A number of Iowa State University Extension specialists are at the 2008 Farm Progress Show Aug. 26-28 at Boone in central Iowa. They will give presentations at various times all day for the three-day show. For a listing and the schedule of the different topics you are interested in, go to www.extension.iastate.edu/shows/fps08/presentations.html.
Among the speakers are ISU Extension soybean agronomist Palle Pedersen and ISU Extension corn agronomist Roger Elmore. This has been a very difficult spring and summer for Iowa corn and soybean farmers. Lots of soybeans were planted late. What does that mean for soybean farmers as we think about all the late planted acres?
"It is very important that farmers realize how far behind their beans are in development. The early planted beans are now approaching R6 growth stage in late August. The late planted beans are still at R3 and R4. That means any time a stress occurs that you can manage, you should take care of it," says Pedersen.
Aphids and bean leaf beetles take toll
Specifically, what kinds of things should farmers focus on given this maturity issue? There is the insect issue and the disease issue. Regarding insects it's been a big year for soybean aphids. They appeared very late in Iowa this year, later than they usually strike. The cooler temperatures have caused soybean aphid populations to explode in the northern part of the state.
Last week we were able to find aphids in the northern two thirds of Iowa. Late planted beans can still be a target for aphids when they are moving around.
Another insect that is causing Iowa soybean growers problems this year in August is the second generation bean leaf beetles. Here in late August, it doesn't take many beetles before you are reaching threshold to justify spraying. The threshold last week was around 15 beetles per 20 sweeps of the sweepnet.
"Those two insects—aphids and bean leaf beetles—I am very concerned about this year," says Pedersen.
Watch for late-season diseases on beans
He adds, "If the wet weather continues here in Iowa in late August, we could see more late-season diseases from the south develop in soybeans. We don't have any good data to show what the economic threshold is for spraying with a fungicide on these late season disease. This may be a year where it would be worthwhile to do some kinds of tests with fungicides on your farm.
Do you think we should have any concerns about soybean rust possibly moving into Iowa late in the season? "No, I don't think so," says Pedersen, "not this year. Rust didn't do much down in the South earlier this growing season. The pattern is very similar to the last four years. I will not be surprised if somewhere in Iowa in late September or early October we will be able to identify rust showing up somewhere in Iowa. But like last year, if it shows up that late in the season it should have any economic effect on soybean farmers at all."
What about field areas not planted this year?
There were a lot of acres in Iowa, mainly drowned out areas of fields that were not planted this year. These areas are kind of swampy right now. Are there any things farmers should be concerned about now as they think about cropping plans for next year?
"A lot of people have talked about maybe putting a cover crop in those areas this fall, just to hold the nutrients so we don't get too much leaching from those areas," says Pedersen. "A cover crop would also keep weeds under control. I think it's a good idea to plant a cover crop. But besides that, the weed pressure is the big concern. It may be worthwhile to seed a cover crop just to hold the nutrients and to also hold the potential soil erosion in check."
Southern rust of corn found in southwest Iowa
Farmers in the southwest third of Iowa should be watching for southern rust on corn, says Alison Robertson, ISU Extension plant pathologist.
On August 13, a field in Taylor County was treated; nearly all plants had southern rust lesions. Southern rust is more of a concern than common rust on corn, and positive identification is critical. Key differences between the two rusts are:
- Southern rust pustules are usually only found on the upper leaf surface, compared with common rust pustules that are found on the upper and lower leaf surfaces.
- Southern rust pustules tend to be round, whereas common rust pustules are typically more elongated
- Southern rust spores are orange to orange brown and pustules tend to be more
densely clustered, while common rust has brick red spores and pustules tend to be more scattered on the leaf
- Southern rust is favored by warm (77 to 82°F) and humid conditions while common rust is favored by cooler temperatures
Does it pay to apply a foliar fungicide?
Foliar fungicides are effective against both common and southern rust. Since southern rust usually arrives in Iowa late in the grain-fill period, in most years the corn crop is not often economically affected, says Robertson. However with delayed crop development this year, southern rust is occurring earlier in the grain-fill period, leading to greater potential losses, so scouting is advised.
Southern rust has a life cycle of three to six days (compared with common rust that has a seven to 10 day cycle). That means under favorable conditions the southern rust pathogen can spread rapidly. There is not a set economic threshold, but producers finding southern rust in fields should closely monitor disease development; scout fields every four to five days.
Long story short: Fields in the southwest third of Iowa should be scouted for southern rust now. Fields should be scouted every four to five days and if more than 50% of plants in a field have southern rust symptoms, treatment may be warranted.