394 BROOKLYN BOTANIC GARDEN MEMOIRS 



From this data it is evident that Bromus Krausei and B. pendulinus 

 are susceptible to the rust on both B. mollis and B. sterilis. From the 

 standpoint of bridging hosts, however, what we need to know is the 

 infecting capacity of uredospores from B. Krausei and B. pendulinus, 

 produced by inoculation with uredospores from B. mollis and B. sterilis. 

 It may well be that B. Krausei and B. pendulinus are merely hosts for 

 the rust on both B. mollis and B. sterilis. At least the evidence is 

 not complete for proving that they are bridging hosts. 



Freeman and Johnson (57) conclude that barley is a bridging host 

 enabling the specialized races of Puccinia graminis on wheat and rye 

 to infect oats. They find that the rust on wheat will not infect oats, 

 but will infect barley; the same is true of the rust on rye. When, 

 however, the rust on barley, produced by inoculation from either 

 wheat or rye is sown on oats, infection occurs to a very slight extent. 

 The data upon which this conclusion is based are as follows: (i) uredo- 

 spores from wheat to barley (26/31),^^ to barley (28/42), to barley 

 (16/16), to oats (2/54); (2) uredospores from rye to barley (23/31), 

 to oats (1/22). As noted before, the barely rust is able to infect all 

 four cereals, but rye and oats less completely than wheat and barley. 

 The indications are that wheat rust and rye rust, as a consequence of 

 growing on barley for one or more generations, are able to infect oats. 

 It is evident, however, that the data obtained are rather meager and 

 very much more extensive series of inoculations should be carried out. 



Evans (44) has crossed a wheat (Bob's Rust Proof) resistant to 

 rust {Puccinia graminis) with another wheat (Wol Koren) which is 

 highly susceptible. Evans found that the hybrid, although of more 

 vigorous growth than either parent, was much more severely attacked 

 by rust than the susceptible parent. It was also noted that in pot 

 cultures in the greenhouse the hybrid produced an abundance of 

 teleutospores, which rarely occurred on either parent under the same 

 conditions. 



Evans next tried to determine the infecting capacities of the rust 

 after it had developed on the hybrid. He found that the rust from 

 the hybrid infected the susceptible parent much more severely than 

 the rust originally found on it. Not only that but the rust on the 

 hybrid readily attacked the resistant parent. Accordingly it is 

 suggested that hybrid plants may play an important part in the trans- 

 mission of parasites from susceptible to resistant varieties by increasing 

 the virulence of the parasite. 



Stakman and Piemeisel (149) record many grasses SiS hosts for 

 more than one race of Puccinia graminis. In fact Bromus tectorum, 



" The denominator of the fraction indicates the number of leaves inoculated 

 and the numerator the number infected. 



