CONTROL OF SOIL-BORNE PLANT DISEASES 247 



the host plant is due to the formation of resistant spores which remain 

 viable in the soil for long periods. Because of this, the growth of many 

 plants requires a long rotation if this system is to be used as a means of 

 controlling the specific diseases. 



Many fungi and bacteria causing plant diseases were at first thought 

 capable of surviving in the soil for an indefinite time, even in the ab- 

 sence of the hosts. It has since been established, however, that, although 

 the majority of these pathogens are facultative saprophytes, some are 

 obligate parasites. The first can be grown easily on sterile soil and on 

 artificial culture media, whereas the second, such as Plasm-odiofhora 

 hrasskae and Synchytrmm endoh'iotkumy have not been cultivated so far 

 upon any artificial media and are known to die out in the soil in the ab- 

 sence of host plants. 



Certain soil-borne plant diseases may be caused by more than one or- 

 ganism. In the pink-root of onions (375), Phoma terrestrls is followed 

 by Fusarium malli; in the take-all of wheat (314), O. graminis is fol- 

 lowed by Fusarium culmorum. This type of sequence occurs with other 

 diseases, where the primary parasite first attacks the root and is followed 

 by a succession of other fungi, both parasites and saprophytes. By means 

 of the direct microscopic technique, the sequence of microorganisms can 

 be demonstrated in the infected roots of the plants. Certain less special- 

 ized parasites are able to live saprophytically on the dead tissues, 

 whereas the saprophytes are found only in the later stages of decom- 

 position. 



It has been suggested (313, 739) that the root-infecting fungi be 

 classified ecologically as soil inhabitants and soil invaders. The first may 

 be looked upon as primitive or unspecialized parasites with a wide host 

 range, their parasitism being considered incidental to their saprophytic 

 existence in the soil. The second group comprises a majority of root- 

 infecting fungi, the more highly specialized parasites. The presence of 

 these in the soil is closely associated with the occurrence of the host 

 plants: in the absence of a host, these fungi die out in the soil, because 

 of their inability to compete with the soil saprophytes. The close associa- 

 tion between this group of organisms and their host plants is believed to 

 be enforced by competition with the microbiological population of the 

 soil (312a). 



