Methods of Control of Plant Diseases 295 



ing the period of its active decomposition, is most effective in de- 

 pressing pathogenic fungi. Inactivation of pathogenic fungi in the 

 soil b\' use of the antagonistic action of soil microorganisms has 

 been variously attempted. Organic manures were added to the soil 

 to control PJiymatotrichum omnivoruin, the root rot of irrigated cot- 

 ton under continuous cultivation in Arizona. With the Rossi-Cho- 

 lodn\- slide technique, it was possible to demonstrate that micro- 

 biological antagonism represents the true mechanism of the control 

 process. Development of saprophytic organisms was most profuse 

 in the slides buried in the manured plots, whereas the mycelium of 

 P. omnivorum was most abundant on the slides in the unmanured 

 plots. King suggested that parasitism of the fungal strands by bac- 

 teria is one of the reasons for the decline of the pathogen in manured 

 soils. Henry believed that the biological control by the soil micro- 

 flora could even be directed against internal seed infection, since 

 appreciable infection of surface-sterilized flaxseed was found to occur 

 in sterilized but not in unsterilized soil. 



Attention has already been called to the fact that numerous 

 observations have been made concerning the favorable effect of 

 bacteria in depressing various plant diseases. This is true, for 

 example, of the addition of bacteria to unsterilized soil exhausted 

 by growing flax; the percentage of plants diseased by F. lini was 

 thereby lowered. The term "bacterization" was applied to the 

 process of treatment of seed with active bacteria to support the 

 plant against pathogenic fungi. It has been suggested that the 

 effect of bacteria on germinating seeds is due to the liberated bac- 

 terial products capable of depressing the development of parasitic 

 fungi. Although not in all cases conclusive, the results fully justify 

 the hope that a better knowledge of the soil antagonists may lead, 

 if not to complete control, at least to a certain amount of control 

 over the numerous plant diseases caused by pathogenic fungi, espe- 

 cially those that persist for a time in the soil. 



Methods of Control of Plant Diseases 



The methods of treatment of soil for the control of injurious micro- 

 organisms are divided into five distinct groups: 



1. Proper rotation, or withholding of the host plant, since various 

 parasites accumulate as a result of continuous growth of the same 

 or closely related plants; use of resistant plant varieties. 



