GENERAL METHODS OF CONTROL 25 5 



rather than food exhaustion were, therefore, considered to be respon- 

 sible for the destruction of the pathogen. 



The Sclerotium rot of sugar beets was found (518) 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 the possibility exists that the 

 suppression is 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 (911), of F. culmorum and H. sativum on wheat (398, 

 695), and of species of Rhizoctonia on citrus seedlings. These patho- 

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

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

 the amount of Texas root rot of watermelons caused by P. omnivorum 

 (lOi). 



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

 could be suppressed by T. lignorum, which parasitizes the fungi (962, 

 966). The addition of Trichoderma spores to acid sterilized soils pre- 

 vented the damping-off of the seedlings. When T. lignorum was inocu- 

 lated into pots containing Helminthosforium sp. and Fusarium sp., the 

 pathogenic action of these organisms was checked and rendered harm- 

 less to plants (60). T. lignorum was also found (17) to be decidedly 

 antagonistic to Rhizoctonia and Pythium, organisms responsible for 

 seed decay and damping-off of cucumbers. 



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

 viridis; seed grains inoculated with the former gave more vigorous 

 growth than uninoculated seed grains (226). 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 Sclerotinia lihertiana (S. sclerotiorum) . The utilization of 

 this organism for the biological control of plant diseases has, therefore, 

 been suggested (415). 



Henry (398) believed that the biological control by the soil micro- 



