76 THE EUSTS OF GRAINS IN THE UNITED STATES. 



and adaptation to varying conditions a rust species widely distributed 

 may form a number of strains or types, differing in physiological 

 reactions; (9) the host plants exercise a strong influence not only on 

 the physiological and biological relationships but in some cases even 

 on the morphology of the uredospore. 



(4) Rust life histories were very incompletely understood up to 

 1864-65, when De Bary demonstrated the heteroecism of Puccinia 

 graminis Pers., but numerous citations in literature show that bar- 

 berries in proximity to grainfields had long been believed harmful. 

 From 1865 life-history work on the Uredinese has made rapid strides 

 and the relationships of many European and American forms of rusts, 

 particularly those of P. graminis, have been demonstrated. 



Whether or not the secidium is an essential stage in the life history 

 of rusts has long been questioned. Many authors believe it serves to 

 reinvigorate the fungus, and this view has been strengthened since the 

 recent discoveries of cell fusions and the origin of the binucleated 

 condition in the aecidium of various rust species. 



To test this invigoration theory continuous culture experiments 

 from the uredospore of six different grain rusts were undertaken by 

 the writers and 52 successive uredo generations of each rust grown 

 without the intervention of any other spore form. At the end of 

 these experiments cultures were as easily made and the rusts grew 

 as luxuriantly as at the first inoculation. For this length of time, 

 at least, there is no need for a sexual generation. 



(5) Whether or not rusts live over winter in the uredo stage has 

 been a mooted question. Investigators in Germany, Denmark, 

 Sweden, England, and the United States have investigated this 

 problem for different rusts with various results. In the United States 

 it has been demonstrated by several investigators that forms of 

 Puccinia graminis and P. ruhigo-vera live over winter in the uredo 

 stage. These results have been reenforced by experiments cited in 

 this bulletin, and the possibility of wintering of the uredo of several 

 rusts in the northern latitudes of the United States has been shown. 



(6) Rusts in the uredo or secidial stages are present in different 

 parts of the country at all times. Like dust particles, which have been 

 proved to be carried hundreds of miles by air currents, these rust 

 spores may be carried from regions where they are plentiful to regions 

 where grain is in a receptive condition. 



That large quantities of rust spores are present in the air at various 

 times has been proved by various investigators and by the writers. 



(7) A severe rust epidemic was prevalent in the important wheat 

 States of the Mississippi Valley in 1904. In an analysis of the 

 climatological conditions of this region for the years 1903, 1904, and 

 1905, during the critical or heading period of the grain and during 



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