166 



Diseases of Economic Plants 



turnips, rutabagas, radish, black mustard, charlock, and 

 other members of the cress family are affected. 



Black-rot was first noted on turnips in 1892 in Iowa, and 

 on cabbage in 1895 in Wisconsin, and is now very dis- 

 astrous in its effects in practically ail sections of the 

 United States east of the Mississippi River, as well as in 

 the more western states. 



Infection arises from the 

 causal bacteria which are 

 often present in the soil from 

 preceding years. These bac- 

 teria gain access to the drops 

 of water at the water pores 

 on the leaf edge. The first 

 sign of the disease appears 

 as a blackening of the veins 

 upon the edges of the leaves. 

 The affected region rapidly 

 enlarges, the blackening ex- 

 tending toward the stalk, 

 and throughout the vascular 

 system. Soon this blacken- 

 ing reaches the stem of the 

 plant and proceeds up and 

 down the stem, thus gaining 

 Fig. 89. — Portion of cabbage stem entrance to other leaves, 

 showing blackened veins. Re- gj^^u reaching the entire 

 drawn after Russell. i - tt n i 



plant. Usually many leaves 



are infected simultaneously. Affected leaves soon yellow 

 and wilt, owing to the obstruction of the water channels, 

 then dry, become parchment-like, and fall to the ground. 

 General infection of the plant is followed by death. Reliable 

 diagnostic characters are the blackened vein areas of the leaf, 

 blackened veins as seen in cross sections of the leaf stalk or 

 of the stem of the plant. Other rots may supervene, giving 

 offensive odors. If plants with the smallest amount of dis- 

 ease are placed in storage, the disease continues to develop, 



