Southern rust of corn has now been observed in different areas of Illinois. Southern rust is one of two different rust diseases of corn that can be observed in the state (the other is known as common rust). Because nearly every corn hybrid grown is susceptible to southern rust, yield reductions can occur if infection takes place early enough in the season. Late-planted corn fields are at the highest risk for yield losses associated with southern rust and should be scouted for the presence of this disease. Warm and humid conditions are most favorable for the southern rust pathogen infection and disease spread.
It is important to be able to differentiate southern rust from common rust, since the latter generally is not considered a threat to yellow dent corn hybrids because most are fairly resistant to common rust. Southern rust pustules generally are smaller than common rust pustules and are orange in color compared to cinnamon-brown in color for common rust. Southern rust pustules tend to be more densely scattered than common rust pustules and more chlorosis (yellowing) around the pustules generally will be observed with southern rust.
Because nearly every corn hybrid is susceptible to southern rust, foliar fungicides are the only management tool available. If corn plants are at the R3 development stage (milk stage) or beyond, then it is less likely that southern rust will cause yield loss; however, on late-planted fields that are not yet at that stage, it is important to continue to scout those fields for southern rust. A southern corn rust observation map for the United States is available through the the IPM PIPE system (http://www.ipmpipe.org/); however, southern rust may be present in counties that are not highlighted on the observation map.