By Travis Faske, Extension Plant Pathologist
Southern rust was detected last week (July 13) in Conway Co. near Plumerville and in Jackson Co. near Newport. In both cases the incidence was low (one or two plants) and the severity was less than 1% on a single leaf in the upper canopy on corn at late dough to early dent. This is the first report of southern rust in the 2018 Arkansas corn crop. Earlier this week (July 16), southern rust was detected in Woodruff Co., near Augusta at a similar level of incidence and severity. This “first report” is about two weeks later than normal as southern rust typically arrives in southern Arkansas in early July. This is a reminder to scout and not a justification for widespread use of a fungicide. Given the growth stage of the majority of the Arkansas crop, southern rust is unlikely to have a yield impact.
Dry conditions have suppressed the spread of southern rust as free moisture as dew or light rain is necessary for spores to germinate and cause infection. 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 the disease consist of high 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. In the absence of these conditions, infection and pustule development will be much slower.