STEM RUSTS OF CONIFERS AND THE BALANCE OF NATURE 527 



This disease cycle gives rise to a host-parasite system characterized 

 by sequential resistance. In other words, mechanisms for resistance can 

 be clearly separated by space and time. Such a situation provides an 

 opportunity for controlled manipulation of the disease cycle. The result 

 is to greatly simplify studies on factors for resistance and to make more 

 efficient the incorporation of several types of resistance into one 

 individual. For example, trees exhibiting resistance known to be based 

 in the leaf could be tested for resistance based in the stem by direct 

 inoculation of the stem with basidiospores , infected leaves, or by bark 

 patch grafting. 



RESISTANCE FACTORS IN THE WHITE PINES 



The published reports of the mechanisms or factors for resistance 

 were previously covered in this Symposium by Dr. Patton. A summary of 

 these factors follows: 



1. Lower needle lesion frequency on secondary needles in some P. 

 strobus individuals. This was also observed in P. monticola by Bingham 

 (1954) . 



2. Hypersensitivity in the foliage of P. ipeuce Griseb., P. armandii 

 Franchet, and P. monticola. 



3. Needle spots only. This includes individuals of P. strobus s 



P. monticola, and P. wallichiana A.B. Jacks, (syn. P. griffithii McClell.) 

 that had needles infected with blister rust that did not subsequently 

 produce cankers. 



4. Corking-out of established cankers by the formation of a wound 

 periderm. Observed in P. strobus and P. monticola. 



5. Bark hypersensitivity in which the fungus is killed before it 

 can enter the stem. This has been observed and reported in P. strobus 

 and P. monticola. 



At this Station, we have new data and have confirmed some older 

 findings on the resistance of western white pine to blister rust as 

 follows : 



1. There are at least two different pathogenic races of C. ribicola 

 with corresponding differential resistance genes in P. monticola. 



2. One host-parasite interaction is indicated by the formation of 

 a yellow needle lesion and host resistance in the secondary needles is 

 inherited as a single recessive gene. 



3. A second host-parasite interaction is indicated by the formation 

 of a red needle lesion and host resistance in the secondary needles is 

 inherited as a single dominant gene. 



4. A third host-parasite interaction is indicated by the formation 

 of a long yellow or yellow-red spot with green islands of epidermal 

 tissue . 



5. The needle lesion frequency trait may be controlled by a single 

 nondominant gene. This work did not take into account races and thus the 



