pH AND GROWTH 19 



pathogen (155, 156, 302, 322). However, it is not certain that the 

 relation is as simple as it appears, and in some wilt diseases tempera- 

 ture affects the host (100, 174) and, probably, other soil organisms 

 which influence the disease (264). 



For a few plant diseases it has been definitefy shown that differences 

 between temperature optima for disease and for growth of the patho- 

 gen occur and reflect the effect of temperature on the host. This has 

 been established particularly for the seedling blight of wheat and 

 corn caused by Gibberella saubinetii (72, 73) and a root rot of sweet 

 clover caused by Plenodomus meliloti (258), and has been suggested 

 for other diseases (114). 



Finally, there are some plant diseases in which the observed action 

 of temperature appears to be exerted on the pathogen, but on spore 

 germination or sporulation rather than on mycelial growth (95, 155, 

 156). It is conceivable, of course, that pathogenicity, as distinct from 

 growth, is affected by temperature, but so far this has not been demon- 

 strated. 



7. pH AND GROWTH 



Under given conditions, a fungus will grow maximally over a 

 certain range of initial pH values of the medium, and will fail to 

 grow at high and low extremes. However, it must be borne in mind 

 that pH is not a unitary factor; the mechanism of its action differs 

 at different concentrations of hydrogen ion. For example, one part 

 of the pH-growth curve may reflect the effect of a low pH on enzyme 

 systems, another the effect of a high pH on metal solubilities. Acidity 

 may also affect the entry of essential vitamins (341), surface metabolic 

 reactions, entry into the cell of organic acids, or the uptake of min- 

 erals (226). 



It is not surprising, therefore, to find that almost any factor in 

 the environment may change the shape of the pH-growth curve. Such 

 factors include temperature (232, 327), time of harvest (175, 210), 

 gross changes in the medium (187, 340), growth factor supply (178), 

 calcium and magnesium levels (314), and nitrogen source (140). 



Furthermore, pH is affected during growth by metabolic activities 

 — raised by absorption of anions or production of ammonia from 

 nitrogenous compounds, lowered by formation of organic acids or 

 absorption of cations. These effects of growth on pH complicate 

 results, particularly in the poorly buffered media commonly em- 

 ployed. It seems probable that many of the broad optima recorded 



