50 BULLETIl^ 957, U. S. DEPARTMENT OF AGEICULTUEE. 



In many cases the petiole became diseased by growth of the 

 mycelium downward from the leaf blade into it, but direct infection 

 of the petiole occurs occasionally. This is shown by the presence 

 of infections on the petiole one-haK inch or more distant from other 

 infections. Microscopic examination by CoUey in such instances 

 has shown the intervening tissues to be entirely free from migrating 

 mycelium. York ^^ had several instances where infection took place 

 well down on the petiole, and no other infection was present either 

 on that petiole or the leaf blade. While many inoculations of petioles 

 have been made by members of the Office of Investigations in Forest 

 Pathology, but few have been successful, as above indicated. 



SORI ON STEMS. 



Evidence of infection of Ribes stems has long been sought. In 

 1917, Posey, Gravatt, and. Colley (112) discovered three uredinia 

 on yoimg stems of Ribes Jiirtellum which resulted from natural in- 

 fection in an outbreak area. Artificial inoculations on young stems 

 of the same species with seciospores produced 18 more uredinia. A 

 single stem infection was produced by Gravatt in the greenhouse 

 upon a young seedling of R. fasciculatum (PL V, fig, 2) . .^Eciospores 

 were used in this case also. While the tender stem was completely 

 girdled, it survived long enough to form wood and finally completely 

 outgrew the disease. Since then Taylor has successfully inoculated 

 with seciospores the stems of young seedlings of Rihes missouriense 

 and of R. americanum in the greenhouse. York has infected stems 

 of young R. glandulosum plants with geciospores and urediniospores 

 out of doors, and has found natural infections on the same species 

 and on R. cynosbati. He has infected a young stem of a 2-year-old 

 plant of R. cynosbati with seciospores in the greenhouse. 



RELATION OF STOMATA TO THE INFECTION OF RIBES. 



A number of investigators of the Uredinales have stated that 

 seciospore and urediniospore germ tubes obtain entrance to their 

 hosts through the stomata (34, 70, 110, 149, 171). 



As heretofore stated, Cronartium ribicola infects the Ribes plant 

 on the lower side of the leaf mostly. Less frequently it infects the 

 petioles, floral bracts, and cotyledons. It may infect young stems. 

 Infection never occurs on the upper surface of the leaf. Examina- 

 tion of a number of different species of Ribes has been made by mem- 

 bers of the Office of Investigations in Forest Pathology. Data on 

 the stomata may be summed up as follows : 



Stomata were present in large numbers on tKe lower surface of leaves of all species 

 examined. Stomata were present in small numbers on the upper surface of leaves of 

 Ribes cereum (60, 78), i?. inebrians (60, 7S),R. odoratum by Marshall, R. orientale (160), and 



31 York, H. H. Op. cit. 



