294 Disease-Producing Microorganisms 



their growth or by attacking and destroying the mycehum of the 

 parasites. The biological control of plant diseases is particularly 

 effective against those organisms which have become highly special- 

 ized to a parasitic form of life. 



Van Luijk obtained biological control of plant parasites by inocu- 

 lation of soil with microorganisms selected for their antagonistic 

 capacity, or by addition of their growth products. Broadfoot em- 

 phasized that antagonism of a saprophyte to a plant pathogen, as 

 measured by growth on artificial media, is not necessarily a measure 

 of the actual control that may be exerted upon the parasite in the 

 soil. Inoculation of soil with an antagonistic organism, such as 

 T. lignorum, may have only a temporary effect in changing the 

 microbiological balance of the soil population. Weindling and Faw- 

 cett attempted to control R. solani by use of T. lignorum, and Cordon 

 and Haenseler by use of B. simplex, with similar effects. Daines 

 reported that T. lignorum produces a substance toxic to S. scabies. 

 This substance is rapidly destroyed in the soil on aeration. It was, 

 therefore, believed doubtful that the fungus could be of much 

 assistance in combating potato scab. 



Fellows obtained field control of the take-all disease of wheat 

 (O. graminis) in Kansas by application of chicken and horse manure, 

 alfalfa stems and leaves, and other organic materials. Garrett at- 

 tempted to prove that the factor chiefly controlling the spread of the 

 pathogenic fungus along the roots of the wheat plant was associated 

 with the accumulation of carbon dioxide and a corresponding lower- 

 ing of the oxygen tension. This could best be maintained by addi- 

 tions of organic manures. Organic matter low in nitrogen was more 

 effective than high-nitrogenous materials; it was, therefore, suggested 

 that the soil microflora uses the mycelium of the pathogen as a 

 source of nitrogen. Addition of nitrogenous materials, either in an 

 organic or in an inorganic form, protected the parasite by offering 

 a more readily available source of nitrogen. Differences in the 

 microflora associated with the decomposition of various composts are 

 believed to be largely responsible for differences in persistence and 

 virulence of pathogens causing root rots of cereals. 



Green manures, when added to soil before planting, cause consid- 

 erable reduction in slime disease of tomato plants. Organic ma- 

 terials high in nitrogen, and supplementary additions of nitrogen 

 sufficient for complete decomposition of the organic material, are 

 most effective. Thom and Morrow found that organic matter, dur- 



