SOIL TREATMENTS AND SEED GERMINATION 9 



SOIL FUNGICIDES AND DAMPING-OFF 



It appears from the foregoing discussion that damping-off is not altogether 

 controllable b\- the adjustment of the environment. It is, of course, controllable 

 by ridding the soil of fungi, as by the use of various chemicals, although there is 

 need of fungicides which will prevent the recontamination of soils for a longer 

 time than do the commonly used volatile chemicals. Steam is subject to the 

 same criticism, and Anderson (6) concluded that Pythium rapidly reinfests soil 

 thus sterilized, the fungus growing up, in tobacco beds, from unsterilized soil 

 below. He suggested the use of formaldehyde instead of steam, but there is no 

 evidence that its fungicidal effect is much less fleeting and it has been noted by 

 Thomas (85) and by others that formaldehyde treatment of soil does not for long 

 prevent the reintroduction of pathogenic fungi. 



This is really no great drawback when only damping-off or early damping-off 

 is to be combated but may be serious with seeds which are slow to germinate 

 or seedlings of species which remain susceptible to infection by soil fungi for a 

 long time. It is true that the susceptibility of most species to damping-off, es- 

 pecially to that caused by Pythium, usually decreases fairly rapidly as plants 

 grow older; but damping-off fungi may injure plants in other ways, and Gratz (39) 

 found that Rhizoctonia may infect seedlings of cabbage until they are more than 

 four months old. 



Once the pathogenic fungi have been introduced into soil which was (steam) 

 sterilized some time before, they may cause even more damping-off than in soil 

 not sterilized at all. This was the conclusion of Hartley (43) and of Abdel-Salam 

 (3) with reference to Pythium and Rhizoctonia respectively. Horsfall (48), 

 too, observed that damping-off may be worse in reinfested soil than it was before 

 steaming, and Abdel-Salam made a similar observation in connection with the 

 use of formaldehyde. 



It is evident, then, that such treatments are more effective in getting fungi 

 out of soil than in keeping them out, and just how soon sterilized soil may be 

 reinfested is a point of some interest. Pythium grew in soil inoculated by Nolla 

 (71) one week after treatment with formaldehyde, and he applied it twice as 

 heavily as is usually recommended. This indicates that soils so disinfected may 

 become recontaminated with fungi (although, of course, they often do not) by the 

 time seeds are planted, especially if, on the grounds of safety, there is some delay 

 between disinfection of soil and seeding. 



We may now, on the basis of the above discussion, consider in some detail the 

 use of a number of chemicals, for more needs to be learned about better methods 

 of using old soil disinfectants and about other chemicals which may have more 

 nearly ideal qualities as soil disinfectants. 



Formaldehyde 



Formaldehyde has been applied to soil in many different ways. Table 3 includes 

 mention of several representative methods, the number of cubic centimeters 

 of comm*ercial formaldehyde^ and of water applied per square foot, and results 

 in control of damping-off. 



-37 percent by weight, 40 percent by volume. 



