TEMPERATURE 



409 



Temperature >■ 



Figure 7. Idealized types of temperature-response curves of spore germination. 

 The distinctive features are exaggerated for emphasis. Curve 1, symmetrical with a 

 broad optimum; curve 2, skewed to the right; curve 3, symmetrical with a sharp 

 optimum. 



tant: early observation yields a curve with a sharply defined optimum, 

 but in time the germination at less favorable temperatures catches up 

 and the optimum becomes much broader. This phenomenon appears 

 to be general (13, 23, 84, 166, 219, 241, 260, 299). 



Second in importance to the time effect as a factor in determining 

 the shape of the temperature curve is the general principle that the 

 limits of germination are narrower if some other factor is non-optimal. 

 Thus, conidia of Sclerotinia fructicola at an unfavorably acid pH ger- 

 minate only at 13-29°C and the curve is sharply peaked at 21°; at a 

 more favorable reaction, pH 2.4, both the total range and the optimum 

 are much broader (283). The depressant effect of low temperature on 

 Erysiphe gram in is germination is virtually abolished in the presence of 

 nutrients (317), and the minimum temperature of germination in Col- 

 letotrichum lagenarium is lower in a nutrient medium than in water 

 (101). The principle applies also to relative humidity, i.e., the tempera- 

 ture curve becomes narrower and sharper as the humidity becomes more 

 unfavorable to germination (118, 286). 



Although different spore forms of the same species usually have at 

 least similar temperature responses (148, 178, 285), the endoconidia 

 and macroconidia of Chalaropsis thielavioides show optima at 9-30° 



