•104 SPORE GERMINATION 



It is probable that only in rather unusual environments, e.g., very 

 acid soils, is pH a natural limiting factor for spore germination. Inso- 

 far as acidity and alkalinity are ecological influences, their effects on 

 mycelial growth and spore formation are likely to be more decisive. 



8. WATER AND GERMINATION 



Available water is one of the primary determinants of spore germina- 

 tion. Consequently, a considerable body of evidence has accumulated 

 on the moisture responses of economically important fungi; several 

 reviews of these data have been published (56, 71, 113, 256). The effect 

 of humidity on spore survival has already been considered. 



Certain problems of method have received less attention than they 

 deserve. Most investigators have reported data in terms of relative 

 humidity; although this practice is justifiable, a clearer picture of water 

 relations emerges when the vapor pressure deficit is taken as the cri- 

 terion (67, 262). The absolute humidity is not a useful value (224). 

 The importance of accurate temperature control, especially in attempts 

 to discriminate between free water and a saturated atmosphere with no 

 free water, has not always been realized. Both static and moving-air 

 systems for the simultaneous control of temperature and humidity have 

 been described (56, 57, 67); the static principle — incubation in a closed 

 system over a humidity-regulating solution — is both simpler and more 

 accurate. 



When germination is measured on solid substrates which take up 

 water — agar, grain, textiles, etc. — the minimum humidity permitting 

 germination is often lower than that in the absence of substrate (4, 22, 

 118). This, we may reasonably infer, means that the water-absorbing 

 capacity of these substrates is greater than that of the spore, but this 

 explanation begs the question of how the spore is able to draw water 

 from the substrate. 



The response to moisture is usually conditioned by temperature. 

 Thus, conidia of Uncinula necator are adversely affected by low humid- 

 ity at 28 °C or above, not at 25° or 20° (67). The optimum humidity 

 for germination of Aspergillus niger conidia is substantially higher at 

 40° than at 30°C (25). In Altemaria citri, both germination time and 

 germ tube length are less affected by relative humidity at the tempera- 

 ture optimum for germination than at non-optimal temperatures (286). 



Excluding the powdery mildew and the rust fungi, to be taken up 

 later, the fungi can be classified according to the minimum relative 

 humidity at which detectable germination occurs. Several, including 

 species of Aspergillus, Penicillium, and a few other genera, form spores 



