Stewart’s wilt disease on field corn revisited
Last fall, an epidemic of Stewart’s disease occurred with foliar symptoms showing up on many field corn varieties in numerous locations across the state (figure 1). Stewart’s disease is caused by the bacterium, Pantoea stewartii, and transmitted to corn by the corn flea beetle when it feeds on foliage and deposits its bacteria-laden feces in the feeding wounds. The flea beetle overwinters as an adult in ground litter and harbors the bacterium in its gut. With the mild winter we experienced, there were concerns that overwintering beetles might cause the more serious wilt phase on field corn seedlings in spring instead of the foliar lesions that appeared in August and September when the corn was in the milk to dent stage. No wilt symptoms were reported on field corn this spring, although one grower did report heavy losses in a field of sweet corn.
However, this fall, Stewart’s disease is appearing again, from infections that took place after tassels emerged. The leaf lesion phase is caused mainly by the first summer generation of flea beetles. At least two overlapping generations of corn flea beetles occur over the summer, with the first generation peaking about mid-June, and the second about four to six weeks later. Although it is less severe and in a more limited geographical area, it still seems to be affecting a number of varieties. Most of the fields affected have been in the greater Thumb area, including Sanilac, Tuscola, Saginaw, Lapeer and Gratiot counties, although I have found some of it in variety trials on campus. So far, representative samples among different seed companies and maturities have been collected from MSU corn variety trials in Sanilac, Cass and Lenawee counties and tested with ELISA (enzyme-linked immunosorbent assay). Samples collected from corn variety trials in Lenawee and Cass counties produced only one positive from Lenawee County, and no positives out of 16 in Cass.
Fourteen out of 15 varieties from Sanilac County have tested positive. Extension educators have identified a number of fields in Saginaw, Lapeer, Tuscola and Gratiot counties with leaf symptoms that are strongly suggestive of Stewart’s disease, but they have not yet been confirmed by ELISA.
Earlier this year, Extension educators in Saginaw and Sanilac counties collected corn flea beetles from corn fields adjacent to locations that were hard hit last year. The beetles came from one field in Saginaw County and one field in Sanilac County, and all were tested individually with an ELISA kit to detect the Stewart’s disease bacterium. Three potato flea beetles collected on June 12 from Sanilac were negative. Ten out of 30 corn flea beetles collected from Sanilac on June 29 were positive, and 20 one of 34 collected on July 16 were positive. From the Saginaw location, nine out of 14 flea beetles collected July 16-21 were positive, and none of five flea beetles collected from the same location on August 25 were positive, although a corn leaf sample collected at the same time from that field tested positive. Although we have not found any evidence that the wilt phase of Stewart’s disease is occurring on field corn, the appearance of the leaf blight phase across many varieties for a second year raises some concern about the possibility that resistance to the disease may be diminishing.