70 r 



1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 6500 

 ELEVATION, FT 



Figure 4 — Actual percentage infection plotted against elevations. Lines 

 A to D represent elevation dines for the geographic areas indicated in 

 figure 1. 



Table 3 — Simple correlations among population means for rust data and growth data 



Symptom of infection 



Three-year height 1 



Leaf length 2 



Percent dead 3 



Elong 4 



Start 5 



Infection 



-0.19* 



0.10 



0.12 



-0.11 



-0.07 



Resistant 



.19* 



-.10 



.12 



.11 



.07 



Categories of resistance: 













No galls or bark reaction 



.03 



-.03 



.24" 



-.16 



.03 



Bark reactions only 



.01 



-.09 



-.16 



.14 



-.02 



Gall death 



.04 



.21* 



.17 



.03 



-.01 



Trees with galls or bark reactions 



-.03 



.07 



-.25** 



.07 



.01 



Rate of gall appearance 



.06 



.15 







.05 



-.17 



Rust-killed trees 



.06 



.19* 



-.05 



.14 



.04 



'Significant at the 5 percent level of probability. 



"Significant at the 1 percent level of probability. 



'3-year height = heig ht in the field ( Rehfeldt 1 986) . 



^eaf length = length of leaf in the field (Rehfeldt 1986). 



3 Percent dead = trees that died in a drought test (Rehfeldt 1 986). 



*Elong = elongation of second year's growth in a greenhouse (Rehfeldt 1986). 



'Start = initiation of growth in a greenhouse = time to reach 2 mm (Rehfeldt 1986). 



Correlations of the rust data and growth characters 

 were all very low and most of them were nonsignificant 

 (table 3). The degree of infection was negatively corre- 

 lated with 3-year-old growth; gall death and rust-killed 

 trees were correlated with leaf length; and presence of 

 galls or bark reactions were negatively correlated with 

 percentage dead due to drought. 



DISCUSSION 



The general level of resistance of this collection of pon- 

 derosa pine populations to western gall rust was high; 

 nonetheless, there were several populations that were 

 highly susceptible. Three resistance categories were 



recognized. The first category of resistance was expressed 

 as trees that had no symptoms. Such trees had either 

 escaped inoculation or had defense mechanisms that 

 prevented the fungus from penetrating the cuticle or cell 

 walls of the epidermis or that confined the infection after 

 penetration, so as to not be easily visible. The second 

 category of resistance comprised trees with easily visible 

 stem reactions. Whatever caused these reactions, possi- 

 bly chemicals toxic to both host and fungus cells, resulted 

 in the death of the fungus. The third category of resis- 

 tance was expressed as typical galls at one of the first 

 three inspections, but that were dead at the fourth 

 inspection. This condition was signaled by a patch of 

 necrotic tissue surrounding and encompassing the gall. 



5 



