pH AND GROWTH 21 



in Table 3 reflect the ability of the fungus to raise or lower the pH 

 of an initially unfavorable medium; data on Colletotrichum linde- 

 muthianum (173) illustrate this phenomenon. 



The values of Table 3 should, therefore, be viewed with some cau- 

 tion; at best they offer a rough guide to choice of media and a possi- 

 ble indication of ecological preferences. They are not to be regarded 

 as absolute figures in any sense. 



A double pH optimum is frequently reported, a minimum appear- 

 ing between two maxima in the curve. This type of response has 

 been exhaustively analyzed in studies on Coprinus spp. (102, 104). 

 It appears that the minimum merely reflects pH-dependent unavail- 

 ability of one or more inorganic elements; provision of iron, zinc, and 

 calcium in available forms eliminates the double optimum and re- 

 places it with a broad single optimum zone. 



An organism may also be able to grow, even quite well, at a pH 

 which does not permit the initiation of growth (189). The metabolic 

 basis of this phenomenon is not clear; again, the artificialitv of the 

 simple pH-growth curve is emphasized. 



In contrast to the bacteria and the actinomycetes, fungi are rela- 

 tively more able to invade acid environments, although we should 

 avoid the common assumption that they are restricted to such environ- 

 ments. In culture, the larger basidiomycetes are often unable to grow 

 at an initial pH above 7.0; examples include most species of Ma- 

 rasmius (182), and Tricholoma (222). Correspondingly, limited 

 ecological studies of these fungi indicate that they are in general 

 restricted to acid environments (338). However, Marasmius rotula 

 grows in culture and is found in nature on substrates of widely dif- 

 ferent pH (182, 338), and several other fleshy basidiomycetes, notably 

 species of Coprinus, actually require alkaline conditions for best 

 growth (104, 120, 312). 



Biological types of fungi may only roughly be characterized as to 

 pH preference. Most plant pathogens — exceptions are evident in 

 Table 3 — grow best in media with an initial pH of 5.0 to 6.5. Species 

 of Streptomyces are marked by a relative intolerance of acidity; very 

 few can grow at a pH lower than 5.0. The same intolerance of 

 acidity appears also in other groups, e.g., the lower phycomycetes 

 (61, 85). The dermatophytic fungi, perhaps because they are usually 

 grown in complex media, seem rather indifferent to pH over the 

 range 4.0 to 10.0 (284, 285). 



Other biological activities of the fungi respond to pH, but not 

 necessarily in the same way as growth. As a rule, the pH range for 

 sporulation is narrower than that for vegetative growth (Chapter 



