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Posted 30 June 2017. PMN Crop News.


Keys to Field Identification of Common Versus Southern Rust in Field Corn


Source: Mississippi State University Extension Article. www.cals.msstate.edu


By Tom Allen, Extension Plant Pathologist, Mississippi State University Extension


Stoneville, Mississippi (June 21, 2017)--Common rust has appeared to be a little more severe and certainly widespread in the MS corn crop this season. Much of that has to do with the cooler temperatures as well as the wet weather experienced. As a field diagnosis, telling common and southern rust apart from one another can be difficult. Early in the season, generally prior to tassel (VT) telling the two rust diseases apart can be complicated. Common rust in the lower part of the canopy can oftentimes look different than the pustules that form on or above the ear leaf. In general, when southern rust is observed the disease can be located on leaves at or above the ear leaf. Southern rust is not normally a lower canopy inhabitant.

 

As outlined below, this information should serve as a general guide to aid in the diagnoses of the two corn diseases. In questionable situations and when leaf samples can be sent to a diagnostic laboratory the spores can be pulled from leaves for a quick comparison with the aid of a microscope. However, be mindful that cannot be done in the field. As with anything in plant pathology there are exceptions to the rules (for example, basing a diagnoses on the presence of the rust on the upper or lower leaf surface). As with the diagnosing of any plant disease rely on observing multiple features of the suspect disease rather than basing everything on one single characteristic.

Common rust

every corn hybrid is susceptible to the disease

generally low in the canopy (bottom three leaves) early in the season (vegetative corn), but can be observed throughout the entire plant canopy

pustules may appear larger

pustules are not generally observed with a halo; however, pustules on the lowest leaves can develop a yellow halo that makes field diagnostics difficult

can be a slight pattern to the disease based on the time of infection and when leaves were still in the whorl of the plant

prefers cooler temperatures (but this is relative)

sporulation that erupts through pustules tends to be brown, dark orange, or russet in color

pustules can form on BOTH sides of the leaf, but this is not the best of diagnostic features associated with the disease

in MS, fungicides have not been warranted to manage common rust since warmer temperatures generally shut down the fungus

Southern rust

every corn hybrid is susceptible to southern rust, but there may be ranges of susceptibility within the commercially available germplasm NOTHING is resistant

generally observed higher in the canopy (ear leaf and above) when first observed

pustules are generally smaller in size, at least compared to common rust

oftentimes pustules are surrounded with a faint yellow halo

no pattern associated with the disease

prefers warmer temperatures than common rust (can be active well into 100F)

sporulation that erupts through pustules tends to be light orange in color

pustules generally (but not always) form on the upper leaf surface

fungicides are effective at reducing potential yield losses; however, timing of application and level of infection are important and likely differ on an annual basis (applications at dent are generally not considered to be economically beneficial)


This article was published by Mississippi State University Extension. Visit http://www.mississippi-crops.com/2017/06/21/keys-to-field-identification-of-common-versus-southern-rust-in-field-corn/ for the original article.