36 WILLIAM Q. LOEGERING 



wheat and the two Ribes -alternating blister rusts of pine, we find two 

 aecial hosts and one uredial host involved in both cases. However, 

 taxonomists have separated two species of pathogens involved in the two 

 blister rusts but only one species in leaf rust of wheat. Yet in both 

 cases the variability in the host rpathogen relationship could be explained 

 on the basis of two corresponding gene pairs. There is no evidence that 

 this is true, primarily because of our inability to transmit the pycnial- 

 aecial stage from plant to plant. 



Obtaining direct evidence by the use of grafts or tissue-culture 

 inoculations may be possible. If so, the methodology suggested by Loegering 

 (1968) could be used to predict the presence of gene-for-gene relationships 

 in pycnial-aecial host :pathogen relationships. Moseman (1966) has reviewed 

 another approach to this problem. Work with barley mildew (Hordeum vulgare 

 L. -.Erysiphe graminis D.C. f.sp. hordei Em. Marchal) has demonstrated clearly 

 that the genetics of one member of the relationship can be determined by 

 study of the genetics of the other member. 



Nelson and Kline (1963) worked with leaf spot of corn [Zea mays L.: 

 Helminthosporiwn carbonian Ullstrup) and leaf blight of oats (Avenae sativa 

 L.:Helmintnosporiwn viotoviae Meehan 5 Murphy). Reaction to these pathogens 

 is controlled by one gene pair in each of the hosts; however, they cannot 

 be crossed to study the relationship of the 2 pairs of genes. H. cavbonwn 

 is avirulent on oats and H. victoriae is avirulent on corn. Nelson and 

 Kline were able to cross the 2 pathogens and obtained a segregation of 

 1:1:1:1 for haploid ascospore cultures with high pathogenicity to both 

 hosts, high pathogenicity to corn but low to oats, low pathogenicity to 

 corn but high to oats, and low pathogenicity to both hosts. From these 

 data we can conclude that the 2 gene pairs in the respective hosts are dif- 

 ferent. D'Oliveira (1966) attempted to do this type of experiment involving 

 the aecial host:P. vecondita Rob. ex Desm. f.sp. tritioi (Eriks.) Carl, 

 relationship, but he was unable to cross either the hosts or the pathogen 

 cultures involved. A similar situation exists with respect to the two 

 i?£&e-9-alternating blister rusts of pine. All attempts to cross pinyon 

 pines with 5-needle pines have failed (C. W. Busche, personal comrrrunioation) , 

 and I am unaware of any attempts to cross the two pathogens. Thus the 

 possibility of a gene-for-gene relationship involving these two pine rusts 

 has not been tested. 



Clearly defined evidence for or against the occurrence of a gene-for- 

 gene relationship between white pine and C. vibicola is not available. 

 Yet the illustrations indicate procedures and points of view which could 

 be useful in obtaining evidence. Such evidence, whether for or against 

 would be useful in attempts to control damage to pine by blister rust. 



Attempt has been made to acquaint the reader with the gene-for-gene 

 concept and point out what a powerful tool it is in studies of any host: 

 pathogen relationship. To utilize it to its maximum in studies of blister 

 rust, foresters and forest pathologists must consider the aegricorpus as 

 well as the fungus and the diseased tree. This approach could be extremely 

 useful even if the relationship of pine:C. ribicola is found to be 

 non-specific. 



