V 
^50 BTJLLETIX 957, U. S. DEPARTMENT OF AGRICULTURE. <■ 
' In many cases the petiole became diseased by growth of the 
mycelium downward from the leaf blacle into it, but direct infection 
of the petiole occurs occasionally. This as shown by the presence 
of infections on the petiole one-half inch or more distant from other 
infections. Microscopic examination by Colley in such instances 
has shown the intervening tissues to be entirely free from migrating 
mycelium. York 31 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 OX STEMS. 
Evidence of infection of Ribes stems has long been sought. In 
1917, Posey, Gravatt, and Colley (112) discovered three uredinia 
on young stems of Ribes MrteUum which resulted from natural in- 
fection in an outbreak area. Artificial inoculations on young stems 
of the same species with aeciospores 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). zEciospores 
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 Tavlor has successfullv inoculated 
with aeciospores the stems of young seedlings of Ribes missouriense 
and of R. americanum in the greenhouse. York has infected stems 
of young R. glandulosum plants with aeciospores and uredinicspores 
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 aeciospores in the greenhouse. 
RELATION OF STOMATA TO THE INFECTION OF RIBES. 
A number of investigators of the Uredinales have stated that 
aeciospore 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 the 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, 78), R. odoratum by Marshall, R. orientale (160), and 
?! York, H. H. Op, eit, 
