GENERAL METHODS OF CONTROL 309 



(903). The application of organic matter to such soils results in the de- 

 struction of most of the sclerotia and mycelium of the pathogen (649). 

 Microbial antagonists rather than food exhaustion were, therefore, con- 

 sidered to be responsible for the destruction of the pathogen. 



The Sclerotium rot of sugar beets was found (552) to be controlled 

 by the application of nitrogenous fertilizers. This effect was believed to 

 be due largely to a change in the metabolism of the fungus or of the 

 host. It was also suggested, however, that suppression may possibly 

 be due to a change in the balance of the soil microbiological population. 



The possibility of suppressing the growth and eliminating the infec- 

 tivity of plant pathogens by utilizing the activities of the soil microflora 

 was demonstrated also for a number of other diseases. It is sufficient to 

 cite the suppression of Monilia jructigena on apples by various fungi 

 and bacteria (933), of F. culmorum and H. sativum on wheat (408, 

 729), and of species of Rhizoctonta on citrus seedlings. These patho- 

 gens are markedly influenced by T. lignorum, a common soil sapro- 

 phyte. A species of Trtchoderma was also found to cause a reduction in 

 the amount of Texas root rot of watermelons caused by P. omn'ivorwm 

 (105). 



The damping-off of citrus seedlings, caused by a number of fungi, 

 could be suppressed by T . lignorumy which parasitizes the fungi (989, 

 992). When T. lignorum was inoculated into pots containing Helmln- 

 thosforium sp. and Fusarium sp., the pathogenic action of these organ- 

 isms was checked and rendered harmless to plants (63). T. lignorum 

 was also decidedly antagonistic to Rhizoctonia and Pythium^j organisms 

 responsible for seed decay and damping-off of cucumbers (14). 



The presence of Gihherella on corn inhibited infection due to T. 

 viridis; seed grains inoculated with the former gave more vigorous 

 growth than uninoculated seed grains (227). On the other hand, T. 

 viridis was found able to attack and to destroy the sclerotia as well as the 

 mycelium of such pathogenic fungi as Corticium rolfsii, Corticium sa- 

 dakii, and Sderotinia libertiana. The utilization of this organism for 

 the biological control of plant diseases has, therefore, been suggested 



(<427)- 



Henry (407) believed that the biological control by the soil micro- 

 flora could even be directed against internal seed infection, since appre- 



