LEAF AND SEEDLING DISEASES 173 



the Continent. The rusts are, for the most part, charac- 

 terized by a very complex life-cycle, and many of them 

 grow at different periods of the year on two different hosts 

 and bear four different kinds of spores in the course of their 

 life-cycle. Species which migrate from one host to another 

 and back again are called heteroecious, to distinguish them 

 from autoecious species, which live always on one host or on 

 the ground. 



The discovery of heteroecism is one of the landmarks in 

 the history of plant pathology. The first rust in which it 

 was demonstrated is the black rust of wheat (Puccinia 

 graminis), a fungus which has caused a great deal of damage 

 to wheat crops all over the world. It had long been suspected 

 that wheat rust was encouraged by the presence of plants 

 of barberry, and a decree was promulgated in Rouen as early 

 as 1660, 1 ordering the destruction of barberry in the vicinity 

 of wheat fields. But it was not till 1865 that de Bary showed 

 that this rust actually infects barberry plants in spring, and 

 that wheat leaves are again infected in early summer by 

 "spores produced on the barberry plant. 



The life-cycle of a typical heteroecious rust will be better 

 understood by following the accompanying diagram (fig. 71). 



Starting from the bottom left-hand corner we have the 

 mycelium in the tissues of the host A (e. g. the wheat plant). 

 This mycelium bears red or rust-coloured pustules on the 

 surface, called uredosori, which contain numerous spores 

 called uredospores. These spores again infect the host 

 A, sending their germ-tubes through the stomata, and 

 giving rise to fresh mycelium which bears more uredospores, 

 and so on through the summer. Towards the end of the 

 season mycelium on the same host produces another kind 

 of spore (dark brown or blackish in the case of the black 

 rust of wheat), called the teleutospore. The teleutospores 

 may appear in special pustules (teleutosori) or side by side 

 with, and in the same sori as, the uredospores. These 

 teleutospores, which are thick walled, are the perennating 

 organs of the fungus. They fall on the ground and remain 

 there during the winter. They contain enough food reserve 

 1 Whetzel, 1918. 



