Corn
What should I do about common rust?
Common rust is a disease that frequents Mississippi corn fields nearly every year. However, infections and associated corn grain yield loss are usually very minimal. Infection normally occurs from mid-May to mid-June.
Common
rust has cinnamon-brown colored round to elongated pustules that frequently
form in bands on the lower part of the leaf, which result from infection
when the leaf was in the whorl. Common rust pustules form on both upper
and lower sides of an individual leaf, distinguishing Common from Southern
rust, which predominately sporulates on the upper leaf surface.
Common rust development requires relatively cool to moderate temperatures (54 to 82 degrees F) and nearly 100% relative humidity for about six hours. For example, Mississippi experienced a common rust epidemic in 1997, which was promoted by sustained cool, cloudy, rainy weather from late May through mid-June. The 1997 corn crop was developing through late vegetative growth stages during this time. Rust development is much more likely in pre-tassel stage corn, because a large whorl provides a humid, protected environment, and young leaf tissue is more susceptible to infection than emerged leaves. After tasseling, all leaves are completely emerged (past whorl stage) and should be relatively immune to further common rust development.
Mississippi research (MAFES Research Report Vol. 22, No. 9) indicates fungicide treatment for common rust control may increase grain yield of field corn, if applied at least one week prior to tasseling on a susceptible hybrid such as Pioneer 3223 or 31B13, especially when grown under irrigated (high-yielding = 150 bushels per acre or more) conditions. Conversely, fungicide application timing within a week of tasseling, or on a hybrid with moderate Common rust resistance, or on corn grown in a dryland (low-yield potential) environment will not likely provide an economic return.
Corn grain yield reductions from Common rust infection rarely exceed 10 - 15 percent.
Thus, producers and consultants should closely scout fields for further common rust development. They should closely monitor the weather forecast before making a fungicide treatment decision. Fungicide application will likely only produce a positive return if common rust is developing on a susceptible corn hybrid, at a growth stage prior to tassel, grown in a high yield potential environment during prolonged cool, humid weather conditions.
