334 DISEASES OF ECONOMIC PLANTS 



often attacked. Since infection cannot occur without mois- 

 ture, weather conditions are of predominating influence. 



The longer the crop grows, the longer the period of sus- 

 ceptibility is maintained. Thus a late-planted crop in 

 Indiana showed half as much infection as a crop which 

 was planted earher, chiefly because it was exposed to con- 

 tagion only about half as long. Moisture and richness 

 of soil predispose to disease by increasing growth and giv- 

 ing a greater number of susceptible points of attack. Corn 

 thicldy planted is more liable to infection because venti- 

 lation is restricted, and thus a more humid atmosphere is 

 created around the plants. 



It has been shown that corn smut can be reduced some 

 16 to 72 per cent by spraying with Bordeaux mixture, but 

 the saving does not warrant the expense. The only practical 

 method is to go through the field several times during the 

 season and cut out and burn all the developing smut masses 

 that can be seen, to thus destroy the spores and prevent 

 the continued spread of the disease. Professor Bessey says : 

 " The cost per annum of gathering and burning the smutted 

 ears ought not to exceed ten or fifteen cents per acre. A 

 smart boy carrying a bag slung over his shoulder ought 

 to be able to earn good wages in smutting corn at ten cents 

 per acre." This practice continued yearly would result 

 in continued diminution of smut. Whether it will be profit- 

 able or not depends upon the amount of smut usuall}^ found 

 in any given community. The use of the silo in which the 

 smut spores are destroyed tends to lessen the amount of 

 smut. The smut fungus can live and even increase in 

 manure. Therefore live smut spores may infect the ma- 

 nure pile and thereby increase the disease in fields to which 



