Skip to main content

Southern Rust, Bacterial Leaf Streak, and Eyespot Are Developing in Corn

Southern rust was found in Yankton County, bacterial leaf streak (BLS) was found in Brule County, and eyespot was found in several fields and counties the week of July 27, 2020. The three diseases were found at low severity levels. However, continued conducive environment (high humidity, dewy nights and warm temperatures), these diseases could reach yield reducing levels. It is important to accurately diagnose diseases before management decisions are made.

Southern Rust

Green corn leaves with dark orange and red rust spots indicative of southern and common rust.
Figure 1. Southern rust (lower leaf) and common rust (upper leaf) compared. Southern rust forms orange/yellowish lesions that are clustered while common rust has dark red scattered lesions on the leaf surface.

Southern rust being found in corn at VT/R1 in South Dakota at this time is the earliest in the last seven years. This disease can be quite severe and can cause significant corn yield loss under favorable weather conditions. Most hybrids are susceptible to this rust. Scouting is encouraged to determine the need for a fungicide application. Most of the fungicides on the market have good efficacy against this rust.

Southern rust is quite distinct from common rust based on the color and arrangement of lesions on the leaves (Figure 1). Southern rust forms dark orange/yellowish lesions that are clustered on a leaf surface while common rust forms dark red lesions often scattered on the leaf surface. Also, when held against the light, southern rust shows a yellow halo around the lesions.

Bacterial Leaf Streak

Green corn leaf with brown steaks across surface.
Figure 2. A corn leaf with initial bacterial leaf streak symptoms.

Bacterial leaf streak (BLS) is a relatively new bacterial disease of corn in S.D. and can be confused with gray leaf spot, a fungal disease which starts to develop after the tasseling growth stage. Since BLS is a bacterial disease, a fungicide application will not protect the corn plants from infection.

Bacterial leaf streak starts to develop as narrow, wavy, tan to brown, long lesions on corn leaves (Figure 2). As symptoms advance, the lesions may expand and coalesce forming larger lesions which maintain wavy water-soaked margins.

Two green leaves; the left with large, orange to brown lesions (bacterial leaf streak) and the right with small, light brown lesions (grey leaf spot).
Figure 3. A) Bacterial leaf streak lesions. Note the wavy margins with a dark brown halo. B) Gray leaf spot lesions. Note how they are relatively shorter and restricted within the veins of the leaf.

The distinction from gray leaf spot is BLS’ wavy, water-soaked lesions (Figure 3-A). Gray leaf spot forms short, regular (rectangular), tan lesions (Figure 3-B). It is not yet known where BLS inoculum comes from. There are some speculations that the bacteria can survive on seed. Fields that have been found with BLS also had volunteer corn showing symptoms of BLS which indicates that the BLS pathogen can survive on corn residue.

Current BLS management recommendations include crop rotation and crop residue management through tillage where practical. No information is available currently on BLS resistance across hybrids.

Eyespot

Two green corn leaves with initial (A) and advanced (B) symptoms of eyespot. Initial symptoms are small, yellow to brown spots of leaf. Advanced are large brown spots and blotches across entire leaf surface.
Figure 4. A) initial eyespot symptoms. B) advanced eyespot symptoms.

Eyespot symptoms start as small, round, brown lesions (Figure 4). As symptoms advance, the lesions develop a tan center surrounded by a yellow halo. These can coalesce to form large dead portions of the leaf. Eyespot is a fungal disease caused by Aureobasidium zeae. This fungus survives in corn residue and infection is promoted by several hours of leaf wetness from rainy weather or prolonged heavy dew. The spores are splashed onto lower leaves from corn residue or blown by wind to upper leaves.

For corn on corn fields with eyespot starting, a fungicide at R1 is effective against this disease. Most of the fungicides registered for corn in South Dakota are effective against eyespot. Susceptibility to eyespot varies among hybrids. Since the eyespot pathogen is residue-borne, crop rotation and, where practical, residue management through tillage can help reduce the inoculum density. For up-to-date information on what pesticides are best suited for diseases in corn please refer to the latest South Dakota Pest Management Guide: Corn.