482 



E. P. VAN ARSDEL 



PREDICTION OF MICROCLIMATES FAVORABLE AND UNFAVORABLE TO RUST 



The white pine blister rust fungus (Cronartium vibioola J.C. Fisch, 

 ex Rabenh.) is sharply limited by meteorological factors in the eastern 

 United States (Charlton, 1963; Van Arsdel , Riker, and Patton, 1956). 



In the Lake States, climate largely determines the distribution of 

 rust. For convenience the disease distributions can be divided into 

 three scales according to meteorological conditions: (1) a macro scale 

 that is determined mainly by latitude and mass area elevation, (2) a 

 meso scale that is determined by elevation range of hills and river 

 valleys, and (3) a micro scale that is determined by the structure of 

 forest stands and the influence of small hills and slopes within the 

 stand (Van Arsdel, 1965a). 



In the large-scale climatic gradation, rust is more general and 

 found on white pines (P. strobus) on all sites in the more northern 

 regions, at higher elevations, and near cold bodies of water such as 

 Lake Superior (Van Arsdel, 1961, 1964). This is shown as zone 4 in 

 Fig. 2. 



Figure 2. Map showing differences in quantity of blister rust 

 spread to white pines in the Lake States. Rust in tops of 

 emergent pines carried from distant ribes are characteristic 

 of Zone 4. The cooler the summer weather, the more favorable 

 for disease spread. 



