Southern rust was detected last week (July 7, 2014) near McGhee, Eudora, and Rector, AR. This was the first report of 2014 season and one week earlier than 2013, when it was found near Caraway. In the south, hot spots had ~ 50 pustules/ leaf (Fig. 1) on a few leaves in the field. The current weather forecast of scattered showers this week in much of the state will provide good conditions for rust development. Given the favorable conditions and how far the spores can spread on prevailing winds, southern rust will likely be found this week in other fields, possibly a county away from these reports. This alert is a recommendation to scout fields because rust has been confirmed in the state and determine if a treatment is warranted rather than a reason for wide spread treatment of corn fields. Below are some key points on southern rust identification, conditions that favor disease, and management considerations.
A key point in disease control is accurate disease identification and southern rust looks very similar to common rust to the untrained eye. Here are a few fundamental on southern rust identification compared to common rust. Southern rust pustules are circular to oval in shape, small (0.2 to 2 mm long) light cinnamon brown to orange in color often encircled by a light-green halo. Typically southern rust sporulates on the upper leaf surface in contrast to common rust that sporulates on both 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. and lower) and southern rust at mid canopy (4 to 5 ft. from ground). Common rust is not a yield-limiting disease and should not be the sole reason to apply a fungicide.
Rust spores are spread from infected corn fields progressively 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. Conditions that favor disease consist of high temperatures (80 to 90+ F), high relative humidity, and heavy dew.
Fungicides are effective at suppressing southern rust; however, there is no economic threshold for a fungicide application. Factors to consider consist of corn stage of growth, yield potential, threat or observation of southern rust in the area, and prolonged conditions that favor disease development. It is generally accepted that a fungicide application at VT/R1 when southern rust has have been observed with good yield potential may be the most beneficial at suppressing rust development and protecting yield potential; however, additional application may be needed for season long crop protection. Field corn within two weeks (50% starch line) from physiological maturity (i.e. black layer) is very unlikely to benefit from a fungicide application.
Among the fungicides labeled for corn diseases, strobilurin fungicides (FRAC code 11) are more effective before disease development (early detection), whereas triazoles fungicides (FRAC code 3) are effective before and after disease development. A list of fungicides and fungicide efficacy table can be found in the MP154. Many fungicides are good to excellent for southern rust control so consider economics (i.e. fungicide cost) if a fungicide is warranted. A higher volume of water (5 to 7 gal/ac) will improve coverage needed to reach southern rust in the mid-canopy.