Southern rust

Common rust

Common rust

Goss's wilt

August 8, 2008

Corn Disease Update

Southern Rust

Southern rust (Figure 1) has now been confirmed in Thayer, Clay, and Hall counties and is probably not limited to those areas. Optimal temperatures for southern rust development are 77-82°F. Drier conditions will help slow the spread of southern as well as common rust, but irrigation and dew provide enough moisture to support infection.

Common Rust

Common rust (Figure 2a) continues to increase across the state, particularly in southwest Nebraska. Like southern rust, common rust needs moisture for spore germination and plant infection, but the optimal temperature for common rust development is 61-77°F, slightly lower than that of southern rust. Pustule development can overlap gray leaf spot lesions (Figure 2b), making it difficult to diagnose both diseases. For more information about how to diagnose rust diseases, see the UNL NebGuide, Rust Diseases of Corn in Nebraska.

Goss's Bacterial Wilt and Blight

After seemingly disappearing from the state for almost 20 years, Goss's wilt and blight (Figure 3) has increased in incidence during the past three to five years. Most of the recent disease outbreaks have occurred in western Nebraska, northeast Colorado, and southeast Wyoming. However, this year Goss's wilt is developing in counties where it hadn't been reported in recent years, such as Platte, Antelope, Hitchcock, Hayes, Phelps, and Red Willow. Increased incidence of the disease can probably be attributed to the frequent hail storms earlier in the growing season that created wounds, which are required by this pathogen for infection. For more information about this disease, including tips for diagnosis, see the UNL Extension NebGuide, Goss's Bacterial Wilt and Leaf Blight of Corn.

Because this disease is caused by bacteria, it can NOT be managed with foliar fungicides. And, since it can be difficult to diagnose and easily confused with other fungal diseases, an accurate diagnosis is essential for effective disease management.

More information about the diseases of Nebraska crops is available at the UNL Extension Plant Pathology Web site, Plant Disease Central. The site also includes sample submission forms and information on how to submit samples to the UNL Plant and Pest Diagnostic Clinic.

Tamra Jackson
Extension Plant Pathologist