254 CONTROL OF SOIL-BORNE PLANT DISEASES 



ganic or an inorganic form, was believed to protect the pathogenic or- 

 ganism against attack by the soil microflora, by offering a more readily 

 available source of nitrogen. Tyner (895) suggested that the differ- 

 ences in the microflora associated with the decomposition of different 

 plant residues are largely responsible for differences in persistence and 

 virulence of pathogens causing root rot of cereals. 



Against some plant pathogens, however, high nitrogenous materials 

 were found to be very effective. Considerable reduction in the slime- 

 disease of tomato plants resulted from the addition of green manures 

 to the soil before planting (904) j organic materials high in nitrogen, 

 as well as the supplementary addition of nitrogenous materials suffi- 

 cient for complete decomposition of the organic matter, brought about 

 greater reduction of the disease. Organic matter was found to be most 

 effective during the process of decomposition 5 after it has undergone 

 extensive decomposition and reached a stage of slow decomposition, 

 when it is usually designated as humus, it becomes comparatively inert 

 (878). 



The antagonistic action of soil microorganisms has been utilized in 

 several areas of the United States for the control of P. omnivorumy the 

 root rot of cotton. It was shown (484-486) that this pathogen can be 

 inactivated when organic manures are added to the soil before the crop- 

 growing season. Eaton and King (223) demonstrated, by the use of 

 the contact slide technique, that microbiological antagonism represents, 

 in this case, the true mechanism of the control process; the develop- 

 ment of saprophytic organisms was most profuse in the slides buried in 

 the manured plots, whereas the mycelium of the pathogen was most 

 abundant on the slides kept in the unmanured plots. The conclusion was 

 reached (345) that manuring definitely controls cotton root rot, as a 

 result of the parasitism by bacteria of the fungal strands of the causative 

 agent of the disease. Continuous growth of cotton on certain neutral or 

 alkaline soils in southern United States was believed to bring about an 

 unbalanced soil population in which P. omnivorum became a dominant 

 organism J this was accompanied by the absence or only the sporadic 

 presence of Trichoder-ma and other molds (878). The application of 

 organic matter to such soils results in the destruction of most of the 

 sclerotia and mycelium of the pathogen (609). Microbial antagonists 



