Fungitoxicity, in terms of inhibition of mycelial growth, was demonstrated with eight chlorinated hydrocarbon insecticides by the culture plate bio-assay method. All of the dosage–response curves thus derived were bimodal rather than linear. Evidence is presented to show that the shape of these curves is governed by two physical properties of the compounds: water solubility (which was determined experimentally) and vapor pressure. The concentration of insecticide in solution governs the initial rise of each dosage–response curve which is extended to a peak because of supersaturation and is followed by a decline as normal saturation is regained. The concentration of toxicant vapor in contact with the organism governs the second rise in the curve to a level dependent on the vapor pressure of the compound. Lindane has a high degree of fungitoxicity because of its relatively high water solubility. Heptachlor, chlordane, and aldrin are also highly toxic because of their vapor pressures. DDT, methoxychlor, dieldrin, and endrin are weakly fungitoxic because of both low water solubility and low vapor pressure.
A means of determining the thiram content of treated soil was developed, applying the paper disk bioassay technique with Glomerella cingulata as the test organism. It was found that thiram persisted in sandy soil for over two months but disappeared from compost soil within one week. Thiram treatment changed the microbiological balance in the soil, the number of bacteria being increased and the fungi decreased for some time. Thiram was shown to be selective in its action against fungi; Penicillium and Trichoderma, being resistant, increased in number after soil treatment. Thiram treatment rendered soil more difficult to infest artificially with Pythium ultimum and also prevented a natural increase in the population of damping-off organisms resulting from repeated cropping. This protection for seedlings persisted longer than did the fungicide in the soil.
The effects of several insecticides and herbicides on the development of early blight and Fusarium wilt of tomato were investigated. Young tomato plants were grown in sand to which solutions or suspensions of these chemicals were applied repeatedly before the foliage was inoculated with spores of Alternaria solani and either before or after the roots were inoculated with spores of Fusarium oxysporum f. lycopersici. On the basis of lesion counts, increased early blight development resulted from applications of lindane, 2,4-D, and isodrin, and decreased disease from endrin, MH, NPA, dieldrin, IPC, dalapon, demeton and aldrin. Lindane, isodrin, and dalapon increased the severity of Fusarium wilt whereas endrin, aldrin, TCA, DDT, and dinoseb reduced it. 2,4-D and MH affected wilt development in a susceptible and a resistant variety in different ways according to the time of application in relation to inoculation, but they did not alter the reaction of an immune variety.
Sterilization brings about significant changes in the physical and chemical, as well as biological properties of soil which thus becomes a different and often more favourable medium for microbial activity. In an investigation of a root rot of greenhouse tomatoes marked differences in numbers of fungi, bacteria, and actinomycetes were noted in both soils and rhizospheres as a result of sterilization with steam, chloropicrin, and formaldehyde. Roots invariably supported much higher numbers of the three groups of organisms studied, thus displaying the common "rhizosphere effect". Numbers of bacteria were considerably greater on infected than on healthy roots.Qualitative differences in fungi and bacteria were also noted in both soils and rhizospheres. Of particular interest was the tendency for bacteria with simple food requirements and those stimulated by amino acids to predominate in the rhizosphere, and for those with more complex nutritional needs to predominate in soils apart from the roots.It is suggested that such nutritional investigations of rhizospheres may be useful in studies on the physiological activity of plant roots.
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