58 J. C. ZADOKS 



4. Physiologic races characterized by differential genes for virulence 

 are not yet known in Cronartium ribicola (Ahlgren, 1968; Peterson 

 and Jewell, 1968) . 2 



From an evolutionary point of view, the stable ecosystem of western 

 white pine has been unbalanced by the introduction of blister rust. The 

 time needed to restore equilibrium is called "relaxation period". A 

 rough guess for this relaxation period is five to ten generations. In 

 evolutionary perspective the strategic problem is far from hopeless. 

 With the possible exception of the chestnut (Castanea dentata) destroyed 

 by the chestnut blight (Endothia parasitica) nature solves its own 

 problems through the action of quantitative genetics. But nobody can 

 wait for the natural solution which takes five to ten generations or 

 about half a millenium. 



MONOGENIC RESISTANCE 



Monogenic resistance tends to be self-destructive (Person, 1966). 

 The sequence of differential resistance over differential selection to 

 differential virulence which destroys the original resistance has the 

 character of a "boomerang effect". Wheat and potatoes can be harvested 

 before the boomerang hits, pine trees probably cannot. 



Wheat breeders tried many tricks to lessen the boomerang effect. 

 One is to incorporate several different monogenes for resistance in one 

 cultivar. The trick seems too laborious for application in tree 

 breeding; moreover it does not work too well. The pathogen can collect 

 the compatible set of virulence genes in one physiologic race. Among 

 the examples, too numerous for citation, is one of a tree rust, a leaf 

 rust of coffee {Hemileia vastatrix; Noronha-Wagner and Bettencourt, 

 1967) . 



A second trick, advocated by Borlaug (1959, 1965), is the composite 

 cultivar. Wheat lines are bred differing from each other in one mono- 

 gene for resistance but identical in all other respects. Seed from 

 different lines is mixed to sow a wheat crop that is uniform in agro- 

 nomical characters but composite with respect to resistance. The trick 

 uses a polycyclic effect and works well when a rust population builds up 

 gradually (Table 2). However, in western white pine blister rust poly- 

 cyclic effects are of little importance and the trick is spoiled where 

 presence of Ribes spp. permits an unrestricted multiplication of 

 inoculum. 



2 Editors note : Later in the symposium (of. Hoff and McDonald, these 

 proceedings) it was stated, on the basis of unpublished data, that at 

 least two pathogenic races of C. ribicola exist. 



