242 



Rust in Wheat. 



[JULY, 



farming is now turning its attention to methods of coping with 

 these pests, and where methods of doing so have been devised 

 their application has become part of the routine work of 

 the farm. The discovery of preventive or curative measures 

 generally demands an exact knowledge of the life histories 

 of the pests, and the study of parasitism is of too recent a 

 growth for many to have been investigated in sufficient detail. 

 In consequence there are still numbers of fungi whose ravages 

 are unchecked year by year simply because we have no means 

 of attacking them. Foremost amongst these are the rusts 

 of the cereals. The necessity for investigating this group 

 of parasites and determining what means can be adopted for 

 dealing with them becomes evident when one knows that 

 Prussia alone lost some £20,000,000 in one year through their 

 attacks. 



We have, in this country, no reliable statistics of the losses 

 due to the cereal rusts. In fact one cannot help thinking that 

 many farmers look upon the parasite as part of the plant itself, 

 for otherwise it is difficult to explain the fact that in a series 

 of returns obtained from all parts of the country with the 

 object of obtaining such statistics many stated that they had 

 not seen rust or mildew on their crops. 



The investigations of Eriksson, Marshall Ward and others 

 during the last decade have added greatly to our knowledge 

 of the life history of these parasites and shown that the problem 

 of dealing with them is not so hopeless as it was considered to 

 be a few years previously. Eriksson's work in particular has 

 brought out a fact of fundamental importance. He has 

 proved that such a rust as Puccinia graminis, the black rust 

 of wheat, though occurring on a large number of the wild- 

 growing grasses cannot pass from these to wheat. If spores 

 are taken from wheat they fail to bring about disease if applied 

 say, to cocksfoot, and vice versa. This same specialization of 

 the fungus to particular host plants is found amongst other 

 species of rusts. The common yellow rust of wheat, Puccinia 

 glumarum, though occurring on wheat and barley cannot pass 

 from the one host to the other. There is one " variety "of it 

 which has specialized for wheat and another for barley, and 

 though identical in all external respects there is some physio- 

 logical difference between them which determines which host 



