TEMPERATURE 863 



great; Phoma apiicola is an extreme with mycelial growth on agar at 

 5-28°C but pycnidium formation only at 13-26° (25). 



The optimum temperature for sporulation of a given species may 

 be either close to the growth optimum, as in Piricularia oryzae (149) 

 and Rhizopus spp. (303), or rather different, lower in Fusarium con- 

 glutinans (287) and higher in Sphaerotheca pannosa (194). 



Different spore forms of a given species often differ in their tempera- 

 ture requirements. Thus, the zygotes of Sporodinia grandis and of 

 Saprolegnia mixta form at lower temperatures than do the correspond- 

 ing asexual sporangia (172, 173). Perithecia of Aspergillus spp. (171, 

 256) and of Ceratostomella (Ophiostoma) fimbriata (17) are produced 

 in abundance at relatively high temperature and are replaced by co- 

 nidia at lower. Pseudopeziza ribis forms conidia at 20-24°C, micro- 

 conidia at lower temperatures (33). 



It must be borne in mind that most of these data are only semi- 

 quantitative, and that the same uncertainties are involved in the defi- 

 nition of an optimum for reproduction as in the definition of a growth 

 optimum (Chapter 1). One such complication is illustrated by the 

 findings of Robbins and Schmitt (245) on Phycomyces blakesleeanus: 

 the failure of progametes to form at 26 °C is the result of the accumula- 

 tion of acid in the medium before the two strains meet. Presumably, 

 progametes would form at 26° if the pH were controlled. 



One may speculate that sexual stages adapted to survive a cold win- 

 ter are formed prefentially at low temperature, and that fungi which 

 must live through a hot and dry season will tend to form their dormant 

 spores at higher temperature. However, there is as yet no ecological 

 or physiological data bearing on this possibility, and the failure of 

 sexual reproduction in Neurospora crassa at high temperature (305) 

 speaks against the idea. The asci of Erysiphe graminis mature some- 

 what more rapidly if chilled briefly, but they do mature without cold 

 treatment (114). The apothecia of Claviceps purpurea appear, from 

 limited data, to develop only if the sclerotia bearing them are exposed 

 to low temperature (170). Zygospore formation in Thamnidium spp. 

 is also best at low temperature (150). 



Temperature-induced zonation has been considered elsewhere 

 with the conclusion that the positive effect of high or low tem- 

 perature on spore formation is probably effected by retardation of 

 mycelial growth. 



Morphological effects of temperature are easily observed; as exam- 

 ples we may cite the finding that the conidia of Cercospora sesami are 

 larger at an intermediate temperature than at higher or lower tem- 



