480 E. P. VAN ARSDEL 



I think I finally got the message across to this reporter. However, 

 all of us working in the various phases of rust-spread research must keep 

 the difference between genetic resistance and climatic escape firmly in 

 mind. I, working in my environmental studies, must be aware of the varia- 

 tions in genetic susceptibility; you, in selecting trees resistant to the 

 rust fungus, must be aware of the importance of microclimatic variations 

 in small distances on the ability of the fungus to infect a particular 

 host plant. 



Considering genetics, most of the evidence seems to indicate less 

 variation in pathogenicity in the fungus than there is variation in 

 susceptibility and resistance in the host plants. However, this varia- 

 tion in host genetics affects the pathogenicity of the fungus. For an 

 example from my own work, in an isolate of race 1 of the rice blast 

 fungus (Piruoularia orizae Cav.), infection occurred in 6 hours at 22°C 

 on Zenith rice and in 6 hours at 20°C on C.I. 8970, an especially 

 susceptible variety (Green, 1958; Green and Van Arsdel, 1956). This 

 temperature difference was enough to affect epidemic development in a 

 major way in Florida. Thus, the minimum environmental requirements for 

 infection varied with the genetic susceptibility of the host. 



While there are not similar data available on rust infection where 

 one research worker has compared the same isolate on pines of different 

 susceptibility, the published reports on white pine blister rust seem to 

 indicate faster penetration and establishment on sugar pine (Pinus 

 lambertiana Dougl.) than on eastern white pine (P. strobus L.). With 

 these bits of evidence in mind, I think we have to assume that minimum 

 environmental requirements for infection can vary from host to host with 

 their genetic susceptibility, and that environmental and genetic effects 

 are always closely linked. 



However, Patton's work on selecting resistant trees and mine on 

 microclimate, in the same region on the same rust on the same host 

 species (P. strobus) , indicate that genetic and microclimatically con- 

 trolled factors of susceptiblity and pathogenicity can be effectively and 

 profitably separated. Microclimates do affect the amount of host infec- 

 tion, regardless of the overall susceptibility of a species to a rust: 

 a tree in a moist site should always become infected more readily than 

 its adjacent neighbors in drier sites. When selecting resistant candi 

 dates you should be sure you have not selected an equally susceptible 

 host located in a microclimate unfavorable to infection. 



While most of my examples will come from work on white pine blister 

 rust, I shall give sufficient examples to show that fusiform rust infec- 

 tion is much more prevalent on slash pines in moist sites than on drier 

 sites in the same pine stand. 



LOCAL VARIATION IN BLISTER RUST INCIDENCE 



We can look at any mass of infection incidence data and make a 

 pretty good estimate of how much climatic variation is occurring in the 

 infection process. As an example we can take the masses of infection 

 data collected in the Lake States by E. E. Honey, H. N. Putnam, Ray 

 Weber and others that I presented at the First International Phyto- 

 pathology Congress in London last year. 



