Earlier this week (June 18, 2012) southern rust was reported in Jefferson Co. As of today, southern rust has been confirmed in nine more counties that include: Arkansas, Lee, Lonoke, Monroe, Prairie, St. Francis, Jackson, White, and Woodruff counties. The highest disease severity was 2 to 4 pustules per leaf on corn in Monroe and Lee counties which was more than 1 to 2 pustules per leaf that was observed in all other counties.
Historically, southern rust starts in the southeastern counties of Arkansas; however, no reports of rust have come from Ashley, Chicot, Drew, Desha or Lincoln counties. It is likely the conditions needed for infection were not met in these counties, thus there is extremely low or no disease at this time.
An earlier publication on this blog has more information on environmental conditions that favor southern rust and management tactics. It is important to know if southern rust is present before a fungicide is applied. Fungicides with a strobilurin only (Headline or Quadris) are only effective if applied before rust is found in the field. Fungicides with a triazole only (folicur, tilt, promimax, evito, domark, and several generics) or triazole + strobulin (stratigo, headline amp, quilt, and quilt xcel) are effective if applied before or after rust is found in a field.
Information provided by Michael Emerson, Program Associate, Plant Pathology