Southern corn rust (SCR) is developing in the lower Mississippi River Valley and moving north. It is not yet detected in the mid-south states of Kentucky and Tennessee or plains states like Kansas. However it will arrive soon. The charts below show counties where SCR has been detected in states that are monitoring the disease. The pattern for 2017 is similar to 2016.
SCR moves and affects the crop differently each year. A series of weather patterns from the Gulf of Mexico disperse spores northward on the winds and storm fronts. Disease severity and loss depend on crop development and weather patterns through the summer. Late planted corn is most at risk. Commercial corn hybrids are not resistant to the disease and vary in susceptibility. Some hybrids are better than others, but disease protection is required in most cases when SCR arrives in pre-tassel to early grain fill stages. It is important to be aware of the disease and use control measures where and when needed. How to identify and what to do about SCR is discussed in this article.
Southern corn rust (Puccina polysora) is moving north and can cover a large geographical area. SCR is usually a late developing disease, but is getting another early start. It begins in the south in regions such as Mexico where susceptible corn is grown through the winter. Rust pores move on the winds into the lower Mississippi Valley and Texas where corn is planted in late February-early March, and then it progresses north in the corn crop. Corn hybrids vary in susceptibility to the disease, none are truly resistant. It matters when the disease begins to develop in the crop. Losses can be severe when infected before tasseling to early milk growth stages. It is a fast developing disease. Warmer temperatures in the 80-90 degree range and frequent rain or heavy dew Increase development.
Description. SCR forms pustules that are light cinnamon or a golden brown to orange color usually surrounded by a yellow halo. Common rust, not an economic threat, is slightly darker cinnamon brown to reddish color. SCR pustules are small about the size of a pin most often on the upper surface of the leaf and often appear in clusters. SCR will also infect leaf sheath, ear shank and husk leaves. Pustules of common rust are large and have rough edges due to erupted epidermal tissue on the upper and lower sections of the leaf. SCR pustules usually start at the mid plant level as opposed to common rust that start on the lowest leaves. Older leaves are more resistant to SCR spore penetration. Sometimes common rust and SCR can be found on the same plants.
Disease cycle is about 5-10 days. Spores are like a powdery cluster around the pustules. Infections are increased when leaves are moist with dews for 12-15 hours. Bright sunshine, warm temperatures and wind aid spore release and dispersal during the day. Several weeks of infections will eventually kill upper leaves. Yield losses can be in the range of 20-50 bushels per acre when disease begins at flowering and is severe. Plants are more susceptible to the disease before tasseling to early grain fill stages.
What to do. SCR causes severe losses when infections begin at or before flowering stage. Fungicides do not control the disease but can suppress it because SCR develops fast and continues late in the season.
- Scout fields prior to the tasseling to early grain fill stages. If 3-5% of leaf area and 50% of the plants are identified with SCR pustules then a fungicide application is warranted
- One must distinguish the difference between SCR and common rust. SCR develops in mid to late season and is an economic threat. Common rust develops in early season during cooler weather and is not a threat.Purdue University publication BP-82 “Common and Southern Rusts” will help you identify and distinguish differences for these rusts.
- Fungicides provide up two weeks of protection. Fungicide applications are most effective when applied VT to R1 growth stage when SCR is present. However, a second application may be required later in the season when disease is severe. Fungicides must be applied to suppress or kill germinating spores on the leaf surface before new pustules develop. Pustules already developing in the leaf at the time of fungicide application will be visible later after fungicide application.
- Most fungicide products control SCR but several are better with longer residual life. Package mixes with mixed modes of action improve disease control and reduce the risk of resistance development. Purdue publication BP-160 “Fungicide Efficacy for Control of Corn Diseases” is a good resource for choosing and understanding the use of various fungicide products.
- Fungicide application at brown silk or later is most effective for rust control. Disease development later in the season at R4 (late dough) or R5 (dent stage-50% starch line) usually do not benefit from fungicide application. Late planted corn is more at risk to the disease, so planting early is always a good strategy.
Summary. SCR is making another early appearance in corn growing areas along the Mississippi River basin. Fungicides provide good protection when applied in advance of the disease when severity is high in the early grain fill stages. Disease control may be required during grain fill stages but before the dent growth stage. There is no plant resistance to SCR but varying levels of susceptibility. Hybrids with moderate to susceptible reactions benefit most from fungicide protection. Late planted corn is more at risk to disease development. Yield loss is more likely during warm summers with frequent rain or heavy dews.
*8-9 = highest tolerance
6-7 = moderate tolerance
4-5 = susceptible
- Bradley, Carl A. 2017. Update on Southern Rust of Corn. Kentucky Pest News. June 20, 2017. https://kentuckypestnews.wordpress.com/2017/06/20/update-on-southern-rust-of-corn-2/
- Wise, Kiersten. 2010. Common and Southern Rusts. Purdue University Publications. BP-82-W. 4 pp. https://www.extension.purdue.edu/extmedia/BP/BP-160-W.pdf
- Wise, Kiersten. 2017. Fungicide Efficacy for Control of Corn Diseases. Purdue University Publication. BP-160-W. 2 pp. https://www.extension.purdue.edu/extmedia/bp/bp-160-w.pdf
Note: The information in this issue is based upon field observations and third party information. Since variations in local conditions may affect the information and suggestions contained in this issue, LG Seeds disclaims legal responsibility therefore. Always read and follow label instructions.
Ratings for Southern Corn Rust are from Product Profiles as published by LG Seeds and from the LG Seeds 2018 Seed Guide.
All pictures are courtesy of Jesse Grogan, LG Seeds
LG Seeds and design are trademarks of SCA Limagrain.
Download a copy of this Technical Bulletin: Tech_341 - Southern Corn Rust