Saturday, August 19, 2017

What the Heck is that Corn Disease?

Several weeks ago, Bryce Sandahl, our Account Manager near the Rockford, Illinois texted me a picture of a disease showing up on corn.  He thought it looked like gray leaf spot, but was not sure because we really had not had the right conditions for disease development.


When I looked at the lesion, it looked shiny, so I immediately thought bacteria.  We looked under the microscope and found bacterial oozing coming from the leaf tissue.  We did an ELISA quickstrip test to test for Goss's wilt (another common bacterial disease in corn) and it tested negative.

Back, in the old days, when I was at the U of I Plant Clinic, we would have assumed it was most likely Stewart's wilt by process of elimination.  But, with new diseases constantly emerging, such as Bacterial leaf streak, we need to be careful!  So, I sent a sample to the U of I Plant Clinic and they confirmed that it was Stewart's wilt by use of primers and sequencing!

Stewart's wilt was also a major threat to sweet corn!  It is vectored and spread by the corn flea beetle. After the recent mild winter, it is possible that  the corn flea beetle populations could be heavier this year.

There can be two stages of this disease:  seedling wilt phase, which affects susceptible young plants and the leaf blight phase, when infection takes place after tassel.  Remember that insecticides in seed treatments like PowerShield® help to control the corn flea beetle in young plants, so this helps to combat the seedling wilt phase of Stewart's wilt.

The main way to control this disease is resistance and if it is found today, the lesions will be small, watersoaked, following veins, with wavy margins (like the picture) and not extend much further than 2 to 3 cm from where the corn flea beetle fed. Before resistance in hybrid corn, the symptoms from Stewart's wilt were much more serious.

Fun Facts:  This disease was thought to be first described in the 1880's by the entomologist, T.J Burrill, who is famous for his work on Fireblight and helping to prove that plant diseases could be caused by bacteria.

Stewart's wilt held much economic importance throughout history because of quarantine restrictions, costs associated with breeding for resistance, and phytosanitary regulations within seed trade.

Lastly, this disease brought back many memories from my U of I college years when working as an undergraduate with Dr. White's corn plant pathology breeding crew.  If you were a short student worker, like me, you got the job of "clapping" the corn, during the seedling stage, with Stewart's wilt inoculum.   Later, these various corn lines were evaluated and rated after disease development.  If you were good, like me, you got to hold the clipboard for the grad student and take notes for them!

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