INFLUENCE OF ENVIRONMENTAL FACTORS 269 



distinctive than morphologic differences between accepted spe- 

 cies in certain other genera. Similarly constant physico-chemical 

 differences amonsj funoi can be demonstrated to exist. Their 

 taxonomic value, like that of other bases, however, remains a 

 matter of dispute. 



Influence of environmental factors on physiologic species. 

 The pathogenic potentialities of fungi are modified by environ- 

 mental factors, as has been demonstrated to the satisfaction of 

 everyone who has worked with plant pathogens. There is evi- 

 dence also that the specialized races that together constitute a 

 species respond differently to a single factor. For example, 

 Waterhouse (1929) has shown that certain physiologic forms of 

 Fiiccinia graminis tritici, P. gramims avenue, P. triticina, and P. 

 simplex are pathogenic to a particular host variety in summer but 

 not in winter. Waterhouse reported that P. simplex was capable 

 of infecting 14 varieties of barley equally potently in winter and 

 in summer, but 8 varieties were resistant under winter weather 

 conditions and susceptible in summer. A similar response to 

 weather was noted by Peturson (1930) in P. coronata avenae. At 

 57° F Red Rust-proof oats were resistant, but at 70° F or higher 

 this variety was susceptible. Ruakura oats were resistant within 

 the range 57° to 77° F, whereas the varieties Green Mountain, 

 White Tartar, and Green Russian were susceptible within this 

 range. Susceptibility to form species 21 of P. gramims tritici was 

 dominant at high temperature in the cross between Marquillo and 

 Marquis wheat, but at low temperature resistance was dominant 

 [Harrington (1931)]. Presumably these effects of temperature, 

 representative of similar observations on other pathogenic fungi, 

 involve the metabolic activities of both interacting organisms and 

 are to be regarded as quantitative rather than qualitative. 



It is well known that certain diseases involve only mature plants 

 or plant parts, whereas others are limited to seedlings or to young 

 tissues. Fomes pini, for example, causes disease of mature conifers 

 and becomes a very important cause of decay in overmature stands. 

 Again, the leaves and fruits of grapefruit and orange are subject 

 to melanose, caused by Diaporthe citri, and to scab, caused by 

 Sphaceloma faivcetti, in the period of 4 to 6 weeks after the petals 

 have fallen but become highly resistant thereafter. The funda- 

 mental causes of differences between young and old tissues in 

 susceptibility to infection by fungi are little understood. Further- 



