WATER 405 



which germinate at relative humidities near 80 per cent (1, 25, 1 18, 132, 

 162, 244, 250, 320). A somewhat larger group of species germinate at 

 humidities of 90-95 per cent and do not require liquid water; exam- 

 ples include Altemaria brassicae, Colletotrichum spp. (51), Ustilago 

 spp. (56), Cladosporium spp. (51, 119, 224), Botrytis cinerea (224), 

 Fomes annosus (225), Septoria ap'ii, Fusanum spp., and Verticillium 

 albo-atrum (217). Still higher humidity is required by such fungi as 

 Venturia inaequalis (56) and Magnusia brachytricJia (277). Most of 

 or all the fungi mentioned in this paragraph germinate better at high 

 humidity than at low, regardless of the minimum value. 



Finally, the spores of many fungi are found to germinate only in 

 liquid water; in this group we may mention conidia and ascospores of 

 Endoconidiophora fagacearum (60), asexual spores of the Peronospora- 

 les (73, 141, 302, 308), and conidia of Schrotinia spp. (56, 73). 



Almost all studies of uredospore germination in rust fungi agree that 

 liquid water is required and that germination is nil even at 100 per 

 cent humidity in the absence of free water (1, 10, 57, 111, 129, 133, 134, 

 267, 308, 322). It is not always realized that temperature control must 

 be extremely accurate if a humidity of 100 per cent is to be maintained 

 without condensation of liquid water. For this reason, we must reject 

 those reports in which accurate temperature control is not specified and 

 germination in the absence of free water is claimed. This stricture does 

 not, however, apply to the work of Clayton (56), who found that uredo- 

 spores of Puccinia coronata and P. graminis germinate at 100 per cent 

 relative humidity, albeit less well than in the presence of free water. 



Aeciospores, teliospores, and sporidia of Gymnosporangium globo- 

 sum all require liquid water for germination (174). 



Alone among the fungi, the powdery mildew fungi form conidia 

 which in the experience of most workers germinate at very low relative 

 humidity, even over desiccants (33, 34, 48, 56, 67, 306). Exposure to 

 liquid water, in fact, inhibits germination and may cause death of the 

 spore by plasmoptysis (67, 167). As a result of these water relations, 

 the powdery mildew diseases are favored in nature by relatively dry 

 conditions (26, 48, 311). 



Yarwood has in a series of papers (306, 310, 312) provided evidence 

 that the ability of these spores to germinate at low humidity is a func- 

 tion of their high water content. Two different methods of determin- 

 ing water content agree in assigning a value of about 70 per cent water 

 to the conidia of Erysiphe polygoni. This is much higher than the 

 water content of other fungus spores. Correspondingly, although dif- 

 ficulties of measurement exist, the density of the spores of powdery 

 mildew conidia is less than that of most other spores. Finally, it has 



