IMPLICATIONS 437 



by species of Penicillium and Trichoderma and the amount of 

 cellulose decomposed. 



The abundance of studies on protein decomposition by soil 

 fungi has yielded data on the various factors that modify the 

 accumulation of ammonia. These factors are known to include 

 aeration, soil moisture, soil type, soil reaction, duration of the 

 incubation period, temperature, nature of the protein complex, 

 and presence of soil minerals, especially phosphates. 



SOIL-BORNE PATHOGENS 



No attempt can be made adequately to summarize the vast liter- 

 ature on the relation of soil-inhabiting fungi to disease in plants. 

 Species of Pythium, Phvtophthora, Aphanomyces, Thielaviopsis, 

 Fusarium, Sclerotinia, Colletotrichum, Gloeosporium, Botrytis, 

 Rhizoctonia, Sclerotium, and Phymatotrichum are among those 

 well-known to be soil-borne and to cause serious destruction of 

 crops. Some of them occur in virgin soils, and others are intro- 

 duced with the culture of the host species. Unfortunately many 

 of them, when once introduced into a field, persist for years, even 

 when susceptible hosts are not planted in these fields for long 

 periods. Pratt (1918) isolated Fusarium radicicola, F. tricho- 

 thecioides, and Rhizoctonia solani from soils in southern Idaho 

 that had never been cropped to potatoes. Rathbun (1918) found 

 Fusarium, a cause of damping-ofl of coniferous seedlings, in vir- 

 gin seed-bed soils. Soils that are "crop sick," on the other hand, 

 may contain a variety of species capable of producing infection 

 [Beckwith (1911)]. 



Infection by soil-inhabiting fungi has been shown to be con- 

 trolled by such factors as temperature, reaction, and interaction, 

 subjects given consideration in Chapters 5, 7, and 12. 



Few cases involving soil-borne human pathogens have been 

 proved. Emmons (1942) determined that Coccidioides immitis, 

 the cause of "valley fever," may be isolated from the soil in re- 

 gions where this disease is endemic. 



IMPLICATIONS 



As a result of the transformation of organic materials into 

 humus by soil fungi, organic acids are produced, and these acids 



