108 SPORE GERMINATION 



with a high optimum have in a sense solved the problem of developing 

 at high temperature without losing their capacity to germinate at low 

 temperature. Spore germination in the truly thermophilic fungi and 

 actinomycetes has not been studied quantitatively. 



Primarily from the data collected by Togashi (285), we can estimate 

 that the "average" optimum for plant pathogenic fungi is about 25°. 

 No similar estimate can be made for saprophytic fungi; probably the 

 figure is somewhat higher. 



Certain groups of fungi can be roughly characterized as a whole by 

 their temperature preferences. Thus, species of Aspergillus tend to 

 have high optima (21, 25), those of Peronospora to prefer rather low 

 temperatures (84, 285). Uredospores of most rust fungi germinate best 

 at 22° or lower (44, 57, 72, 183, 266, 268, 285, 322), although exceptions 

 to this rule have been reported (111). Limited data on Coprinus spp. 

 suggest a rather high optimum, 30-35° (146). 



Conidia of the powdery mildew fungi as a rule germinate best at a 

 moderately low temperature, about 21°. However, reported optima 

 for different species range from 6° to 28°, and within one species, Ery- 

 siphe cichoracearum, strains with optima of 15° and 28° occur (317). 

 Similarly, physiologic races of other parasitic fungi, e.g., Puccinia gluma- 

 rum (268) and Tilletia spp. (171), may differ considerably in their re- 

 sponses to temperature. Intrageneric variations in response are exem- 

 plified in Rhizopus: individual species have optima as low as 27° and 

 as high as 44° (298). 



Figure 7 depicts representative types of curves of total spore germi- 

 nation versus temperature. The most frequent shape of curve is 

 roughly symmetrical — or is slightly skewed to the right — with a wide 

 optimum zone; this curve is usually found in studies of the rust fungi 

 (44, 57, 266, 268, 269) and has been reported for such divergent types as 

 Fusarium lini (31), Ustilago spp. (16), Coccomyces hiemalis (148), and 

 several Hymenomycetes (260). An extreme example is found in the 

 response of conidia of Magnusia spp. (276). 



No one type of curve can, however, be said to characterize all fungi. 

 The two most common modifications are (1) curves which are skewed 

 to the right, like most mycelial growth curves (53, 115, 167, 317, 318), 

 and (2) curves with a sharp optimum rather than a broad optimal 

 zone (71, 93, 129, 143, 296). More rarely, a curve skewed to the left has 

 been reported (48, 84) from studies of fungi with low optimum tem- 

 peratures. 



The diversity of types of curve becomes less surprising when one 

 considers the number of factors that have been shown to affect the 

 shape of the curve. Of these, time of observation is the most impor- 



