Southern rust

Southern rust photo submitted with sample from Vernon County, Mo. 

Editor’s note: The following was written by Nathan Kleczewski, University of Illinois Extension field crop pathologist, for the university’s Illinois Field Crop Disease Hub blog.


How should we go into 2020 as far as managing Southern rust is concerned?

First, know your hybrid. Although the majority of corn hybrids lack adequate resistance to Southern rust, there are a couple hybrids that can be planted in parts of Illinois that have decent resistance to this disease.

Second, monitor the disease. You can track disease occurrence in real time by accessing the corn IPMPIPE page at https://corn.ipmpipe.org/southerncornrust/.

Remember that P. polysora is an obligate pathogen and needs to blow into the state each season, unlike grey leaf spot or even tar spot. Thus, by paying attention to where the disease is, knowing your hybrid, and knowing the stage of growth, you can make informed management decisions.

Lastly, it is important to accurately identify the disease.

Southern rust is caused by Puccinia polysora, which produces fuzzy, raised structures called pustules on leaves and stalks of corn. They are mostly found on the upper leaf surface, which can help distinguish it from the less damaging common rust.

Pustules contain thousands of small orange spores. Spores can be dispersed miles on air currents, allowing the disease to spread rapidly.

Under hot, humid conditions, spores of the fungus can infect susceptible corn, and symptoms can be observed within 3-4 days. Within 7-10 days, spores are produced and can be dispersed.

The cycle of spore-infect-spore can continue as long as conditions are conducive and corn plants are green.

Conditions that favor disease development include hot temperatures (morning low of 75°F and daytime high of 93°F) and at least 4 hours of consecutive leaf wetness. Outside of these conditions, disease progress can occur, but at a slower rate.

Southern rust does not overwinter in Illinois and blows into the region from warmer regions. In years where it develops to a significant degree early in southern regions, it can move into Illinois during critical stages in crop growth.

In general, we see the disease move in most years in late July or early August. This means that in years when plantings are delayed, the disease can arrive on time but plants may be at greater risk for yield loss because the earlier infections occur the more yield can be impacted.

Experience from our Southern colleagues indicates that stalk integrity isn’t likely to be affected unless you see significant infections during the vegetative stages of crop development.

Now that we are on the same page about this disease, what about management? Let’s start by considering management decisions.

Experience from the South indicates that trying to hold off an application until VT/R1 if possible is going to give you the highest likelihood of coming out even or ahead of this disease. If you apply during the vegetative stages, realize that means that you might need to come back again and make a second application. You now have likely doubled your application costs.