308 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 (920) 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 (928) 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; after it has undergone 

 extensive decomposition and reached a stage of slow decomposition, 

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



It has been recommended (767) that organic composts be used in 

 forest soils in order to overcome the infertility resulting from the ac- 

 cumulation of toxic substances. The addition of the compost changes the 

 microbiological population of the soil, followed by the destruction of 

 the toxins. 



The antagonistic action of soil microorganisms has been utilized for 

 the control of P. omnivoruniy the root rot of cotton. This pathogen can 

 be inactivated when organic manures are added to the soil before the 

 crop-growing season (506). By the use of the contact slide technique, 

 microbiological antagonism was found (226) to represent the true 

 mechanism of the control process; the growth 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 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; this was accompanied by the 

 absence or only the sporadic presence of Trkhoderma and other molds 



