380 PHYSIOLOGY OF THE FUNGI 



The toxicity of the metabohc products of Fusarium vasinfectum was 

 found to be dependent upon the medium on which the fungus was cultured 

 (Rosen, 192G). Filtrates of cultures grown in a medium containing 

 potassium nitrate and sucrose were highly toxic to cotton plants, while 

 filtrates from cultures grown in a medium containing ammonium lactate, 

 sodium asparaginate, and glycerin were not toxic. The filtrate of the 

 nitrate-sucrose medium contained nitrites. Solutions of chemically pure 

 sodium nitrite were also decidedly toxic to cotton plants. We may 

 assume that the action of this fungus in converting nitrates to nitrites 

 is the same within the host plant as it is in the culture vessel. 



Thus, there seems to be abundant evidence that the metabolic products, 

 including enzymes and toxins, of a given fungus vary both in kind and in 

 amount with the pH and composition of the culture medium. On the 

 other hand, the evidence that the same situation exists in nature is 

 extremely scarce. One may speculate, however, that the types of nutri- 

 ents furnished by the host cells and the pH of the cell sap may also influ- 

 ence the metabolic products of the fungus in the host plant. If this is 

 true, a given fungus may find the nutrients and environment supplied by 

 one host particularly favorable for the production of a disease-inducing 

 toxin or enzyme. If the host is unable to inhibit the action of these 

 substances, disease may result. The natures of both the pathogen and 

 the host determine the severity of the disease. This hypothesis may 

 help to explain, in part, the variation in intensity of parasitism of a 

 fungus on its different hosts. While there is little evidence to support 

 this idea at present, it is hoped that experimental work will be conducted 

 to test its merits. 



The possibility that the presence of vitamins may affect pathogenicity 

 has been suggested (Pehrson, 1948; Prasad, 1949). There is no evidence 

 that deficiencies for vitamins are correlated with either parasitism or 

 pathogenicity, and vitamin deficiency may be excluded as a factor leading 

 to the parasitic habit. Likewise, there seems to be little or no correlation 

 between the nitrogen requirements of fungi and the parasitic habit. 

 Nonliving organic materials in nature are sources of vitamins and organic 

 nitrogen just as are the living plants. For example, Ustilago striiformis, 

 a highly parasitic fungus, is self-sufficient with respect to vitamins, and 

 some isolates are capable of utilizing nitrate nitrogen, while Phycomyces 

 hlakesleeanus, an obligate saprophyte, is deficient for thiamine and is 

 unable to utilize nitrate nitrogen. 



Opposed to the destructive parasites discussed above are the balanced 

 parasites, which, in general, have a strong power to penetrate mechan- 

 ically but whose chemical actions on the host are relatively weak. Most 

 of the filamentous balanced parasites produce intercellular mycelium, 

 sending haustoria into the host cells. These serve as food-absorbing 



