Southern rust was confirmed this week (June 18, 2012) on corn in Arkansas. This confirmation is about one week earlier than that of 2010 and nearly one month after Southern rust was found this year in Louisiana. Southern rust was confirmed in four fields near Pine Bluff, AR. Rust pustules were found in the mid to upper canopy (~ 4 to 5 ft from the ground) with a disease severity of one to two pustules per leaf. Furthermore, disease incidence was approximately one of every twenty plants in these fields. Common rust was observed in all fields on the oldest leaves near soil and northern corn leaf blight was found on one plant in a field. Though microscopy is often needed to confirm early infection of Southern rust, symptoms can be helpful to different southern rust from common rust of corn.
The fungus that causes southern rust is Puccinia polysora. Pustules are light cinnamon brown to orange in color, smaller (0.2 to 2 mm long) on the upper leaf surface and circular to oval in shape (Fig. 1). Note the lack of sporulation on lower leaf surface (Fig. 1). Southern rust pustules can be found on leaves, stalks, and husks on ears of corn. Spores from these pustules (urediniospores) are yellowish to golden in color and elipsioid in shape (Fig. 3). Questionable samples should be submitted to the Plant Health Clinic through county extension offices.
Common rust is caused by the fungus P. sorghi. Pustules of common rust are elongate, golden brown to cinnamon brown in color and found on upper and lower leaf surface (Fig. 2). Spores are chestnut brown or golden brown and obovoid in shape (Fig. 3).
Rust spores are windblown from infected corn leaves and are blown progressively northward during the growing season. Free water as dew is necessary for rust spores to germinate and infect a corn leaf. Symptoms appear about 3 to 6 days after infection and by 7 to 10 days the pustules may rupture to expose mature rust spores. Based on the pustule development on the corn near Pine Bluff, the infection was probably 7 to 10 days ago. Conditions that favor disease consist of high temperatures (80 to 90 F), high relative humidity, and frequent rainfall.
Resistance is limited thus producers commonly rely on fungicides to manage this disease. Fungicides used to manage this disease are from the triazole or strobilurin fungicide group (Table 1). A common action threshold is only a few pustules per leaf though it can vary depending on host growth stage and environmental conditions.