PLANT DISEASES 285 



adapted to spreading the disease through the wheat 

 field in the summer time. 



As the wheat plants begin to ripen just before harvest, 

 the fungous thre-ads which have produced the summer 

 spores now begin to produce spores of a darker color, 

 which are provided with thick walls and which in the 

 pustule look black. These are the winter spores and 

 are commonly called the black rust. They are really 

 only the winter stage of the stem rust. These winter 

 spores are not blown about by the wind, but remain on 

 the straw over winter. 



The summer stage, commonly called red rust, and 

 the winter stage cause a great deal of damage to the 

 wheat, first by breaking open the epidermis, or outer 

 covering, and allowing the water to evaporate from the 

 stem. In the second place, where the wheat plant is 

 badly infected with the disease, a great deal of nutri- 

 tion is taken by the parasite. This becomes especially 

 dangerous in the stem rust of wheat when the rust 

 covers the stem just under the head, as is the case in 

 an ordinary epidemic of rust. 



If now we follow the life story through the winter 

 stage, we find that the winter spores which remain on 

 the wheat straw will not germinate until the following 

 springtime. They then send out a little thread from 

 which about four tiny spores are cut off. These spores 

 are blown by the wind. If they fall on the leaves of 

 a barberry plant (which is an ornamental, thorny 

 shrub very commonly raised and introduced in most 

 parts of the United States for hedges and other orna- 

 mental purposes), they will grow by sending out an- 

 other thread and penetrating into the leaf of the bar- 

 berry, where a mycelium is built up. If they do not 



