FURTHER DATA ON THE ORANGE-RUSTS OF RUBUS 



By L. O. KuNKEL * 

 Pathologist, Cotton, Truck, and Forage Crop Disease Investigations, Bureau of Plant 

 Industry, United States Department of Agriculture 



In 1 91 6 the writer showed that there exists in the United States two 

 orange-rusts on species of Rubus (i).^ Morphologically these rusts 

 closely resemble each other in their caeoma stages, but in the behavior 

 of the orange spores when germinated and in life cycle they were shown 

 to differ. During the last two seasons further observations were made 

 on the orange-rusts, and it is the object of the present paper to report 

 the results obtained in this study. 



Atkinson (2) has performed some experiments which to him seemed 

 to indicate that there is only one orange-rust on species of Rubus in the 

 United States. He admits that the orange spores show two distinct 

 methods of germination but attributes this to the influence of tempera- 

 ture. According to his view, promycelia are produced at high tem- 

 peratures and germ tubes at low temperatures. He suggests that this 

 may explain the behavior of the orange-rusts in different parts of the 

 country. In the north and in mountainous regions where the spring 

 temperatures are relatively low the aeciospores produce germ tubes, 

 while in southern sections of the country where temperatures are high 

 they produce promycelia. 



The writer (7) has previously rifeported the behavior of the aeciospores 

 of the two orange-rusts when germinated side by side at a room tem- 

 perature of about 25° C. This experiment seemed to show conclusively 

 that the aeciospores of the two rusts differ in manner of germination. 

 Nevertheless, in view of Atkinson's results some further germination 

 tests have been made. 



GYMNOCONIA INTERSTITIALIS ON BLACK RASPBERRY 



In the fall of 191 6 the writer collected the telia of Gymnoconia inter- 

 stitialis on leaves of wild black raspberry plants growing on the Virginia 

 side of the Potomac River near Washington, D. C. These telia showed 

 that the long-cycled rust is present in the locality just mentioned. Since 

 that time many collections of orange-rust have been made from both 

 wild and cultivated Rubus plants in the vicinity of the city of Washing- 

 ton. A study of these specimens has shown that the rust on the black 

 raspberry is always long-cycled while the rust on the blackberry and 



• The writer wishes to acknowledge here the help he has received from many colleagues who have 

 offered suggestions or have sent him living specimens of the rusts. 

 ' Reference is made by number (italic) to *' Literature cited," p. 512. 



Journal of Agricultural Research, Vol. XIX, No. 10 



Washington, D. C. Aug. 16, 1920 



Qt Key No. G-202 



