32 A STUDY IN CEREAL RUSTS 



from, a number of pustules were measured in determining the averages. 

 The fact that pustules are produced only with difficulty and that the 

 spores are smaller on resistant varieties would seem to indicate that 

 the fungus is not vigorous and cannot develop extensively although it 

 may gain entrance into the leaf tissues. 



COMPARATIVE VIRULENCE OF AECIDIAL AND LONG-TIME 

 UREDOSPORE INOCULATIONS ON RESISTANT FORMS 



Various observers have remarked on the reinvigorating power of the 

 aecidial stage of Puccinia graminis. Plowright (1882, p. 234) was of 

 the opinion that much more damage was done by aecidial infections 

 than by infection by uredospores which had been reproduced for a 

 number of successive generations. Bolley (1889, p. 13) states that the 

 aecidium, being a sexual product, should be considered as functionally 

 reinvigorating. He also reasserts this principle in a later work (1909, 

 p. 182). Arthur (1902, pp. 68 and 69 and 1903, p. 17) observes that 

 primary uredospores have a greater disturbing effect on the host than 

 do long-time uredospores. Freeman and Johnson, on the other hand, 

 cite experiments (1911, p. 33) to show that when the aecidial and 

 teleutospore stages were excluded for 52 generations the fungus still 

 retained its power of infection. The fact that sexuality in the rusts has 

 been definitely established would make it seem reasonable to suppose 

 that there would be a reinvigorating power. However, Barclay (1892, 

 pp. 8 and 40) states that in India there are no barberries for "enormous 

 distances" from fields of wheat in which Puccinia graminis is quite 

 destructive. McAlpine (1906, p. 58) points out that Puccinia graminis 

 probably causes no more damage in any country in the world than it 

 does in Australia where barberries are practically absent and aecidia 

 .have never been found. The rust is quite serious in South Africa, but, 

 according to Pole Evans (1911), the aecidtal stage is absent. 



Comparative trials were made with aecidiospores, primary ured- 

 ospores, and long-time. uredospores. The varieties used were not in 

 all cases the most resistant, since no seed of some of the more resist- 

 ant forms was available when the aecidia appeared. Trials were made 

 on Minnesota No. 163, Kubanka 1516, lumillo 1736, and einkorn 2433. 

 The two last-mentioned forms are fairly resistant. The long-time ured- 

 ospores used represented the twenty-fourth generation on wheat. A 

 number of trials were made and the results were not always uniform. 

 The incubation period of the fungus when developed from primary 

 uredospore or from aecidiospore inoculations was slightly longer than 

 when developed from long-time uredospores. The pustules developed 

 from long-time uredospores were apparently smaller and more numer- 

 ous, while those from aecidiospores and primary uredospores averaged 



