ENVIRONMENT AND WHITE PINE BLISTER RUST INFECTION 



485 



4- UP APGONNE . 1967 

 MAXIMUM TEMP. DEC C 

 ONE CLEAR DAY JUNE 



Figure 5. Temperature variations in a forest opening 280 ft. 

 in diameter in 70 ft. tall trees (4:1::D:H ratio). The 8°C 

 warming above the shade temperature would raise a July or 

 August day temperature of 27°C to 35°C--a temperature fatal 

 to blister rust. 



In smaller openings with a diameter half the height of the 

 surrounding trees, the opposite is true. In the Lake States the sun 

 never shines onto the ground in such an opening after August 10. Conse- 

 quently, the ground is always cooler during the day than on nearby open- 

 field or tree-covered sites. It is warmer at night because the closely 

 surrounding trees hold in the heat and impair outward radiation. The 

 plants growing in the opening are side-shaded from insolation and are 

 generally cooler than the surrounding air because of their radiant heat 

 loss to the open sky. This means that the cooled plants are often 

 cooler than the dew point, and dew periods are greatly extended. The 

 locally supercooled air also increases available moisture. Thus, the 

 small opening has a cool, very wet, local climate with a reduced tempera- 

 ture range that is extremely favorable to rust infections on ribes , 

 teliospore and sporidial production on ribes, and sporidial infection on 

 white pine. 



Except for small openings and a few other especially favorable sites 

 described later, rust is rarely present in Zone 1, Fig. 2. On pines in 

 small openings in the southern Lake States, rust is almost invariably 

 present. In the northern Lake States where the macroclimate is more 



