southern rust corn disease
WILL RUST RETURN? If southern rust returns to cause problems in the Midwest in 2017, it will be because it blew in from outside the region. The pathogen doesn’t overwinter in the Midwest.

10 facts about corn rust

Corn Illustrated: Southern rust hammered cornfields in 2016 in areas where farmers rarely worry about it.

Flip through the 2017 DuPont Pioneer Agronomy Sciences Research Summary and one article may catch your eye. It’s simply titled "Southern rust of corn." If you were one of the unlucky people whose corn was nailed by southern rust late last year, you will be drawn into the story immediately. Even if your fields escaped, you’ll still find the report useful for managing corn disease in the future.

Steve Butzen, agronomy information consultant for Pioneer, prepared the report. Here are 10 facts about rust in corn any producer ought to know.

1. Southern rust is a fungal disease. So is common rust, which occurs more frequently in the Corn Belt. However, when southern rust occurs, it is often more destructive than common rust, Butzen says. Yield loss estimates in southern Indiana in 2016 exceeded 50 bushels per acre, according to ag retailers.

2. Southern rust is favored by high humidity and temperatures in the 80s and 90s. That’s why it’s more frequent in Southern states. But when the wind blows spores into the Midwest in a summer like 2016, it can become an issue. Common rust prefers cool to warm and moist conditions, with temperatures of 60 to 77 degrees F.

3. Southern rust can spread rapidly. New infections may occur every seven days, Butzen reports. Individual fields can be damaged very quickly, and epidemics can spread over large areas. The southern rust outbreak in 2016 in the Midwest came primarily in late August and early September.

4. Fungicide applications may be warranted for southern rust. These can help minimize crop damage. Fields where farmers didn’t apply fungicides to save costs tended to get hit hardest in 2016.

5. Less photosynthesis occurs after leaf damage from southern rust. There is less functional leaf area after infection, Butzen explains. Photosynthesis is reduced, and less sugar is produced.

6. Southern rust can weaken cornstalks. Plants will use stalk carbohydrates to fill kernels once they can’t make enough sugar to keep up. Stalks are weakened, and the potential for stalk rot rises.

7. Where and when infection occurs matters. If the infection is confined to lower leaves, yield losses will be lower. In hard-hit areas in 2016, infection raced through the entire plant. 

8. Planting date can impact yield loss from southern rust. Later-planted fields are at higher risk to southern rust damage, Butzen notes.

9. Rotation and tillage don’t help control southern rust. The disease spores don’t overwinter in the Midwest. That’s the good news for growers there. The bad news is a new batch can blow in the following season and move into the region.

10. Corn breeders screen for resistance to southern rust. Pioneer corn breeders screen both inbreds and hybrids for southern rust, Butzen says. Hybrids in the commercial lineup carry a rating for rust resistance, from 1 to 9, with 9 being most resistant on Pioneer’s scale. Most hybrids in the current lineup rate between 3 and 5, Butzen reports. That’s because genetic resistance to southern rust is limited.

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