TEMPERATURE AND GROWTH 15 



parent optimum temperature for radial growth of Sclerotinia fructicola 

 is affected by pH (301). 



It is clear that temperature optima and ranges reported are valid 

 only under specified conditions of time, medium, and method of meas- 

 urement. There is no single temperature optimum of growth. Never- 

 theless, we shall continue to employ the concept of an optimum tem- 

 perature, on the grounds that it is empirically useful and that gross 

 differences in apparent optima provide a basis for ecological studies. 



A given metabolic process — respiration, antibiotic production, or 

 vitamin synthesis — does not, of course, necessarily respond to tem- 

 perature in the same way as does growth. Studies on penicillin produc- 

 tion in particular support this generalization (49, 227), as do studies of 

 bacterial growth and metabolism (78). 



Over the range in which growth is proportional to temperature, the 

 relation may be expressed by the value of Q 10 , defined as the ratio of 

 the growth rate at a given temperature to that at a temperature 10°C 

 lower. The values found by Fawcett (92) in studies on four fungi are 

 comparable to those reported for bacterial growth (298). 



The shape of the temperature-growth curve is somewhat affected by 

 the time of incubation. Growth at suboptimal temperatures is often so 

 slow that it is not apparent at all at a time when cultures at more 

 favorable temperatures are growing vigorously. Growth at supra- 

 optimal temperature may begin rapidly but slow down or cease en- 

 tirely after a period of time (34, 223, 278, 286). Both of these factors, 

 of course, affect the curve, so that use of only one incubation period 

 may introduce error. 



Most fungi make at least some growth over a 25- or 30-degree range 

 of temperatures, but narrower ranges are known, e.g., in Merulius 

 lacrymans (53) and Capnodium salicinum (98). Narrow ranges are 

 often associated with low optima, but the reverse is not true — many 

 fungi with low optima grow over a normally wide range of tempera- 

 ture. 



Temperature optima for plant pathogenic fungi have been collected 

 by Togashi (307) and are depicted graphically in Figure 6. Not all of 

 these reports can be accepted, and details of the distribution are unduly 

 affected by popular incubator temperatures, but it can be seen that 

 most members of this ecological group have optima in the region 

 20-30°C and about half have their optima between 26 and 30°C. 



The relation between temperature and habitat is not yet entirely 

 clear. Many fungi are probably world-wide in distribution, i.e., their 

 growth is limited by factors other than temperature. Species of Allo- 

 myces, on the other hand, are clearly confined naturally to the tropical 



