover crops were found to contain representatives of several different genera of fungi, a specific fungus was dominant in each as was the nematode, Pratylenchus pratensis, in timothy and clover. This build-up of specific organisms appeared to be correlated with the incidence and severity of the disease in the roots of the strawberry plants that followed in the respective series.However, in strawberry plants grown in the variously treated soils, fungal infection was negligible and, on the whole, not related to that of the preceding cover crop. An exception to this was the heavy infection by the mycorrhizal fungus (Rhizophagus sp.?), following timothy and corn, but a lack of correlation, in many cases, between the presence of the fungus and discoloured and necrotic tissue indicated that other agencies might be responsible for the injury. In timothy and red clover treated soils, nematodes, particularly Pratylenchus pratensis, might have been an important factor.Fewer bacteria were found adjacent to roots of healthy plants than to those of diseased ones. Qualitative differentiation on the basis of nutritional requirements indicated a striking relationship between the incidence of certain groups of bacterial isolates and the severity of disease attack. The equilibrium between presumably "harmful" bacteria and the innocuous, normally occurring rhizosphere types is designated the Bacterial Balance Index. There are marked differences in the microbiological equilibria of the different soil series; increased severity of root rot is associated with a fall in the Bacterial Balance Index.
Soybean and red clover, grown as cover crops and incorporated into strawberry root rot soil, showed a marked difference in ability to control the disease on variety Premier. Soybean caused a striking reduction in the incidence of root rot and a drastic shift in the bacterial equilibrium of the soil. Red clover had little effect on the severity of the disease or the general microflora of the soil.A study of "rhizosphere effects" reveals that the characteristic differences between the resultant bacterial equilibrium of the soils in which the two leguminous plants were grown, could not be attributed to influences exerted by the latter in the living state. However, the bacterial types favoured during decomposition in experimental cultures of tissues of red clover and of soybean, each inoculated with root rot soil, were identical with those isolated from root rot soil with which red clover and soybean, respectively, had been incorporated. In contrast to the putrefactive decomposition of red clover, soybeans apparently underwent a carbohydrate breakdown that could be reproduced essentially in culture by the substitution of glucose for soybean tissues. Beneficial changes in the bacteriology of actual root rot soils could be induced by the decomposition of pure carbohydrate in place of soybean. The favourable alteration in the bacterial equilibrium was accompanied by a corresponding modification of the fungous flora such that potentially pathogenic forms were replaced by presumably innocuous ones. These carbohydrate treated soils were capable of producing strawberry plants with well developed healthy root systems. The ability of soybean to control strawberry root rot therefore seems to depend primarily on a carbohydrate type of breakdown in diseased soil, causing a highly favourable shift in the microbiological equilibrium. The decomposition of red clover, on the other hand, did not under the same conditions induce these salutary effects.
Bacteria of the rhizospheres of flax and tobacco were found to possess more complex nutritive requirements than those in the corresponding control soils. The roots of even young seedlings favour the development of those types that are dependent upon a supply of thiamin, biotin, and amino nitrogen for their growth, thus suggesting that the roots may excrete significant amounts of these stimulative substances. The "rhizosphere effect" was more pronounced with susceptible than with resistant varieties of either flax or tobacco. A greater difference was found to exist between the rhizosphere and the control soil where the latter is poor than where it is richly supplied with organic matter, since liberation of growth substances by plant decomposition permits a limited development of the more typically rhizosphere forms, apart from the zone of influence of the growing plant.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.