PATHOLOGY .AND GENETICS OF WHITE PINE BLISTER RUST 439 



Resistance in the bark is illustrated by recovery from established 

 stem infections. Struckmeyer and Riker (1951) described the anatomy of 

 wound periderm that served effectively as a barrier to prevent the fungus 

 from progressing further into the tissue and resulted in a final corking- 

 out of the infected area. Degrees of resistance were associated with 

 capacities of trees to differentiate a cork cambium. This type of resis- 

 tance is observed commonly in both eastern and western white pines. The 

 reaction was a commonly used criterion of resistance in testing parent 

 selection and progenies of eastern white pine in Wisconsin's program 

 (Patton and Riker, 1958; Patton and Riker, 1966) , and the same reaction 

 has been found in progenies from crosses in Heimburger's program in 

 Ontario (Boyer, 1964b) . The typical corking-out reaction has also been 

 reported for western white pine by Bingham, Squillace, and Wright (I960). 



An additional bark reaction was classified by Bingham et at. (1960) 

 as a hypersensitivity whereby a necrotic sunken lesion develops at the 

 base of an infected needle. This also is a common reaction of eastern 

 white pine. 



Hypersensitivity may also play a role in foliage resistance. According 

 to Hoff (1966), Boyer reported a hypersensitive reaction in the foliage of 

 a single Pinus peuoe Griseb. specimen. Hoff observed a similar type of 

 reaction in P. armandii Franch. needles, in which there was premature death 

 in the epidermis and mesophyll, and in P. monticota Dougl . , where there was 

 evidence of host cell death in association with host cell proliferation. 



Some or all of these types of resistance may be present in different 

 selections. A low frequency of needle lesions is one major measure of 

 resistance (Patton, 1967). When several other criteria, including that 

 of being completely disease free, were used in judging resistance in 

 progenies of crosses between resistant eastern white pine parents, the 

 percentage of resistant trees in progenies of better-than-average resis- 

 tance-transmitting selections ranged from 16 to 70%, compared with about 

 10% in the control group. Such criteria included needle spotting but no 

 subsequent canker development, needle death but no resulting stem infec- 

 tion, development of a necrotic sunken lesion around the needle base 

 (hypersensitive reaction in the bark) , and corking-out of small incipient 

 stem cankers with complete recovery (wound periderm formation) (Patton 

 and Riker, 1966) . 



NATURE AND MECHANISMS OF RESISTANCE 



Although the gross symptomatology of resistance reactions has been 

 described, little is known of the biochemical mechanisms involved in the 

 interaction between host and parasite in the expression of resistance 

 reactions. Some indications have been given of the possible association 

 of phenolics or growth regulators with resistance, but since this aspect 

 of resistance is covered in another paper in this panel no further details 

 will be given here. One area still deemed important for further study is 

 the resistance reaction expressed by inhibition of vescicle formation 

 through interaction of host and parasite at the guard cells of the 

 stomata (Patton, 1967) . 



The growth in culture of cambial explants infected with C. vibioota 

 (Harvey, 1967) suggests that axenic culture of the rust fungus may be a 

 technique that will prove useful in studying the nature of the host- 

 parasite relationship or the physiology and biochemistry of resistance, 

 and in screening selections and progenies for resistance. 



