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Southern rust of corn makes its first appearance of the 2020 cropping season in Arkansas
Author: Travis Faske, Extension Plant Pathologist

By Travis Faske and Terry Spurlock, Extension Plant Pathologists

Southern rust was detected last week (July 6) in Arkansas, Jefferson, and Lincoln counties.  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 as for the past few years has been detected in early July.  This newsletter is a reminder to scout, and not a justification for widespread fungicide use.  The current counties were southern rust has been detected can be monitored on the Corn ipmPIPE website.  Currently (July 12, 2020) southern rust has been reported in Alabama, Arkansas, Georgia, Louisiana, Mississippi, and Texas (Fig. 1).

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 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.

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.

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 “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.

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