GENETIC VARIATION IN RESISTANCE TO CRONARTIIJM AND 

 PERIDERMIUM RUSTS IN HARD PINES 



Bohun B. Kinloch, Jr. 

 Pacific Southwest Forest and Range Experiment Station, Forest Service 



U. S. Department of Agriculture 3 Berkeley 3 California 



ABSTRACT 



.Among the four major and closely related species of southern 

 pines, susceptibility to Cronartium fusiforme ranges from high 

 to near immunity, Kith interspecies hybrids exhibiting a 

 similar range. Inheritance of resistance in hybrid progenies 

 is apparently complex and dependent on the genotypes of individual 

 parents. Within species, a distinct pattern of racial variation 

 in resistance exists in loblolly pine {Pinus taedd) , but both 

 that species and slash pine (P. elliottii) show even greater 

 variation among individual parent trees. Heritability of resist- 

 ance is high and relatively stable to varying conditions of 

 rust hazard. 



Evidence of resistance to other pine stem rusts is limited but 

 similar. Racial variation in susceptibility to C. quercuum is 

 evident in jack pine (P. banksiana) , and both racial and indi- 

 vidual tree differences in susceptibility to Peridermium pini 

 are known in Scots pine (P. sylvestris) . 



Little is known of mechanisms of resistance to most of the pine 

 stem rusts, though earlier work on P. harknessii indicated a 

 physiological basis, depending on the rate of response of 

 cortical tissues to invasion by rust mycelium. The most resist- 

 ant reactions resulted from rapid necrosis of affected cells, 

 similar to hypersensitive type reactions of other plant rusts. 



Variation in pathogenicity on pines, one of the most critical 

 problems, has been found in P. pini and C. fusiforme, but 

 relatively few studies have been made. Potential variation 

 depends largely on the sexual behavior and extent of gene 

 exchange in the various rust populations. 



New approaches to the analysis of genetic interactions between 

 hosts and pathogens in wild, heterozygous populations are dis- 

 cussed in the context of the gene-for-gene theory and illustrated 

 by a case study of another tree rust. Through knowledge and 

 recognition of properties inherent in a complementary genie 

 system, analysis of this host-parasite combination was possible 

 in only one generation of breeding. The applicability of this 

 type of approach to pine rusts is discussed. 



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