The Appearance of Southern Rust

Published: July 11, 2018

Southern rust (Puccinia polysora) is a relatively common, annually occurring disease of corn that can result in significant yield loss if infection is severe. Generally, it is observed in many, if not all, of our neighboring southern states before appearing in Missouri fields. This year is an exception. On Tuesday (July 10th), the University of Missouri Plant Diagnostic Clinic confirmed the presence of southern rust in a sample collected from West Central Missouri (Figures 1 and 2). It is the first confirmed report of southern rust in the United States this season.

Figure 1 Southern rust confirmed in Vernon County. Note the densely packed, orange-tan pustules on the upper leaf surface. Photo capture by sample submitter.

Figure 2 Spores of Puccinia polysora magnified 400 times. These spores are found in the rust pustules that break through the leaf surface and are responsible for the orange-tan color. Photo by Kaitlyn Bissonnette.

Since southern rust is a "tropical" disease that requires a living host to survive, it does not overwinter in Missouri but rather in warmer climates. The spores are blown north on wind currents into the corn producing states each year. The unseasonably warm daytime temperatures that have been occurring across much of the state also mean warmer nighttime temperatures when relative humidity is at its highest. These conditions create the ideal environment for the development of southern rust, when spores are present. The overall incidence (percent of plants affected) and severity (percent of an infected plant that is affected) of the disease is relatively low. However there are a few things to keep in mind when scouting for or managing this disease.

Puccinia polysora favors temperatures around 80°F with high relative humidity for four or more hours. As the fungus multiplies in the host tissue, raised structures (pustules) form and masses of spores erupt through the leaf tissue. The circular (or oval) pustules are orange to tan in color, commonly form on the upper leaf surface, and are densely clustered. As the season progresses, the pustules can change from orange-tan to a brown or black color. Rust is often initially observed in the mid to upper plant canopy along field borders or at the ends of rows where spores can easily land on the leaf surfaces. Southern rust can be easily confused with other leaf diseases of corn such as Common rust (Figure 3) or Physoderma brown spot (Figure 4). It is important to properly diagnose southern rust so as to not make unnecessary (and often costly) fungicide applications. Suspected southern rust samples can be sent to the MU Plant Diagnostic Clinic for confirmation.

Figure 3 Common rust pustules that can be confused with southern rust. Photo by Craig Grau.

Figure 4 Physoderma brown spot of corn. Can be mistaken for southern rust of corn, but leaf tissue remains intact. Photo by Daren Mueller.

Management of southern rust is dependent upon the crop stage when it is first detected and the environmental conditions. Up to R3 (milk stage), there can be benefit from applying a preventative fungicide application. However, it is important not to apply a fungicide prematurely. Infection in the early R stages can result in more substantial yield losses than in the later R stages. Take note of the current distribution of southern rust in the area (through the iPIPE website), the growth stage of the crop, and the environmental conditions as they are the most important factors to take into consideration when making decisions to apply fungicides. With no other confirmed reports of southern rust in Missouri (or any other state) to date, disease pressure is relatively low, so exercise caution when making costly management decisions.

More information on Southern rust, look-alikes, and its management can be found at the Crop Protection Network.