Siii\ i\al of Plant Pathogens in the Soil 



287 



infected pastures safe for use. A better understanding of the an- 

 tagonists that are responsible for the rapid destruction of pathogenic 

 organisms in the soil may throw hght upon this problem and im- 

 prove the methods of control. 



Fig. 114. Influence of the hydrogen-ion concentration on the incidence of potato 

 scab (from Dippenaar). 



Survival of Plant Pathogens in the Soil 



The organisms causing plant diseases can be divided into five 

 distinct groups: fungi, actinomycetes, bacteria, animal forms, and 

 viruses. They are all found in the soil, and most are able to survive 

 there for long periods, especially in the presence of the host plant. 

 The fungi are by far the largest and most important group. 



These fungi belong to the Myxomycetes {Plasmodiophora brassi- 

 cae, causing club root of cabbage), Phycomycetes (Phytophthora 

 infestans, Aphanomyces laevis, Syncliytriiim endohioticinn, Pytliium 

 debaryamim) , Ascomycetes {Botrytis cinerea, Sclerotinia trifoliorum, 

 Corticium vagiim). Fungi Imperfecti {Phoma betae, Verticilliiim 

 alboatriim, Hehninthosporium gramineum, Fusarium lini, Fiisarium 

 vasinfectiim) , and finally certain Basidiomycetes, including smuts. 



Various fungi have been isolated from both cultivated and virgin 



