           |
|
-
- Soil Temperature May Hold
Key to Stewart’s Wilt Severity
-
- By Willie Kirk, Beth Bishop,
Rob Schafer and Adam Byrne
Michigan State University Stewart’s wilt is a disease of corn, caused by the bacterium Erwinia stewartii (E. F. Smith) dye. It is transmitted to corn seedlings during feeding by the corn flea beetle.
Bacteria invade the vascular tissue of the plant, disrupt nutrient and water flow, and cause leaf lesions, leaf wilting, barren stalks or plant death. Disease severity depends on the variety and on plant age at the time of infection. Early plantings of highly susceptible seed corn inbreds and sweet corn hybrids are likely to have the most severe symptoms.
The bacterium is carried in the digestive tract of the corn flea beetle. Stewart’s wilt severity is thought to be dependent on the overwintering survival potential of infected corn fleas beetle. This has been predicted by a risk index, defined as the sum of the monthly average temperatures during December, January and February. Indices less than 90 predict no risk for Stewart’s wilt, and greater than 100 predict severe risk.
The overwintering habitat of the vector is not well understood, but is thought to be in the upper soil profile and in the root zone of grasses; however, the predictive index was based on ambient air temperature. The temperature profile of the upper soil, as well as the duration of temperature below a critical threshold, may result in a more accurate prediction of corn flea beetle survival.
Growers aware of the index considered it unreliable and did not use it, according to a survey of sweet corn growers in Michigan.
Temperatures were monitored at three elevations (soil at six inches in depth, soil surface and air at 36 inches above soil) at four sites within fields in Macomb County, Monroe County and St. Joseph County (tree or ditch line, margin of grass beyond the headland, at the edge of the field and 30 feet into the field). At Centerville in St. Joseph County data was only recorded for January and February.
Soil surface temperatures were close to 32oF and their accumulation over the three-month period was close to 90oF. However, average air temperature was fairly consistent at each site within the field but was below 32oF and the accumulated temperature over the period was about 80oF. Soil and surface temperature accumulations indicate a severe to moderate risk of beetle survival, however air temperature accumulations would predict a low survival potential for beetles. Soil surface temperatures were also close to 32oF and their accumulation over the three-month period was close to 90oF. Average air temperature was fairly consistent at each site within the field but was below 32oF and the accumulated temperature over the period was about 80oF. Soil and surface temperature accumulations show a severe to moderate risk of beetle survival, but air temperature accumulations would predict a low survival potential for beetles.
Soil temperatures at each location within the field remained on average close to 32 oF and the accumulated average temperatures were >100oF. Soil surface and air temperatures were generally more variable than soil temperature (compare magnitude of error bars. Soil surface temperatures were also close to 32oF and their accumulation over the three-month period was about 100oF. However, average air temperature was fairly consistent at each site except at the position 30 feet within the field but all temperatures averaged 32oF on a daily basis. The accumulated temperature over the period was about 100oF (111oF 30 feet into the field). Soil, soil surface and air temperature accumulations indicate a severe risk of beetle survival in Southwest Michigan.
Temperatures were only recorded from Jan. 1 to Feb. 29, 2000 due to an equipment failure. The soil temperature at each location within the field remained on average close to 32oF and the accumulated average temperatures were close to 60oF. Given the accumulation of temperatures at other southern locations it was likely that the average temperatures in December were close to 32oF also.
Therefore, average temperature accumulations close to 60oF may indicate the survival of the corn flea beetle. Soil surface and air temperatures were generally more variable than soil temperature. Soil surface temperatures at all locations were also close to 32oF and their accumulation over the three month period was close to or greater than 60oF. However, average air temperature was fairly consistent at each site within the field but was below 32oF and the accumulated temperature over the period was about 80oF.
Soil and surface temperature accumulations indicate a severe to moderate risk of beetle survival although air temperature accumulations would predict a low survival potential for beetles in St. Joseph County.
-
-
|
|
