By Travis Faske, Extension Plant Pathologist
Southern rust was confirmed earlier this week (June 23, 2016) from a commercial corn field south of Eudora in Chicot County. This is the first report of southern rust for the 2016 cropping season, which is about two weeks earlier than in 2015 when it was detected in three counties in central Arkansas. Disease severity and incidence was low (<1%) on corn at R3 growth stage. Given the current and long range forecast of dry weather conditions, the southern rust in this field is unlikely to move much over the next few weeks, so this incidence is not a major threat to the majority of the 2016 corn crop. This report is a reminder to scout and NOT a justification for widespread use of corn fungicides. Below are a few key points on rust identification, conditions that favor southern rust development, and considerations for fungicide use.
Southern rust pustules are circular to oval in shape, small (0.2 to 2 mm long) orange to light brown in color often encircled by a light-green halo (Fig. 1). Several pustules are often found clustered on a leaf when initially detected in the field (Fig. 1). Southern rust sporulates on the upper leaf surface (Fig. 2), while common rust sporulates on both upper and lower leaf surfaces (less on lower than upper). Common rust pustules are elongate, brick red in color and typically found in the lower canopy (3 ft. and lower); however, this year it is common to find it in the upper canopy. Southern rust is almost always found above mid canopy (4 to 5 ft. from ground).
During the growing season, rust spores are spread from infected corn fields progressively northward. Free moisture as dew or light rain is necessary for spores to germinate and infect corn. When conditions favor disease, 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) and extended periods of light rain or heavy dew. In the absence of these conditions infection and pustule development will be much slower.
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 field, and prolonged conditions that favor disease development. It is generally accepted that a fungicide application between VT – R2 when southern rust is detected in the field AND conditions favor disease is the most beneficial time to apply a fungicide to protect yield potential. Yes, fungicides protect yield potential, but do not “add” yield, which is determined by genetic makeup and adaption of a hybrid to a growing area. Depending on corn maturity a fungicide may not be necessary or beneficial to protect yield potential. Given the current weather pattern, corn will be moving quickly from silking to late stages of corn maturity, so be mindful of corn maturity when considering a protectant. Field corn within two weeks (50% starch line) from black layer will not benefit from a fungicide application to protect yield potential.
Of the fungicides labeled for corn diseases, strobilurin fungicides (FRAC code 11) are more effective before fungal infection (early detection in the field – trace amount of disease), while triazoles fungicides (FRAC code 3) are effective before and after fungal infection (post-infection activity reduces severity, but does not “cure” the plant of disease). Many products contain a combination of both strobilurin and triazole fungicides. A list of fungicides and the fungicide efficacy table can be found in the MP154. Many fungicides are very good to excellent for southern rust control, but a higher volume of water (5 to 10 gal/A by air and 15 gal/A by ground rig) will improve coverage needed to protect the upper- to mid-canopy from southern rust development. If you have questions about southern rust please contact Travis Faske at firstname.lastname@example.org.