93 



are produced from this infection. The connection between 

 the fungus on the barberry and that on wheat was thus 

 established, and it was shown that, although the appear- 

 ances of the fungus on the two host plants differ so 

 materially, each represents a stage in the life history of 

 one fungus. 



Since De Bary's remarkable discovery very many 

 rust fungi, showing this type of life history, have been 

 studied. The rust which frequently appears on the leaves 

 of the pear, for example, is the aecidiospore stage of 

 Gymnosforangmm which produces swellings on the 

 branches of juniper, where the teleutospores arise. The 

 Blister-Rust of the Weymouth or white pine, a disease 

 which is very prevalent on the continent of Europe, is the 

 aecidiospore stage of a fungus w^hich forms its uredospores 

 ajid teleutospores on the leaves of currants. Similarly, 

 the Gooseberry rust is the cluster-cup stage of a fungus 

 which forms the uredospores and teleutospores on the 

 leaves of sedges. Many other similar cases occur as 

 diseases of cultivated plants and trees. 



When the facts became known for the wheat rust it 

 W'as thought that in order to eliminate the disease from a 

 given district all that was necessary was to destroy the 

 barberry plants in the district. It was soon found, how- 

 ever, that in some countries, for example, Australia, where 

 the rusts of wheat are most destructive, the barberry is 

 almost unknown. In such cases it seems likely that the 

 rust fungus is able to maintain itself without passing 

 through the barberry. It probably does this by means 

 of the uredospores, which have been shown to be capable 

 of resisting a mild winter. 



Apart from the destruction of the barberry other means 

 of combatting the wheat rust had to be devised. Up to 

 the present this has proved a very difficult problem. The 

 most satisfactory progress has been attained along the 

 lines of breeding disease-resistant wheats. It has long 

 been known that certain wheats are much more resistant 

 to rust diseases than others. The chief difficult v lay in 

 the fact that the most resistant forms, e.g., certain semi- 

 w41d wheats, were almost valueless as crops. In recent 

 years, however, much has been done, especially by Pro- 

 fessor Biffen,of Cambridge, to produce by crossing, w^heats 

 which combine qualities of rust-resistance with good 

 cropping and milling capacities. The subject is neverthe- 



