Soybean rust has the potential to reduce yield by as much as 50% when environmental conditions are favorable. That fact alone is reason enough for most U.S. soybean farmers to want to know more about this foliar disease. Soybean rust is a relative newcomer to American fields, with the first recorded case occurring in 2004. Since then, Phakopsora pachyrhizi, the fungus that causes soybean rust, has infected more than 95 species of plants and spread across the southeastern U.S. You can track the annual infection reports of soybean rust at: https://soybean.ipmpipe.org/soybeanrust/historical-end-of-season-maps/
📸: @International Institute of Tropical Agriculture via Flickr/Featuring visible soybean rust symptoms.
Soybean rust (sometimes referred to as Asian soybean rust) is a difficult disease to control. The fungus that causes soybean rust produces spores that easily disperse in the wind, making it nearly impossible to prevent the fungal pathogen outright. P. pachyrhizi also has an abundance of hosts. Apart from soybeans, which are the second most cultivated crop in the U.S. by bushels, the fungus also feeds on jicama, yam bean, snap and dry bean, yellow lupine, cowpea, and kudzu, an invasive weed that has already spread to 32 states. These two factors, combined with soybean rust’s ability to limit yield in its namesake crop, have already led both public universities and private companies to begin developing rust-resistant varieties.
When left untreated, soybean rust causes yield loss due to premature defoliation—the loss of leaves. This damage inhibits photosynthesis, slowing or even stopping the process of converting sunlight into energy. Modern management of soybean rust largely relies on tracking movement and the use of fungicides when detected in your area.
📸: @Alfribeiro via Canva/Featuring damage caused by soybean rust lesions.
Soybean rust is the name given to a fungal disease that affects a wide range of plants, but it is primarily known in the U.S. for its impact on soybean crops. The fungus P. pachyrhizi invades and feeds on plant cells, causing damage that results in soybean rust. The effect on soybean plants is a growing reddish-yellow discoloration that can make it look as though the plant is rusting.
P. pachyrhizi cannot survive outside of a living host, so all soybean rust infections begin at the site of another infected plant. When conditions are warm and humid, the fungus produces two types of spores: urediniospores and teliospores. Only urediniospores are known to cause infection. They disperse in the wind, sometimes traveling great distances, and typically settle on the leaves of maturing soybean plants. Soybean rust epidemics are most commonly observed during extended periods of rain and in temperatures between 60 and 82 F.
📸: BASF internal photograph/Featuring a demonstration graph of the soybean rust life cycle.
While most “rust” pathogens enter plant tissue through small stomatal openings and penetrate cells after they’ve entered stems or leaves, soybean rust urediniospores penetrate plant cells directly, which may help explain why this fungus can infect such a broad range of hosts. Spores require 6 to 12 hours of leaf wetness to germinate and penetrate plants.
Soybean plants can be susceptible to rust at any growth stage, but infections are often observed during critical soybean growth stages. Infected plants often produce fewer pods and seeds, and seeds are often lower in weight as well. Yield losses can vary greatly depending on environmental conditions.1
Once a soybean rust infection takes hold, it begins to form gray-green, tan-to-dark-brown, or reddish-brown lesions that burst through the plant tissue. These spots or bumps tend to be angular and 2–5 mm in diameter. These lesions most commonly appear on leaves but can also be found on petioles, pods, and stems.
Soybean rust is a multicyclic disease. Each infected plant has the potential to produce more spores, which can be carried on the wind to infect other plants. Every lesion has the potential to launch spores within 7 to 14 days. Severe weather events like hurricanes are thought to be able to rapidly disperse spores across a wide area. When these spores come into contact with soybean leaves, pods, stems, or other plant material, they penetrate the tissue and begin feeding. The energy the fungus gains from the plant’s nutrients helps it grow and repeat its life cycle.
Soybean rust lesions lead to defoliation (damage to leaves) in soybean plants. The more extensive the defoliation, the greater potential for yield loss. When fungicides are applied after 10% disease severity (10% of leaves have rust), the effectiveness of the fungicide is significantly reduced. One study demonstrated yield reduction of 17% when fungicide application was delayed until one week after soybean rust was first detected. The yield reduction rose to 38% if treatment was delayed for two weeks.2 Thankfully, soybean rust does not overwinter in freezing temperatures.
📸:@Forest and Kim Starr via Wikipedia/Featuring Raised lesions commonly found on the underside of leaves.
Timely scouting of soybean rust is a core component of successful management. As mentioned above, crops can suffer increasingly severe yield reduction the longer the fungus goes untreated.
To spot the fungus, begin by searching the plants’ lower, first leaves. Look for mosaic-like areas beginning to appear at or around the flowering stage. Lesions may appear anywhere on the plant but are most commonly visible on the underside of leaves. More mature plants may begin to show signs of damage as pods set, and the infection can spread to the middle and upper leaves of the plant.3
Be aware that early signs of soybean rust infection can be easily confused with bacterial pustules, bacterial blight, and brown spot. These diseases also cause raised spots on the underside of soybean leaves. Experts require a hand lens or dissecting microscope to distinguish between them.
Fungicide applications applied between the R3 and R5 soybean growth stages provided the most consistent yield protection. Fungicide resistance should be a consideration with diseases of this type. Be sure to use more than one mode of action if you plan to apply several fungicides in a growing season. If soybean rust has not been identified within 100 miles of your field, preventive measures are unlikely to be needed. However, regular scouting should be done throughout the growing season to monitor for foliar diseases.4
📸:@fotokostic via Canva/Featuring an example of a sprayer applying fungicide to growing soybeans.
Selecting the right fungicide and applying it within the right window of time is pivotal. We recommend these treatments, along with some key considerations:
Priaxor® Fungicide offers effective control and consistent yield protection when applied between the R2 and R4 growth stages before disease development. The timing and number of applications needed depend on the environmental conditions, including how early in the season the infection sets in, the amount of rainfall, and the average temperature.
Fast-acting and long-lasting Revytek® Fungicide gives you proven protection without trade-offs. It should be applied to soybean rust prior to disease development in 14-day intervals, generally between the R2 and R4 growth stages.
It is also worth noting that traits with genetic resistance have been identified for soybean rust; however, it has not been an effective long-term strategy for countering yield loss.
While we may not be able to keep soybean rust out of our fields, we can stay informed, practice proactive management strategies, and look forward to advancements like rust-resistant varieties. Tools are available to help you monitor the pathogen’s footprint, like this integrated pest management map.
For additional guidance on fungicides, management, and prevention, remember to consult your seed retailer, a nearby extension office agent, or a seed company professional like your regional BASF representative.
📸:@Maksud_kr via Canva/Featuring healthy soybeans thriving after proactive management.
See more examples of controlling fungus and other pressures for soybeans on Grow Smart® Live:
Before mixing components, always conduct a compatibility jar test. Always consult respective product labels for specific mixing instructions. The most restrictive label applies.
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Endnotes
This content is being brought to you in partnership with Grow Smart® Live and contributing guest authors. BASF provides the information in this article as a service to its customers; however, the views expressed by guest writers are their own and do not necessarily reflect the views of BASF.
Always read and follow label directions. Grow Smart, Priaxor, and Revytek are registered trademarks of BASF. Copyright 2024 BASF Corporation. All rights reserved.
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