WINTER kksistancp: of the ukedo. 19 



writi'i ill nine to twelve days. The incubation period is iniieli short- 

 ened 1)V usino- o-erniinatinj;- spores in the inoculations. In the month 

 of October, at Manhattan, Kans., an infection was produced in this 

 way in three days. 



It was attempted by numerous inoculations with the uredoform, 

 chielly at Washington, D. C, to transfer the rust from willow to Cot- 

 tonwood and the reverse, but always without success. An interesting 

 feature of theso experiments was the discovery of the fact that the 

 Cottonwood most common in Washington, known as South Carolina 

 poplar, could not be infected by the uredoform from the conuuon 

 Western cottonwood, though these two poplars are classed by some as 

 being the same species. Moreover, the rust does not occur in nature 

 on the South Carolina poplar, but is very abundant on the Western 

 cottonwood, and even occurs in Washington on the few indivitlual 

 trees of that type growing in the city. 



WINTER RESISTANCE OF THE UREDO. 



In another bulletin" the writer has given in detail the ol)servations 

 and culture experiments proving the successful wintering of the uredo 

 in the orange leaf rust of both wheat and rj^e. In this connection it 

 is easy to see a number of closely correlated facts, which may mutually 

 explain each other: (1) As shown under the preceding topic, the uredo 

 of black stem rust may infect a number of different hosts, and there- 

 fore has a manifoldly greater chance of propagation with the same 

 number of uredospores than if there were but one possible host; (2) as 

 also shown, the uredo of orange leaf rust is restricted in eyery case to 

 but one host, or at most to but one genus, and a much greater produc- 

 tion of uredospores is therefore necessary for the life of the species 

 in this stage; (3) as a corresponding matter of fact it is well known 

 that the uredo of the latter rust exists usually in very much larger 

 quantity than that of the former; (i) on the other hand, the teleuto- 

 sporic stage is the prevailing form of the stem rust, which fact makes 

 this rust usually the more damaging of the two, as the teleutospores 

 infest the steni chielly, thus more directly interfering in plant nutri- 

 tion; (5) the stem rust is proyed to be connected with the barberry 

 rust, thus giving it an additional chance for increased propagation, 

 and this through the medium of the teleuto stage instead of the uredo; 

 (6) finally, the uredo being the prevailing form of the leaf rust, and 

 no secidial form being known in this country,^ it would seem necessary 

 that this stage of that rust should be yery hardy in order to endure 

 extremes of cold and drought and preserve the life of the species. 

 Previous investigations of the writer and others have amply proved 

 that this is the case. In the meantime it is found that in other species 



«Cereal Rusts of the United States, Bui. 16, Div. Veg. Phys. and Path., U. S. 

 Dept. of Agriculture, pp. 21-23, and 44, 45. 



& Except in the case of the form on Elymus virginicus already mentioned. 



