PLANT DISEASES 287 



much like the summer spores. They are formed in 

 little cups, which are known as cluster cups, and are 

 blown off the barberry plants by the wind. These 

 spring spores, or cluster cup spores, will not infect 

 another barberry plant, but will be wasted unless they 

 fall on a wheat plant. Here they send out the usual 

 thread and infect the wheat leaf or stem, and build up 

 a mycelium inside of the wheat leaf, which now pro- 

 duces the summer spore with which we started. 



The life story, therefore, is seen to be quite complex. 

 Summer spores are produced on the wheat plant in 

 midsummer and up to harvest time. The same myce- 

 lium will then produce the black stage or winter spores. 

 These live over on the straw of wheat until spring, when 

 they grow, and by means of small spores which arc 

 blown by the wind, cause the infection of barberry 

 plants, and on these barberry plants the spring spores 

 are produced. The spring spores are then blown to the 

 wheat plants and again cause the summer spores. 



As has been before mentioned, the life stories of the 

 different rusts of cereals are not all similar. The wheat 

 stem rust, as described, lives on two plants, the wheat 

 plant and the barberry plant. Some rusts, as, for in- 

 stance, one of the oat rusts, lives on the oats and on 

 the buckthorn. 



Another possibility of the life history needs to be 

 mentioned. The rusts of cereals are able also to infect 

 certain wild grasses which live through the winter. The 

 summer stage of the rust on these grasses is sometimes 

 formed late in the fall and may live over the winter 

 under the snow so that the summer spores on these 

 grasses may appear early in the spring and start an- 

 other infection, when they are possibly carried back 



