ENVIRONMENT IN RELATION TO WHITE PINE BLISTER RUST INFECTION 



E. P. Van Arsdel 

 Texas A&M University, College Station, Texas, U.S.A. 



ABSTRACT 



Pine trees can be free of blister rust infection either because 

 they are growing in a climate unfavorable to rust or because 

 they are genetically resistant to the rust. The climatic escape 

 is hundreds of times more common than genetic resistance in the 

 American white pines. The minimum time and temperature required 

 for penetration by an isolate of the rice blast fungus {Vivicularia 

 ovizae) differed significantly from one rice variety to another. 

 This illustrates an interrelationship between environmental 

 influences and genetic susceptibility. In the pine rusts, 

 the minimum conditions for infection might, for example, be 

 less limiting in sugar pine {Finns lambertiana~) than eastern 

 white pine {Finns strobus) . As an example of local variation 

 in blister rust incidence due to environmental differences, 29 

 Lake States plots with a median of 250 trees each were used. 

 These were regularly examined for rust incidence for periods 

 exceeding 10 years. In the 4 years prior to alternate host 

 {Ribes spp.) eradication, the infection incidence on the 29 

 plots varied from to 118 cankers/100 trees/yr. The mean was 

 67 cankers/100 trees. After eradication the variation was 

 reduced and the infection incidence averaged 1.34 cankers/100 

 trees. In warmer zones, white pine blister rust is largely con- 

 fined to locally cool, wet openings in the forest and at the 

 bases of slopes. In cool zones the rust is more abundant when 

 the pines are open to the sky in small openings. Trees escaping 

 infection are usually under trees, in large openings, and where 

 sea breezes carry the spores out over the water. Fusiform rust 

 on slash pine was also favored in small openings and was rarer 

 under overstories . It was rarest in large openings. Small 

 openings are those that have diameters less than the height of 

 the surrounding trees . 



While I was at the University of Wisconsin in 1954, a reporter from 

 the Milwaukee Journal called me and asked me about all those blister rust 

 resistant white pines I had found in southern Wisconsin. He thought it 

 was wonderful that while Patton and Riker had found a few dozen resistant 

 trees, I had found hundreds without rust. I tried to explain to him the 

 difference between climatic escape and resistance. I was finding trees 

 that had escaped from blister rust because the trees were growing in a 

 climate unfavorable to blister rust, while Patton was finding trees that 

 were genetically resistant to the rust. 



479 



