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Beginner

Scientific Name
Erwinia stewartii

Identification

• Plants infected before the 5-leaf stage become stunted, wilt and die
  
• Later infections cause pale green or yellow stripes, running parallel to the leaf veins
  
• Wilt-infected plants exhibit brown, mushy tissue inside the stem
• This disease is transmitted by the corn flea beetle- a small (2 mm, 1/12 in.), black beetle

Often Confused With
Magnesium deficiency
Northern corn leaf blight
Three-leaf die back

Period of Activity
Yield losses are most significant when susceptible varieties are infected before the 7-leaf stage. On susceptible varieties, losses at this stage may range from 40- 100%.

The risk of Stewart’s wilt is based on winter temperatures. The winter temperature index is determined by adding the average monthly temperatures for December, January and February. This prediction model is a good guideline, however it is not 100% as it does not take parameters like snow cover and soil temperature into consideration.

Winter Temperature Index	Risk of Early Season Stewart’s wilt
Below 27°C (80°F)	Very low
Between 27 and 29.5°C (80 and 85°F)	Light to moderate
Between 29.5 and 32°C (85 and 90°F)	Moderate to severe
Above 32°C (90°F)	Severe

Scouting Notes
Examine 20 plants each at 5 locations. Look for, and record the presence and number of corn flea beetles.

Thresholds

Variety	Number of beetlesin 100 plants	Feeding Injury
Susceptible varieties	6	10 -25%
Resistant varieties	2

 

Advanced

Scientific Name
Erwinia stewartii

Identification
There are two phases of the Stewart’s wilt infection. Plants infected before the 5-leaf stage become stunted, wilt and die. The cut stem of an infected plant will exhibit brown, mushy tissue at the growing point. When the two cut-ends are pressed together and pulled apart, filaments of sticky material may be visible.

Later infections cause pale green or yellow stripes, running parallel to the leaf veins. These stripes may become brown along the centre with a light green margin. The margins of wilt lesions are wavy or uneven. Later infections do not necessarily result in damaged stem tissue and yield reduction.

Often Confused With
Magnesium deficiency
Northern corn leaf blight
Three-leaf die back

Biology
Stewart’s wilt is transmitted by the corn flea beetle. The bacteria overwinters in the gut of adult flea beetles, which survive through the winter in protected areas. Mild winters can result in higher beetle numbers.

Overwintering adult flea beetles feed on corn in the seedling-to-whorl stage. Once the bacteria are injected into the leaf, they colonize the plant’s vascular tissue, preventing the movement of water and nutrients into the leaves. Since the new growth is affected, wilting and death often occur from the top down.

Period of Activity
Yield losses are most significant when susceptible varieties are infected before the 7-leaf stage. On susceptible varieties, losses at this stage may range from 40- 100%.

The relative risk of Stewart’s wilt is based on winter temperatures. The winter temperature index is determined by adding the average monthly temperatures for December, January and February. This prediction model is a good guideline; however it does not take parameters like snow cover and soil temperature into consideration.

 

Winter Temperature Index	Risk of Early Season Stewart’s wilt
Below 27°C (80°F)	Very low
Between 27 and 29.5°C (80 and 85°F)	Light to moderate
Between 29.5 and 32°C (85 and 90°F)	Moderate to severe
Above 32°C (90°F)	Severe

Scouting Notes
Begin scouting at the spike to 4-leaf stage. Examine 10 plants at each of 10 locations. While walking the field, look for wilted plants and the presence of corn flea beetles. Inspect the root system and growing point of any symptomatic plants. Record the location and number of infected plants.

Thresholds

Variety	Number of beetles in 100 plants	Feeding Injury
Susceptible varieties	6	10 -25%
Resistant varieties	2

Management Notes

• Mild winters favour flea beetle survival and increase the risk of problems the following season.
  
• There is no control for this disease, however, many varieties have excellent resistance to the Stewart’s wilt bacteria.
  
• Corn flea beetle control may help prevent the spread of the Stewart’s wilt bacterium.
  
• Use insecticide seed treatments targeted towards corn flea beetle on any susceptible or moderately susceptible varieties.