VEGETABLE AND FIELD CROPS 249 



Upon the infected seedling's leaves dark opaque spots 

 may be noted, especially upon holding the leaves against 

 the light. The portions of the leaf above such spots soon 

 die, and the spots rupture, exposing sooty, powdery masses. 

 Badly infected seedlings succumb early, while others may 

 survive until harvest. The disease is more fatal if attended 

 by dry weather, which reduces the vigor of the host plant. 

 Plants surviving the disease show smut in the outer leaves, 

 often also within. Diseased bulbs dry up and rot. In 

 handling they may be readily distinguished from sound 

 bulbs. 



The spores are liberated in abundance in the soil, where 

 they remain alive for many years. There is no record of 

 soil once smutted having become free of the smut. The 

 disease may be carried to adjacent fields by insects, also 

 by the means suggested under soil diseases. There is lit- 

 tle or no danger of disease carriage by the seed. 



Since onion sets are immune, the effects of the smut can 

 be avoided by sowing seed in soil that is free of disease and 

 then transplanting into the field. With this mode of opion 

 culture the only difficulty is to secure plats free from in- 

 fection, and this is not often a serious one. In extreme 

 cases disinfection of the soil in plats may be resorted to. 

 (See soil disinfection, p. 54.) On farms once infested all 

 refuse (screenings, tops of infected sets,) should be col- 

 lected and burned to reduce the amount of spores. Where 

 soil known to be infected is to be planted, formalin^ 

 (1 pound in 25 to 33 gallons of water) should be used 

 and applied with a drip attachment on a seed drill at the 

 rate of 500 to 700 gallons of solution for each acre for 



' Selby, A. D., Ohio Agr. Exp. Sta. Bui. 131, p. 51. 



