212 



DISEASES OF ECONOMIC PLANTS 



rapidly enlarges, the blackening extending toward the stalk," 

 and throughout the system of veins. Soon this blackening 

 reaches the stem of the plant and proceeds up and down 

 the stem, thus gaining entrance to other leaves, and finally 

 reaching the entire plant. Usually many leaves are in- 

 fected 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 infec- 

 tion of the plant is fol- 

 lowed by death. Other 

 rots may supervene, 

 giving offensive odors. 

 If plants with the small- 

 est amount of disease 

 are placed in storage, 

 the disease continues 

 to develop, resulting in 

 complete loss. 



Reliable diagnostic 

 characters are the blackened vein areas of the leaf, black- 

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

 the stem of the plant. 



Infection arises from the causal bacteria which are often 

 present in the soil from preceding years. These bacteria 

 gain access to the drops of water at the water pores on the 

 leaf edge. Hence they enter the plant and parasitize it. 



Fig. 96. — Portion of cabbage stem showing 

 blackened veins. Redrawn after Russell. 



