282 ASSOCIATIVE EFFECTS AMONG FUNGI 



sphaeria herpotrichoides, Flenodomus 7/ieliloti y and Wofnoimda 

 grandnis. 



Arrillaga (1935) made all possible combinations on potato dex- 

 trose agar of 12 species of fungi associated with disease or decay 

 of Citrus fruits, with the result that Diaporthe cirri checked the 

 growth, especially of Phytophthora parasitica and P. citrophthora. 



Causes of antagonism. It is apparent from the reports of 

 these studies that the range of interaction between fungi extends 

 from complete indifference of both members of the pair on the 

 one extreme to very active inhibition on the other. Since these 

 effects are manifest indifferently between members of all classes 

 of fungi, it appears improbable that one and the same proximate 

 cause is responsible for all. Instead a variety of causes has been 

 suggested, and evidence in their support has been submitted. Some 

 of these causes are exhaustion of nutrients, modification of their 

 balance or concentration, differential in optimal pH, which may 

 be the result of metabolic products formed by one of the species, 

 differential in optimal temperature, production of excretory prod- 

 ucts, which cause staling, production of toxic substances, and 

 aversion. Not all need be considered in this discussion, nor need 

 evidence in their support be reviewed. 



The term staling is applied to the well-known phenomenon in 

 which the growth rate of a fungus on an artificial medium grad- 

 ually decreases and eventually ceases. This phenomenon is not 

 the result of an exhaustion of nutriment but of the presence of a 

 progressive increase in amount of products of metabolism. Niki- 

 tinsky ( 1904) grew on liquid media repeated crops of Pemcillhim 

 glaucum, P. griseim?, Mucor stolonifer, Aspergillus flavzis, Sac- 

 charomyces cerevisiae, and S. rosaceus. At intervals the mycelial 

 mat was removed by filtration, dried, and weighed. The medium 

 was then sown with the same or a different species, and the mat 

 was again removed. This procedure was repeated until the me- 

 dium would no longer support growth. He observed that, when 

 ammonium chloride was used as the source of nitrogen, the inhibi- 

 tion set in quickly, and the medium became increasingly more 

 acid. To such media he then added alkali, and the media again 

 supported good growth. When he employed ammonium tartrate 

 as the source of nitrogen, the medium became stale less quickly, 

 the XH S being used as the source of nitrogen and the tartrate radi- 

 cal as the source of carbon. When peptone was used, the media 



