Southern rust was found this week (July 19, 2013) near Caraway, AR. This first report of 2013 is one month later than in 2012 when it was found near Pine Bluff. Clusters of cinnamon brown pustules were observed at mid canopy (~4 ft from ground) with a disease incidence of less than 1% of the leaf area. Pustules were somewhat dull and not actively sporulating, which was possibly related to dry conditions but were still identifiable from common rust (Fig. 1). Given the current weather forecast of scattered showers in the state, it is likely southern rust will be actively sporulating by next week and much easier to spot in the field. This does not mean all fields should be protected with a fungicide, but it is a reason to scout for southern rust since it has been detected in the state. Below are some key points on southern rust identification, conditions that favor disease, and management practices.
Southern rust pustules are circular to oval in shape, small (0.2 to 2 mm long) light cinnamon brown to orange in color. Typically southern rust sporulates on the upper leaf surface in contrast to common rust, which sporulates on upper and lower leaf surfaces (Fig. 2 & 3). Common rust pustules are elongate, golden brown to cinnamon brown in color. When scouting a field look for common rust in the lower canopy (3 ft. to bottom leaf) and southern rust at mid canopy (4 to 5 ft. from ground). Southern rust can be found on the edge of fields; it is not always necessary to walk rows, but at least scout along the edge. Questionable samples should be submitted to the Plant Health Clinic in Fayetteville through county extension offices.
Rust spores are windblown from infected corn leaves which progress northward during the growing season. Free moisture as dew or light rain is necessary for spores to germinate and infect. Symptoms appear about 3 to 6 days after infection, and by 7 to 10 days, the pustules rupture to expose mature rust spores (Fig. 2). Conditions that favor disease consist of high temperatures (80 to 90+ F), high relative humidity, and frequent rainfall. Based on the pustule development on the corn near Caraway, the initial infection was probably ~14 days ago and has already caused secondary pustules; note the clustered pattern (Fig. 1). Dry conditions probably aided in suppressing disease development. The current weather forecast is more favorable for southern rust, and pustules will actively sporulate after a rain or heavy dew period making them much easier to spot in the field. Fungicides are effective at suppressing southern rust though there is no economic threshold for a fungicide application. The corn disease working group has compiled some fungicide efficacy data which is available here: Fungicide efficacy table. Producers should consider yield potential, hybrid susceptibility, growth stage, and the weather forecast when southern rust threatens. It is very likely the majority of the corn in Arkansas will mature before rust is an issue. A fungicide application at tasseling or silking when southern rust has been observed on a susceptible hybrid with good yield potential, may be most effective at suppressing disease development; however, additional applications may be needed for season long crop protection. Field corn within two weeks from physiological maturity (i.e. black layer) is very unlikely to benefit from a fungicide application.