174 HOW PLANTS BENEFIT AND HARM MANKIND 



plants from an economic standpoint are the rusts, smuts, and mil- 

 dews which prey upon grain, corn, and other cultivated plants. 

 Some fungi are also parasitic upon fruit and shade trees. The 



chestnut canker, a fungus 



recently introduced on 

 chestnuts planted near 

 New York city, has within 

 five years practically de- 

 stroyed all the chestnut 

 trees within a radius of 

 twenty miles of the city, 



Corn smut, a fungus parasitic on corn ; the black 

 mass consists almost entirely of ripe spores. and IS estimated to have 



done $10,000,000 damage 



already. Damage extending to hundreds of millions of dollars is 

 annually done by the fungi. 



Wheat Rust. Wheat rust is probably the most destructive parasitic 

 fungus. For hundreds of years wheat rust has been the most dreaded 

 of plant diseases, because it destroys the one harvest upon which the 

 civilized world is most dependent. For a long time past the appear- 

 ance of rust has been associated with the presence of barberry bushes in 

 the neighborhood of the wheat fields. Although laws were enacted nearly 

 two hundred years ago in New England to provide for the destruction of 

 barberry bushes near infected wheat fields, nothing was actually known 

 of the relation existing between the rust and the barberry until recently. 

 It has now been proved beyond doubt that the wheat rust passes part of 

 its life as a parasite on the barberry and from it gets to the wheat plant, 

 where it undergoes a complicated life history. The wheat leaf, its nour- 

 ishment and living matter used as food by the parasite, soon dies, and no 

 grain is produced. Some wheat rusts do not have two hosts, living only 

 on the wheat and wintering over by means of thick-walled spores which 

 remain in the stubble or in the ground until the young wheat plants 

 appear the following year. 



Mildews. Another group of fungi that are of considerable economic 

 importance is made up of the sac fungi. Such fungi are commonly called 

 mildews. Some of the most easily obtained specimens come from the lilac, 

 rose, or willow. These fungi do not penetrate the host plant to any depth, 

 but cover the leaves of the host with the whitish threads of the mycelium. 

 Hence they may be killed by means of applications of some fungus-kill- 

 ing fluid, as Bordeaux mixture. 1 They obtain their food from the outer 



1 See Goff and Mayne, First Principles of Agriculture, page 59, for formula of 

 Bordeaux mixture. 



