18 CULTIVATION AND GROWTH 



miscellaneous one, examples of which appear in Table 2; it includes 

 saprophytes — especially coprophilic forms — and several wood-destroy- 

 ing fungi. The second group, the true thermophiles, is defined by the 

 ability to grow at 50°C or higher and the inability to grow at tempera- 

 tures below about 30°. Relatively few such fungi are known; ex- 

 amples include Thermomyces lanuginosus (Monotospora lanuginosa) 

 (135a, 313), Thermoascus aurantiacus (201), Penicillium duponti 

 (240), and Chactomium thermophile (169). No study of the funda- 

 mental basis of this behavior in fungi has appeared. 



Truly thermophilic forms also are found among the actinomycetes, 

 reviewed by Bernstein and Morton (18). The obligately thermophilic 

 Micromonospora vulgaris is distinguished by the high resistance of its 

 conidia to heat damage (87). Both fungi and actinomycetes are active 

 in decomposition of compost at 50°C (320). 



Killing of fungi by high temperature is considered elsewhere (Chap- 

 ter 13); spores are the usual experimental material. It need only be 

 noted here that mycelium is easily killed by elevated temperatures and 

 that many fungi die slowly when held in culture at a temperature just 

 above the maximum for growth. 



Fluctuations in temperature induce two types of zonation, the more 

 common of which results from the presence of zones of reproductive 

 structures (Chapter 11). In a second type of temperature-induced 

 zonation the zones are entirely mycelial; this phenomenon occurs in 

 Cercospora beticola (60) and in other fungi (20, 124). 



The data compiled by Togashi (307) indicate that the optimum 

 temperature for spore germination is generally very close to that for 

 growth in culture. There are instances, however, in which spore 

 germination is less inhibited than growth by supraoptimal tempera- 

 tures (185, 330). Spore germination at temperatures too low for 

 growth has been reported in Rhizopus spp. (330), and is presumably 

 common. 



The relation between a fungus-incited plant disease and temper- 

 ature is rather more complex, inasmuch as at least two organisms are 

 necessarily involved. In general, storage and transit rot diseases are 

 most severe at the temperature which favors mycelial growth of the 

 pathogen; examples include the decay of sweet potato by Rhizopus 

 spp. (172, 330) and storage rots caused by Rhizoctonia sola?ii (170, 

 332). Typically, however, the pathogen will grow in culture at tem- 

 peratures below the minimum for infection (33). 



The vascular wilts of plants caused by Fusarium spp. often have 

 the same pattern, i.e., the disease optimum is close to that of the 



