YELLOW RUST 67 



view we may adduce the fact that Eriksson could infect Berberis 

 vulgaris with teleutospores obtained from many grasses (Wheat, 

 Oat, Barley, Rye, Arrhenatherum elatius, Agropyron repens, 

 A. caninu/m, Dactylis glomerata, Agrostis stolonifera, Elymus 

 arenarius, Poa compressa, P. pratensis, Aira caespitosa, Bromus 

 secalinus, and many others, non-British) while Bolley was able 

 to infect a large number of the grasses with spores taken from 

 a single Barberry hedge. 



The economic importance of the matter lies in the fact that, 

 if the specialisation is as strict as Eriksson maintains, the corn- 

 crops cannot often (in the absence of Berberis) be infected by 

 rust on other cereals or on the wild grasses in the neighbouring- 

 hedges. This excessive strictness, however, no one else is 

 prepared to admit : in any case there are obviously plenty of 

 "bridging" species which would enable the rust to get at the 

 corn at the second step, if not at the first. The reason why we 

 cannot, so far, attribute any very great accuracy to the state- 

 ments regarding specialisation is that the conditions required 

 for infection are demonstrably very complex and at present ill- 

 understood, so that a negative result, even when repeatedly 

 occurring, often proves nothing whatever. This is manifest 

 from Eriksson's own complaints about the " capriciousness " of 

 the germination of the spores, and from the frequent recurrence 

 of such remarks as this — " Uredospores from Aira caespitosa 

 would not always infect Aira caespitosa." There is another 

 very important conclusion that can be drawn from this survey, 

 viz. that the life-histories of hetercecious rusts must always be 

 worked out separately for each country in the world. 



A second example of specialisation is given by the Yellow 

 or Golden Rust (P. glwmarum) : it is divided by Eriksson into 

 five biological races : 



1. f. Tritici — on Wheat alone 



2. f. Hordei — on Barley alone 



3. f. Secalis — on Rye (perhaps also on Wheat) 



4. f. Elymi — on Elymus arenarius alone 



5. f. Agropyri — on Agropyron repens, 



but of the last Eriksson remarks that he could not infect this 

 host with its own uredospores. With the exception mentioned, 



5—2 



