384 ROBERT F. PATTON 



PATTON: But we have also observed the same phenomenon with grafts 

 from 4- up to 40- and 50-year-old susceptible trees. In our inoculation 

 tests, as trees from which we collected scions increased in age the number 

 of infections on the scions was reduced considerably. In other words, it 

 was much harder to get infection on a susceptible old tree than it was on 

 a susceptible young tree. I'm not saying that as a tree grows older it 

 cannot become infected; we know that in time a lot of these older trees 

 do become infected. 



MCDONALD: As I recall, however, in a table of your 1967, 14th IUFRO 

 Congress Paper 1 you showed only a small difference--of 0.6 substomatal 

 vesicles per cm of needle length in a 4-year-old seedlings and 0.4 in 

 40-year-old grafts. 



PATTON: Yes, I don't consider these differences particularly signifi- 

 cant. The main thing demonstrated by this table was the difference in 

 amount of infection we have always obtained between primary and secondary 

 needles. 2 Even though the difference between a secondary needle on a 

 5-year-old and a 40-year-old tree may not be shown by this table, there 

 does still appear to be some difference in susceptibility of the trees as 

 shown by incidence of stem cankers. 



ZUFA: I wish to describe a technique we use at our Research Station 

 in Ontario. We raise test seedlings in 3/4"x3" slit polystyrene tubes, 

 arranged with seedling families replicated and randomized in 200-tube 

 flats. Seed are sown in the greenhouse in January or February and moved, 

 through cold frames, to the outdoors. First-year tubed seedlings are 

 inoculated in late August using inoculation techniques developed by Dr. 

 Heimburger. Shortly after becoming dormant they are moved back into the 

 greenhouse. By the second January (seedling age 12 months) we are able 

 to detect infection with certainty, not only spots on the needles but 

 also of the blister rust mycelium in the seedling stem. Last spring we 

 classified the seedlings according to heavy, medium and light needle 

 infection classes, finding mycelium in all the heavy and medium class 

 seedlings but only in half of the light infection class seedlings. 

 Differences in infection of seedlings in different families were really 

 surprising. The worst family showed 96% heavy and medium infection, the 

 best only 23% heavy and medium infection. We think that on the basis of 

 such a finding we may be able to judge the transmitting ability of trees 

 even in such a short, 12-month-from-seed, period. Later after we moved 

 the 2nd-year seedlings out of the greenhouse, into the nursery, and back 

 into the greenhouse by the next January (24 months after seed germination) 

 aeciospores were produced on the seedlings. I wish to have your comments 

 on these findings. We infected the seedlings while still very young, but 

 they didn't die, at least not all of them. And we found quite surprising 

 differences between the families. 



1 Editor's note: See Table 1, Patton, R. F. 1967 3 preceding 

 Literature Cited section. 



2 Editor's note: The table being discussed shows 11.0 substomatal 

 vesicles per cm of needle length in 2- to 6-month-old primary needles vs. 

 0.6 in 4-year-old seedling secondary needles. 



