286 ASSOCIATIVE EFFECTS AMONG FUNGI 



Buller (1924) has compiled a list of hyperparasites of more than 

 50 species. They include members in the Chytridiaceae, Alucor- 

 aceae, Pyrenomycetes, Agaricaeae, Polyporaceae, and Fungi Im- 

 perfecti. Little of a fundamental nature is known about the an- 

 tagonisms in any of them. 



Evidence of antagonism between fungi in soils. The soil 

 constitutes the normal habitat of many species of fungi. Such 

 factors as texture, organic content, acidity, moisture, temperature, 

 and character of the vegetational cover, are known to influence 

 the presence or absence of a particular species and its relative 

 abundance. How these interrelated factors influence competi- 

 tion between soil fungi remains largely unknown, but undoubt- 

 edly the fungus flora is never in equilibrium. The type of ob- 

 servations that have been made on these problems is indicated in 

 the following discussion. Millard and Taylor (1927) observed 

 that potato scab, caused by Actinomyces scabies, was eliminated 

 in fields containing large amounts of organic matter resulting from 

 green manuring. Under these conditions the development of a 

 saprophytic species, A. precox, was favored, and it was able to 

 suppress the pathogen. A proximate cause appears from the 

 studies by Sanford ( 1926). He noted in cultures that the limiting 

 acidity for germination of A. scabies was about pH 5.3 and that 

 the optimum reaction was pH 8.5. From these results it may be 

 anticipated that acidity from decomposition of green crops that 

 have been plowed under would be unfavorable for the scab patho- 

 gen but might be favorable for other microorganisms to the ex- 

 tent that they would predominate and manifest their antibiosis. 



Fungi causing root rots are known to survive in the soil for 

 varying periods in the absence of their host plants. Supposedly 

 they live under these conditions as saprophytes. Hence it follows 

 that the incorporation of organic matter should increase their 

 incidence, but this anticipated result is not invariable. Other soil- 

 inhabiting species have been shown to modify prevalence of the 

 root-invading pathogens, as the work of Sanford and Broadfoot 

 (1931, 1934) on Ophiobolus gr ami iris, Helminthosporiitm sativum, 

 and Fusarium culmorum illustrates. Not only were soil-inhabit- 

 ing saprophytes able to modify pathogenicity in their pot cul- 

 tures but also similar effects were secured by the use of filtrates 

 from cultures of the saprophytes. Presumably toxic products 

 caused this inhibitory action against the pathogens. Similarly 







