WHITJ>PIXE BLISTER RUST. 47 



slowly at first, but later developed strong germ tubes in considerable 

 quantity. Scarcely a spore in water germinated. As a result of his 

 extensive experience with the rusts, Klebahn says: 



... in other words, I believe it possible for spores which do not germinate in 

 water to infect leaves of the host plant, and it seems to me to be expedient to distin- 

 guish between "infection power" and "viability" of spores more sharply than is 

 ordinarily done. 



Gravatt had experiences somewhat similar to the above in his in- 

 oculations with urediniospores of Cronartium rihicola in 1917. So 

 pronounced has been our general experience in this regard, that many 

 germination and longevity culture tests made in 1918 and 1919 were 

 duplicated by check inoculations on favorable hosts so far as possible. 



LOCATION OF THE INFECTIONS ON RIBES PLANTS. 



SORI ON THE LEAVES. 



The usual place for uredinia and telia of Cronartium rihicola to 

 form is on the lower side of the Ribes leaf blade. It is rather excep- 

 tional for them to appear elsewhere. Nevertheless, they are occa- 

 sionally found on the upper side of the leaf blade. They have been 

 noted there by Gravatt in the greenhouse and by several out-of- 

 door workers on Block Island. The following species have been seen 

 with uredinia or telia on the upper leaf surface: Rihes alpestre, R. 

 aureum, R. cereum, R. fasciculatum, R. fontenayense, R. hirtellum, 

 R. odoratum, and also the horticultural varieties R. aureum var. 

 Utah Yellow and R. vulgare var. White Imperial. It must not be 

 concluded that because fruiting bodies are found on the upper surface 

 of leaves that the infection took place there. On the contrary, in 

 every case seen, it was very evident that the fungus had attacked 

 the infected leaf beneath, and the attack had been so intensive that 

 some sori were pushed through to the upper surface. There never 

 were as many sori on the upper surface as there were on the lower 

 one, nor were they so old. 



Some inoculations have been made in Europe to determine if infec- 

 tion may take place on the upper surface of Ribes leaves. So far 

 as known to the writer they are here summarized : 



In 1913 Ewert (37) brought four potted plants of Rihes nigrum 

 into the greenhouse. On April 10 he inoculated the leaves of one 

 branch of plant 1 on the lower surface only with fresh seciospores. 

 The plant was inclosed in a glass cylinder as a moist chamber. 

 Another branch was used as a check. On April 28 the inoculated 

 branch bore uredinia upon 11 leaves. On April 15 he inoculated 

 the leaves of a third branch, but did not inclose it in a moist chamber. 

 On April 20 there was no sign of infection, and it was then inclosed 

 in a moist chamber. On May 20 all the inoculated leaves bore 

 uredinia. The control remained healthy. 



