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Southern rust of corn makes season’s first appearance in Arkansas

• By Travis Faske and Terry Spurlock •

Southern rust was detected last week (July 6) in Arkansas, Jefferson, and Lincoln counties, Arkansas. Disease incidence and severity was low (one leaf on two plants with 30-40 pustules/leaf; Fig. 2) on corn at blister to milk growth stage (R2-R3).

The timing is about right for southern rust — for the past few years, it has been detected in early July. This 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 Corn ipmPIPE website. Currently (July 12) southern rust has been reported in Alabama, Arkansas, Georgia, Louisiana, Mississippi and Texas (Fig. 1).

IPM map

Figure 1. Distribution of southern rust (red colored counties) were currently (July 12, 2020) positive.

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 three to six days after infection and by seven to 10 days the pustules rupture to release rust spores.

Conditions that favor disease: warm/hot temperatures (morning low of 75 degrees Fahrenheit 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.

southern corn rust table

Figure 2. Southern rust pustules on upper corn leaf surface and benefit of a fungicide to protect corn yield potential in fields where southern rust is detected.

The “timing table” is a guideline on the benefit of a fungicide to protect yield potential at various corn growth stages with the assumption that southern rust is present in the field and conditions favor disease development (Figure 2). See MP 154 for fungicides efficacy to control southern rust in Arkansas.

Please contact tfaske@uaex.edu for comments or questions.

Drs. Travis Faske and Terry Spurlock are a University of Arkansas plant pathologists.