STEM RUSTS OF CONIFERS AND THE BALANCE OF NATURE 531 



If we can assume that C. vibicola is native to Asia, then popula- 

 tions of P. avmandii 3 P. w alii chi ana, P- kovaiensis Sieb. and Zucc, and 

 P. sibivioa Du Tour, all moderately to highly resistant to C. vibicola, 

 also appear to be excellent examples of balanced, native systems. 

 Spaulding (1929, 1956) concluded that the fungus was native to eastern 

 Siberia on the fairly resistant P. sibivioa; but from his data the origin 

 might well have been in the range of any of the even more resistant Asiatic 

 white pines (especially P. avmandii, P. kovaiensis, and P. wallichiana) . 

 Also, the high resistance of P. cerribva L. implicates the European Alps as 

 another possible gene-center for white pine blister rust (Schellenberg, 

 1904; GSumann, 1950; Fassi, 1960) but the published evidence assembled 

 by Spaudling (1929) indicates that the entry of the rust into the P. cerribva 

 stands of the Alps was the termination of the advance of the rust across 

 Europe . 



Good examples of imbalance are the North American white pines-C. 

 vibioola systems. These systems are obviously unbalanced in favor of the 

 fungus. If this condition existed for long and if native populations of 

 the white pines contained no resistance genes, the white pines would not 

 survive within the range of the fungus. A lack of resistance in the 

 American chestnut (Castanea dentata (Marsh.) Borkh.) to chestnut blight 

 (caused by Endothia pavasitiaa (Murr.) A. and A.) will likely result in 

 the extinction of the host species. 



BALANCED SYMBIOSIS 



If the natural host-parasite systems are considered on the basis of 

 population interaction, and if damage is measured by interference with 

 reproductive processes, then we may be able to define the concept of 

 natural systems in terms of an established concept. We suggest using 

 symbiosis, realizing that this concept is generally restricted to inter- 

 actions between individiauls . But for the lack of a more descriptive 

 term, we wish to take the literal meaning--living together of dissimilar 

 organisms--and extend it to include living together of populations of 

 dissimilar organisms. We can now describe the natural system as being in 

 a state of "balanced symbiosis" and define the two interacting species 

 as symbionts. 



Thus far we have considered only the biological balance of a system-- 

 that is, the ability of a species to survive. But man is primarily 

 interested in the economic aspects of this balance. Therefore, he is 

 concerned with loss in productivity expressed as direct mortality or cull, 

 For instance, the damage of C. fusifovme Hedge. § Hunt ex Cumm. on P. 

 taeda L. (loblolly pine) and P. elliottii Engelm. (slash pine) is not 

 biologically severe (the systems have survived) but the economic loss is 

 quite high. However, for many of the pine and indigenous rust systems 

 the economic loss is not great enough to justify a breeding program to 

 increase rust resistance even though the loss in a particular year may 

 be substantial (Peterson and Jewell, 1968; Fassi, 1960; Bakshi and Singh, 

 1967) . Obviously a system such as the North American white pines-blister 

 rust which is biologically out of balance in favor of the rust will show 

 high economic losses. 



