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Corn+Soybean Digest

FAQs On Soybean Rust

Q: How far and fast can this disease spread?

A: Soybean rust dispersal is highly dependent on environmental conditions. Once the pathogen is present, abundant spore production occurs during wet leaf periods of at least eight hours (including extended dew periods) and moderate temperatures of 60-80°F. The spread of the disease within a field can occur quickly. Long-distance dispersal is dependent on wind patterns and weather conditions and is the subject of current research.

Q: How does Asian soybean rust rob yield?

A: The rust fungus starts as a spore that germinates in water on a leaf. The resulting fungal germ tube can enter through stomates or directly penetrate a leaf cell where it continues to grow through leaf tissue.

Initially, infection appears as a light area on the leaf (usually the underside). Eventually, leaf tissue is killed, resulting in lesions.

The lesions reduce yield by destroying photosynthetic area. The fungus also diverts water and nutrients from the soybean plant compounding photosynthetic losses.

Q: How much yield could I lose from rust?

A: Like any other soybean management problem, yield losses from SBR are variable. Yield losses up to 80% have been reported in other parts of the world.

This level of yield loss is rare, but 50% loss is not uncommon during severe outbreaks. It's also possible that under less severe disease outbreaks little or no detectable yield loss may occur.

Q: What can I do if soybean rust is threatening my soybean yield?

A: Properly timed and applied fungicides can reduce yield losses from SBR.

A selection of strobilurin and triazole fungicides has full or Section 18 labels for soybean rust control. It's prudent to include a fungicide application in crop budgets.

Q: Can I cure soybean rust infections by applying fungicide?

A: The strobilurin fungicides prevent new infections and are most effective as a preventative application before infection has occurred. The triazole fungicides can cure early stage rust infections and are preferred as a first application when rust symptoms are visible.

None of these fungicides can cure advanced lesions in a leaf. Both strobilurin and triazole fungicides can be viewed as preventative.

However, these products have a limited period of control and more than one application of fungicide may be required if SBR occurs early in the growing season.

Q: Can I plant a soybean rust resistant variety?

A: No. Although differences in susceptibility have been found among varieties and several genes have recently been identified that have a role in SBR resistance, rust resistant varieties are not currently available.

Q: Will tillage or crop rotation help prevent Asian soybean rust?

A: No. Rust spores are not long lived and this fungus needs live host tissue to survive.

Since the fungus cannot overwinter and each season's initial infection results from windborne spores, crop rotation within a field and tillage are irrelevant to control.

Q: How do I know if my fields need to be treated?

A: Unfortunately, SBR management requires applying control before symptoms are easily observed. Additionally, in the critical early stages soybean rust resembles other diseases.

Unlike soybean aphid, management of SBR is best handled with a preventative approach and on a regional, rather than individual field basis.

Stage of soybeans and weather forecasts may reduce the need to apply fungicides.

Q: Do I need a new sprayer?

A: It depends on your current equipment's ability to deliver sufficient water volumes (20 gal./acre) at adequate pressures. Minimally, you may need to switch to different nozzles.

Controlling SBR with fungicides requires good coverage of the soybean canopy.

This requires small to medium droplets with sufficient water volume and pressure to reach the lower part of the canopy.

Droplet size, water volume and pressure for effective fungicide applications will be much different than the low volume, low pressure nozzles that create coarser, non-drifting droplets desired for application of glyphosate.

Think more along the types of equipment needed with contact herbicides.

Proper sprayer setup becomes increasingly important as the density of the soybean canopy increases.

Q: Where can I get more information as it becomes available?

A: or

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