FUNGI 



149 



84. Rusts. Rusts are destructive parasites that attack 

 almost all seed-plants, but those that attack the cereals 

 are of special importance. Wheat, oats, rye, and barley 

 all have their rusts; and in the United States there is 

 a yearly loss of several million dollars on account of the 

 ravages of the wheat-rust alone, scarcely a field being en- 

 tirely free from the pest. Naturally these parasites have 

 been investigated persistently; but while very much has 

 been learned about their life-histories and behavior, no 

 remedy has been discovered. It has been found that certain 

 varieties of wheat resist the rust better than others, and 

 that varieties ripening early escape serious injury; and 

 these facts may lead to the breeding of resistant and early 

 races. 



The life-history of a rust is usually very complex, since 

 there are several phases in the history, and all the phases 

 may not occur on the same host plant. Since wheat-rusts 

 are better known than any other, 

 one of them may be used to illus- 

 trate the life-history. 



While the leaves and the stems 

 of wheat are growing, the mycelium 

 of the parasite is burrowing among 

 the tissues of infected plants. About 

 the time of harvest, numerous sporo- 

 phores arise from the mycelium and 

 reach the surface of leaves and 

 stems, each sporophore producing 

 at its tip a reddish spore (Fig. 140). 

 These are the summer spores, and they occur in such great 

 numbers that they form rusty-looking lines and spots, 

 giving name to the disease. The summer spores are scat- 

 tered freely by the wind; and those falling upon other 

 plants germinate immediately, the new mycelium pene- 

 trating the host plant and beginning its ravages. By 



