RESISTANCE OF WHITE PINES IN EASTERN NORTH AMERICA 259 



Repeatedly, materials that remained healthy in the inoculation beds 

 have shown blister rust infection in the nursery and in the test areas. 

 This is especially the case with P. strobus and its hybrids with other 

 species. 



P. peuoe Griseb. presents one of the simplest cases of rust resis- 

 tance. The results of our tests are presented in Table 1. There are 

 four populations, raised from seeds obtained from Finland, Yugoslavia, 

 Bulgaria and Hungary. All populations contain some susceptible seedlings 

 2 to 6 years after inoculation. The population from Finland shows the 

 largest proportion of resistant seedlings, 87%, and remains stable there- 

 after. The other populations show much lower proportions of resistant 

 seedlings (19%, 33% and 47%). It is perhaps significant that the popula- 

 tion from Finland, raised from seeds obtained from the arboretum Mustila, 

 where blister rust is known to be present, shows the highest proportion 

 of resistant seedlings and remains stable. Is this the result of natural 

 selection for resistance after one generation of continued attack? 



P. peuoe x P. strobus . --The results of crossing 27 P. peuoe clones 

 with one "wild" P. strobus, the grafts of which were found susceptible to 

 blister rust, are shown in Table 2. Only eight seedlings out of 278 were 

 blister rust free 2 to 5 years after inoculation. Moreover, 6 of the 8 

 healthy seedlings later died of unknown causes--possibly due to blister 

 rust . 



The results of crossing 12 P. peuoe clones with another "wild" P. 

 strobus, the grafts of which remained resistant in our tests, are presented 

 in Table 3. Seven of the resulting hybrid progenies remain stable after 

 2 to 3 years of inoculation, while 5 continued to segregate susceptible 

 seedlings up to 8 years thereafter. The resistance varies from 3 to 42%. 



P. griffithii McClell. (syn. P. walliehiana A.B. Jacks . ) - -Tests 

 with seedlings of this species (Table 4) are confounded by the poor 

 climatic adaptation of most strains to our growing conditions and the 

 resulting great losses among the seedlings due to causes other than blister 

 rust. Out of 125 seedlings of a provenance from E. Punjab (WP 78), one 

 resistant seedling was obtained from among 3 survivors after 6 years of 

 testing. Another population, from an elevation of over 11,000 feet in 

 W. Pakistan, resulted in 31 resistant grafts among 32 survivors out of 

 105 original grafts. The scions for this population were collected, one 

 from each plant, from young natural regeneration and subsequently grafted 

 at Maple. In the 5 populations tested, resistance to blister rust varied 

 from 23% to 97%, four populations remaining stable 3 to 7 years after 

 inoculation, while one population continues to segregate susceptible 

 seedlings. Unfortunately, the population with the highest resistance is 

 a high-elevation type of poor growth form and growth rate. Several 

 grafted clones obtained from parks, arboreta and botanical gardens are 

 reasonably well adapted and flower profusely. It is possible that progeny 

 tests with these, now in progress, will identify some good resistance 

 transmitters for future breeding work. 



P. griffithii x P. strobus . --The results of tests with these hybrids 

 are shown in Table 5. All seedling populations sufficiently tested thus 

 far are results of natural hybridization of P. griffithii with P. strobus. 

 Of the 8 populations tested, 4 have been eliminated entirely, mostly by 

 blister rust. The remaining 4 populations show resistance ranging from 

 15% to 46%, i.e., in some cases higher than in P. griffithii. It is 

 possible that their female parents have been subjected to some natural 



