VAN ARSDEL 



favorable for rust, such small openings frequently contain only the 

 carcasses of dead trees. Other rusts, such as Coleosporiwn needle rust 

 on red pine, are concentrated in small openings. Similarly, in an East 

 Texas area on 35, l/100th-acre plots, the 14 in small openings averaged 

 77 fusiform rust cankers per 100 slash pine trees, the 2 in large openings 

 averaged 7 cankers per 100 trees, and the 12 under a pine overstory 

 averaged 12 cankers per 100 trees. All l/100th-acre plots had oak present. 

 This high incidence of rust on pines in small openings depends to a greater 

 extent on the high moisture than on the favorable temperature. On the 

 other hand, white pines under thin canopies such as aspen have very 

 little rust throughout the Lake States. 



These observations, coupled with tests where additional moisture 

 was added, indicate that condensed water on super-cooled leaves is most 

 favorable to rust infection. Whereas much has been written about the 

 relation of spore germination to relative humidity, few workers have con- 

 sidered what relative humidity might mean under a given set of field 

 conditions. Rogers (1957), in his work on snapdragon rust, did consider 

 moisture conditions other than humidity. He showed that under dew-forming 

 conditions when the leaf is colder than the air, the water formed on 

 leaves at 85% relative humidity. Wellington (1950) , in his studies of 

 the effects of radiation on the temperatures of insectan habitats showed 

 that leaves can be 10°C warmer or 2°C cooler than the surrounding air. 

 These observations show that the ability of the spores to infect the plant 

 depends not so much on the relative humidity of the air as on whether the 

 temperature of the leaf is below the dew point. Regardless of relative 

 humidity, when the temperature of the leaf is below the dew point of the 

 ambient air, there will be dew on the surfaces of the leaf and the 

 adhering spores. Except for short periods before the dew dries off in 

 the morning, when leaf temperature is above the dew point there will be 

 no free water on leaf undersurfaces , even in rain. In the general 

 forest, pines out from under the overstory crowns in the small openings 

 will have many times more rust than their tree-covered neighbors. 



Some interesting information that emphaszes these radiation- 

 temperature-moisture relationships was collected as a part of systemic 

 fungicide study. Data on the compass direction of the canker from the 

 center of the tree, collected for several thousand cankers, showed that 

 cankers were not distributed randomly. Most of them were on the west 

 side of the tree opposite the rising sun where dew could persist a little 

 longer. Cankers are also slightly more abundant on west slopes and to the 

 west of a radiation obstacle like a forest edge. 



AIR DRAINAGE --SLOPE EFFECTS 



Local topographic variation influences microclimate to favor or 

 hinder rust infection. Cliffs and steeper hills interrupt incoming 

 radiation in the manner previously described for trees. However, this 

 local radiation effect by hills is mostly a daytime phenomenon. More 

 important to the night-spreading blister rust fungus are the air-drainage 

 influences of these slopes. Cold air accumulates at the bases of hills 

 and flows away from the shoulders, making a warm slope at the top of 

 the hill and a cold pool at the base. Where the bases of two hills come 

 together in a narrow valley, a very cold pool accumulates (and often 

 starts a down-valley wind) . In southwestern Wisconsin where the general 

 climate is too warm for blister rust spread, rust infection is usually 

 present in these cold pools (Van Arsdel et at., 1961). 



