440 ROBERT F. PATTON 



THE GENETICS OF RESISTANCE AND PATHOGENICITY 



INHERITANCE OF RUST RESISTANCE 



The experience of workers with both eastern and western white pines 

 indicates that resistance is inherited in a polygenic manner (Bingham 

 et al. t 1960; Heimburger, 1962). From some of their earlier work, 

 Bingham et at. (1960) suggested that the heritability of rust resistance 

 may be fairly high. Their figures for broad sense heritability were 0.87 

 and for narrow sense heritability 0.69 (later reduced to 0.60 - Hoff, 

 1966) , and they estimated the genetic gain between F^ and F 2 generations 

 to be about 20 to 24%. Selection and screening procedures for both 

 parents and progenies of eastern and western white pines have proved 

 relatively efficient. But only about one in four tested selections of 

 western white pine exhibited general combining ability for seed- 

 transmission of resistance and about one in six of eastern white pines 

 (Bingham, 1966; Patton and Riker, 1966). Additional information on 

 heritablity of resistance in western white pine is included elsewhere in 

 these proceedings. 



The numbers and kinds of genes involved in white pine blister rust 

 resistance are not yet well known, but most information at present points 

 to the importance of genes with additive effects. Heimburger (1962) has 

 suggested that P. monticola is less polygenic than P. strobus L. and that 

 the former possesses a smaller number of resistance genes, each stronger 

 in action. In some crosses of P. wallichiana A.B. Jacks, (syn. P. 

 gviffithii McClell.) with selected P. strobus, high proportions of seed- 

 lings free of blister rust indicated that major genes might be influencing 

 resistance, probably through suppression of a (probably recessive) gene or 

 genes, inhibiting growth of the rust from needles to stem. Bingham (1969) 

 suggested that the association of specific resistance reactions witti 

 discrete resistance genes be a major topic for investigation in the near 

 future. Additional data on identification of particular resistance genes 

 are being prepared for publication in these proceedings or elsewhere. 



SEXUALITY OF C. RIBICOLA 



At the same time we are concerned with variability in the host, as 

 expressed by host resistance, we must also consider the possibility of 

 variability in the pathogen, particularly as expressed by variability in 

 pathogenicity. Such variability is based first upon mutation, and new 

 races commonly appear as the result of recombination of genes largely 

 through sexual processes (Walker, 1963). If virulence of C. vibioola 

 is subject to relatively rapid change by sexual means, it certainly 

 would be extremely important to take that into account in our resistance 

 breeding programs. 



The reproductive phenomena in C. ribicola are still incompletely 

 understood. Pierson (1933) reported a fusion of pycniospores with 

 certain filamentous hyphae in pycnia of the rust on western white pine. 

 Then Buller (1950) deduced that proper pycniospore exchange was necessary 

 for aeciospore production, that the rust was heterothallic. Hirt's 

 investigations, although they were still inconclusive (Hirt, 1964), 

 presented evidence that conflicted with this belief. A major objective 

 of Hirt's work was to determine whether exchange of pycnial exudate was 

 essential to the production of aecia. Experimental limitations and 

 certain assumptions necessary in his work allowed for the possibility of 

 errors in his conclusions. His study of the formation of the pycnium 



