Teliospores appear on the old uredial infection usually after about 2 weeks of 

 cool weather. The dicaryotic nuclie fuse to become 2N and upon germination meiosis 

 occurs producing four IN basidiospores • These spores close the cycle by infecting 

 white pine. 



Germ tubes produced by germination of the basidiospore generally enter the pine 

 via stomata of the secondary needles. Less important infection courts are stomata of 

 the primary leaves and stomata of succulent stem tissue. 



In the secondary needles, the fungus produces a large mass of mycelium that 

 causes the cells of the pine needle to change color and this becomes visible as a 

 "needle spot." The fungus then grows toward the stem where it grows profusely within 

 cortex and phloem tissue. 



Often there is little or no growth impact on pine resulting from fungus growth. 

 The damage is the result of the killing of branches or the whole tree after the 

 fungus completely encircles the stem. Death probably results from disruption of the 

 phloem and cortical tissues as the fungus produces the aeciospore stage. 



RESISTANCE IN WESTERN WHITE PINE 



Mechanisms of resistance in western white pine are most easily described if 

 presented according to the infection and growth sequence of the fungus as it develops 

 starting in the secondary needles. 



The first mechanism is prevention of needle infection. In the past, trees with 

 no needle spots were thrown out as escapes. But some recent data (Hoff and others in 

 press) show that there is a continuous increase of this type from the most susceptible 

 white pine {Pinus ayacahuite) , with 1 percent uninfected seedlings, to the most 

 resistant (Pinus parviflora) , with 96 percent. In this same test, 24 percent of the 

 Pinus monticola seedlings were uninfected. However, the trait appears to be a 

 threshold character because it varies with the intensity of the inoculation. In 

 another test (Hoff and others 1973) the seedlings were 1 percent clean and F2 ' s 

 were 15 percent clean, indicating that a selection had been made for the trait. 



The frequency of needle spots also varies. There is a tenfold difference 

 between number of spots on seedlings of families with low and high numbers of spots 

 (table 1 and Hoff and McDonald in press) . The data resemble those that might be 

 produced by a single nondominant gene. Families can be grouped into classes (low, 

 medium, high), according to frequency of needle lesions. 



Needle spots also come in a variety of colors, shapes, and sizes. We have 

 published data and photographs of some (McDonald and Hoff 1975) . So far, we have 

 observed the following phenotypic variations in needle spots caused by the blister 

 rust fungus : 



1. Yellow needle spots of normal size; rust fungus with normal virulence. 



2. Yellow needle spots of normal size; rust fungus with very high virulence (observed 

 in Oregon, Region 6, USDA Forest Service, blister rust program). 



3. Yellow needle spots, very small size, 



4. Red needle spots, normal size; rust fungus with normal virulence. 



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