ENVIRONMENT .AND WHITE PINE BLISTER RUST INFECTION 491 



Van Arsdel, E. P., J. R. Parmeter, Jr., and A. J. Riker. 1957. Eleva- 

 tion effects on temperature and rainfall correlated with blister rust 

 distribution in southwestern Wisconsin. Phytopathology 47: 536. (Abstr.) 



Van Arsdel, E. P., A. J. Riker, T. F. Kouba, V. E. Suomi , and R. A. Bryson. 

 1961. The climatic distribution of blister rust on white pine in 

 Wisconsin. U.S. Dep. Agr. Forest Serv. , Lake States Forest Exp. Sta. 

 Paper 8". 34 p . 



Van Arsdel, E. P., A. J. Riker, and R. F. Patton. 1956. The effects of 

 temperature and moisture on the spread of white pine blister rust. 

 Phytopathology 46: 307-318. 



Van Arsdel, E. P., F. df. Stearns, and W. A. Main. 1968. Temperature in 

 a circular clearing in the forest. Bull. Amer. Meterol. Soc. 49: 

 301. (Abstr.) 



Wellington, W. G. 1950. Effects of radiation on the temperature of 

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FLOOR DISCUSSION 



KREBILL: The one thing that does concern me about your work is that 

 it seems to relate mostly to the clear weather situation while earlier 

 workers put more emphasis on the importance of precipitation. Would you 

 care to comment further about the importance of precipitation in dissemi- 

 nation of basidiospores of C. rnbicolal 



VAN ARSDEL: In the Lake States, you have to have precipitation 

 involved. You have to have a couple of wet days to get enough moisture, 

 but in a rainy, cloudy situation, you do not get condensation on the telia 

 on the bottom of the ribes leaf, therefore, it's not wet enough for release 

 of sporidia. For the release of sporidia, you have to put an agar plate 

 near the leaf and supply extra water or provide another source of moisture 

 in rainy weather. So the release actually occurs when it clears off; when 

 the leaves become cooler than the air and the water condenses on the under 

 side of the leaf. So it does occur in clear weather between rainy spells 

 at night rather than when it's actually raining. I put an awful lot of 

 observations into that, and this is my understanding of how it works now. 



ZUFA: You mentioned in conversation that nitrogen affected the 

 susceptibility to the rust. We have made a similar observation. Could 

 you add anything to these observations? 



VAN ARSDEL: Well, there is a lot of work on Pirioulavia and various 

 rusts. I didn't want to get into it too much. I haven't made a specific 

 review, but there is quite a bit of information available. Richard F. Watt 

 of the Forest Service North Central Forest Experiment Station found this 

 on his fertilizer plots with Chrysomyxa in black spruce in the Lake States. 

 Pure nitrogen does increase susceptibility, and this is something I have 

 just accepted from my years of work with the rust. It's sort of like 

 putting corn on a fertile field. It grows better than corn on a poor 

 field. The rust is an obligate parasite, and if you make the host a 

 little healthier, it grows better. 



ZUFA: I wonder if this really increases the susceptibility or if 

 the rust was already in the tree living with it in a kind of symbiosis, 

 and the nitrogen affected only the appearance of the blisters. We had 

 trees on which blister rust did not show up even after repeated inocula- 

 tions, but 5 to 6 years later when a nitrogen fertilizer was applied on 

 such trees the blisters suddenly appeared. 



