Pathogenicity of Stewarts wilt bacterium to corn

​Stewarts wilt and leaf blight is caused by the bacteria species Pantoea stewartii (= Erwinia stewartii).  This bacterium can only enter the corn plant through injury almost always associated with feeding by corn flea beetle.  The beetle picks up the bacterium while feeding on infected leaf tissue of corn or other susceptible grass species.  The bacteria can remain in the insect’s intestine for long periods, including between seasons. 
 
Once injected in a corn leaf, the bacteria produce specific enzymes that attack cellular endoplasmic membranes.  This results in leakage from the cytoplasm of affected cells, providing nutrition for increasing the number of bacteria within the tissue.  If not stopped by the host cells, the bacteria plug the xylem tissue, causing a wilt of a young susceptible plant or elongated wilted lesion in more mature plants.
 
Plant defense system similar to other resistance methods, initially involves chemically detecting the presence of the intruder. After this detection, salicylic acid is produced causing the production and release of enzymes that effectively destroy the bacteria’s enzymes. This dynamic interaction, all controlled by genetics of the pathogen and of the host, affects the speed of cell destruction and the effect on the host plant.
 
Extreme susceptibility is most common in sweet corn varieties and few field corn inbreds. Resistance mostly involves several genes, probably affecting the detection of this bacterium and the speed of reaction, limiting the damage to a few cells.
 
Biology of the vector insect greatly affects distribution of Pantoea stewartii.  Corn flea beetle (Chaetocnema pulicaria) overwintering success is affected by winter temperatures, surviving best in dry warmer temperatures.  In the USA, these insects are favored in the mid-southern states.  Sweet corn vulnerability to this disease increases with late planting because of the increase of the insect.  Populations of this insect has decreased in recent years because of the increased use of a systemic insecticide seed treatment on corn.  Consequently, the disease has also reduced.
 
Stewarts wilt biology includes that of host, bacterium and the vector insect.