Southern rust was detected earlier this week (July 9) in Woodruff Co. near Augusta. Disease incidence and severity was low (one leaf on two plants with 30-40 pustules/leaf) on corn at blister growth stage. The first or second week of July is when southern rust is typically detected in Arkansas, so this “first report of 2019” is on time.
This announcement is a reminder to scout, and not a justification for widespread fungicide use. The current counties where southern rust has been detected can be monitored on the NEW corn ipmPIPE website.
Dry conditions will suppress the spread of southern rust as free moisture (dew or light rain) is necessary for spore germination and infection. When conditions favor disease, symptoms appear about 3 to 6 days after infection, and by 7 to 10 days, the pustules rupture to release rust spores. Conditions that favor disease: warm/hot temperatures (morning low of 75°F and daytime high of 93°F + 4 hr of consecutive leaf wetness) and extended periods of light rain or heavy dew. When these conditions are not met disease development will be much slower.
Fungicides are effective at protecting corn yield potential, but given the price of corn, many are considering the benefit of yield protection before applying a fungicide.
The following table is a guideline on the benefit of a corn fungicide to protect yield potential at various growth stages with the assumption that southern rust is detected in the field and conditions favor disease development (Figure 1). See MP 154 for fungicides efficacy to control southern rust in Arkansas.