PLANT DISEASES. 257 



from a special seed plot, away from the general crop, 

 from which smutted heads are carefully removed as 

 soon as they appear. Such a seed plot would also be 

 useful for improving the quality of grain by selection. 

 Loose smut of wheat and loose smut of barley can be 

 controlled in this way. 



Corn Smut stands alone in its class. Its continued 

 spread during the season can be controlled by the 

 prompt removal and destruction of the first smut 

 masses before the spores are shed. Heavy dressings 

 of barnyard manure on corn land tend to increase the 

 amount of smut. 



Grain Rusts. The rust fungi attack leaves and 

 stems of cereals. Orange-colored summer spores are 

 quickly formed in elongated granular pustules. They 

 spread the disease rapidly from plant to plant. Later 

 dark brown winter spores are produced; these may 

 carry the fungus over winter; but often the rusts live 

 through the winter as mycelium in the leaves of fall 

 planted grain crops or of protected wild grasses. 

 While the rusts cause enormous loss each year, no sat- 

 isfactory method of control is known. It is hoped 

 that the problem will be solved by breeding varieties 

 resistant to rust. Such are the so-called rust-proof 

 varieties of oats planted in the South; but unfortu- 

 nately these varieties are not satisfactory in the North. 

 Since rust becomes worse from week to week as the 

 crop matures, the amount of loss can be reduced by 

 hastening maturity. Low, moist situations should be 

 avoided, as well as rank growth, such as results from 

 too much nitrogenous manure. 



Cotton Wilt, Black Rot or Black Heart. The 

 leaves of affected plants, sometimes of a single branch, 

 wilt or turn yellow and fall off while nearby plants re- 

 main normal. The effects are most marked after the 

 cotton is in bloom. The disease is due to a fungus 

 which lives in the soil and enters the roots of cotton, 

 growing upward in the water vessels of the wood of 

 root and stem, clogging them and producing a charac- 



