Plant-Disease Antagonists 



293 



Tarlk (U. Efkkct of Tricliodcrma Ugnorum on Germination' and Gkowth of 



Uaulky Infected with Uclminthosporium sativum in Sterilized Soil 



(from Christensen) 



* H = II(lniintlinsj>oriiuii, T = Triclindcrma, SI = soil infusion. 



bacterial product checked the disease. The same principle was 

 found to hold true for oranges infected with PeniciIIiiim itaUcum. 

 The injurious action of certain common soil bacteria upon Pseudo- 

 monas c'ltri, the cause of citrus canker, has also been reported. 

 Wheat seedlings were protected from infection by Helminthosporium 

 and flax seedlings from Fusarium by use of antagonistic bacteria. A 

 watermelon disease caused by Phymatotrichiim omnivorum was re- 

 duced when certain fungi (Trichoderma Ugnorum) and bacteria 

 were present in the soil together with the pathogen. The severity 

 of the seedling blight of flax, caused by F. lini, was diminished when 

 the pathogen was accompanied in the soil by certain other fungi. 

 The pathogenicity of H. sativum on wheat seedlings was suppressed 

 by the antagonistic action of TricJiothecium roseum, which is be- 

 lieved to produce a toxic substance. 



The role of microbiological antagonism in the control of soil-borne 

 plant diseases has been outlined as follows: The soil population is in 

 a dynamic biological equilibrium. When a certain crop is grown 

 continuously, various parasites capable of attacking the roots of that 

 crop multiply. Organic manures stimulate the development of vari- 

 ous saprophytes in the soil. These multiply at the expense of the 

 pathogens and are able to check their activity, either by preventing 



