44 



THE RUSTS OF GRAIN'S IN THE UNITED STATES. 



Table III. — Summary of experim£nts to determine the vitality of successive uredo genera- 

 tions of various grain rusts — Continued. 



t>UCCINIA GBAMINIS HORDEI ON WHEAT FROM BARLEY— Continued. 



'Series sent from Washington, D. C, to Minneapolis, Minn. 



2 Not recorded. 



3 Experiment discontinued. 



The lowered percentage of successful infections in July and August 

 of 1907 and 1908 is noticeable and was due to the e.xtreme heat in 

 the greenhouses at the time of inoculation. The uredospore germi- 

 nates either not at all or not nearly so well at the excessive tem])era- 

 tures of 90° to 100° F. and over, which then existed during parts of 

 each day, as it does in more moderate temperatures, 55° to 75° F.; 

 the germ tubes are injured and the host plants themselves become 

 drawn and weak, reducing the chances for infection very markedly. 

 Puccinia coronata, however, is noticeably resistant to heat and 

 P. ruhigo-vera on wheat is a close second, while P. gramims on oats 

 is injured (juickly and P. graminis on rye is killed by excessive tem- 

 peratures. 



216 



