Southern rust has been confirmed this week (May 21, 2012) on corn in the northeastern parishes of Madison, Richland, and Franklin, Louisiana. Disease severity in a few fields was medium to high (15 to 20%) thus growers applied a fungicide to manage the disease. Traditionally, southern rust occurs later (~ one month) in the growing season in the southern states. There has been no report of southern rust in Arkansas, but growers in the southeastern counties should continue to monitor their fields.
The fungus that causes southern rust is Puccinia polysora. Symptoms are similar to common rust (caused by the fungus P. sorghi) with southern rust pustules being lighter in color, smaller (0.2 to 2 mm long) and circular to oval in shape (southern rust and common rust pustules are in the photo). The cinnamon brown to orange pustules are abundant on upper leaf surfaces and sparse on lower leaf surfaces. Southern rust pustules are less powdery than common rust, which sporulate on upper and lower leaf surfaces. On corn southern rust pustules can be found on leaves, stalks, and husks on ears.
Rust spores are windblown from infected corn leaves and are blown progressively northward during the growing season. Free water as dew is necessary for rust spores to germinate and infect a corn leaf. Symptoms appear about 3 to 6 days after infection and by 7 to 10 days the pustules may rupture to expose mature rust spores. Conditions that favor disease consist of high temperatures (80 to 90 F), high relative humidity, and continuous rainfall. Current weather forecast for Arkansas of hot and dry conditions should minimize the spread of southern rust.
Fungicides effectively control southern rust, and those labeled for use on corn are primarily from triazole or strobilurin fungicide groups. See MP 154 for trade names from these fungicide groups. No fungicide application is recommended at this time for Arkansas corn producers for suppression of southern rust.