Watch Out for Defoliation From Asian Soybean Rust

Asian soybean rust, caused by the fungus Phakopsora pachyrhizi, is an aggressive soybean disease that destroys leaf tissue and inhibits photosynthesis, resulting in premature defoliation, early maturation and reduced yields. The Asian soybean rust pathogen affects legume crops but does not affect corn and other grain crops. This disease is more common in the Southern soybean growing regions of the United States. If established early under favorable weather conditions, the disease can severely damage an entire soybean crop.

Asian soybean rust
Asian soybean rust most commonly appears on the leaves, starting in the lower canopy. The small, water-soaked lesions, which turn color from gray to tan or brown, can also develop on petioles, pods and stems.

Identification and Lifecycle

The first symptoms of Asian soybean rust are small, water-soaked lesions that gradually increase in size and turn color from gray to tan or brown. The symptoms spread quickly from the lower leaves to the middle and upper parts of the soybean plant as it matures and sets pods. Although lesions may be found on petioles, pods and stems, they are primarily found on soybean leaves.

Favorable environmental conditions for Asian rust development occur when temperatures range from 54 to 84 degrees F, with humidity above 90 percent for 12 hours or more. Germination of spores and soybean plant infection requires six hours of continuous moisture, such as rain or dew, on the leaves. At that time, the germ tube penetrates the leaf and begins to grow within the plant tissue. The fungus grows as a parasite inside the leaf, taking in water and nutrients. Spores begin to produce on the lower sides of leaves about seven days after infection. As more spores are produced, they rupture through the leaf surface, causing a lesion called a pustule.

The Asian soybean rust lifecycle is short – about nine days. Once infection is established in a field, soybean rust can rapidly spread. For up to 20 days, a single pustule can generate hundreds of spores. In this way, millions of spores may be produced in an infected field.

Spores exposed on the leaf surface are transported by wind and can travel long distances at high elevations. Each spore that lands on a susceptible host, such as a soybean plant, can cause a new infection. Although spores may settle out of the air in still conditions or be destroyed by rain, the number of spores flying over a soybean field may nevertheless be quite significant, and infection can become widespread.

The Asian soybean rust fungus does not survive in crop residue. Spores may survive for about 40 days, but they are unlikely to survive Midwest winters. The only place the fungus is thought to survive winter in North America is in southern regions, where green foliage exists in host plants.

Crop Damage

As multiple lesions form on soybean leaves, infected areas turn yellow and, eventually, leaves drop. Asian soybean rust begins on the lower canopy and, if left untreated by fungicides, spreads until all plant leaves are infected. In severe cases, diseased plants may completely defoliate. Loss of leaf tissue results in fewer and smaller soybean seeds.

Yield reduction depends upon when disease infection begins and how quickly it spreads. Research shows that 80 percent yield loss or more can occur with ideal environmental conditions for Asian soybean rust infection.

Managing Asian Soybean Rust

Farmers have several options to manage Asian soybean rust, including variety selection, scouting, cultural practices and fungicides. These best management practices also help minimize the potential for fungicide resistance. Following seasonal reports on disease occurrences from county Extension agronomists is another helpful practice.

Variety selection

All U.S. soybean varieties are thought to be susceptible to Asian soybean rust. Varieties differ in susceptibility, and selection can be used to help protect against the disease. The relative maturity of a soybean variety influences the onset of Asian soybean rust. Symptoms appear earlier in the growing season with earlier-maturing varieties.


Timely diagnosis is needed to control soybean rust. National monitoring efforts in many states, such as by local Extension agents, help indicate where Asian rust is occurring. If reports indicate rust infections, soybean growers and crop consultants should plan frequent scouting. Experts recommend scouting these areas first:

  1. Early-planted fields
  2. Early-maturing soybean varieties
  3. Low-lying or protected fields with extended dew periods

According to University of Wisconsin Extension, symptoms should be scouted with a 10x or 20x handheld magnifying lens. Backlighting helps in detecting early symptoms. Check leaves on the lower canopy, where leaves stay wet longer. Look for gray spots on the undersides of lower leaves and along veins. The spots can also been seen on the upper sides of leaves and on petioles, stems and pods. Refer to disease identification guides, as Asian rust symptoms may be similar to other soybean diseases such as brown spot and bacterial blight. Also look for emerging pustules, which resemble mini volcanoes with a pore.

Cultural practices

Practices such as residue management, cultivation and crop rotation have little effect on Asian soybean rust.


Fungicide applications offer the best management option. Early application, before infections have a chance to develop, is recommended because preventing an Asian soybean rust epidemic is much more effective than trying to stop one. Multiple applications may be needed, depending on how early in the season rust develops and if weather conditions are favorable (humidity, optimum temperatures and extended dew periods.)

Spray-droplet size is important to provide adequate coverage of upper and lower leaves. Smaller droplets can provide more thorough coverage, but smaller droplets are moved more easily by wind currents and inversion layers. Medium spray droplets – 250 to 300 microns – provide better coverage. Always used full labeled rates and follow label instructions.

Bayer solutions

Also, timely foliar fungicide applications help protect soybean plants from fungal diseases like Asian soybean rust through the season. For farmers, a fungicide decision is a matter of considering production needs, past history in the fields, commodity prices, proper timing and risk management.

Delaro® fungicide offers an unmatched broad-spectrum control. Its advanced formulation delivers best-in-class dual mode of action residual and improved plant health. In soybeans, the ideal time to first apply Delaro is prior to disease development, between R2 and R5, with applications repeated as necessary. Disease-susceptible cultivars and seed soybeans are most likely to benefit from a Delaro application. For cost efficiency, Delaro can be tankmixed with insecticides and applied in the same trip across the field.

Before applying any fungicide, please read the entire label for the best possible results and to confirm that the product is effective on the disease you need to control. Not every product is suitable for every situation, and correct application technique willensure the best results.

©2019 Bayer Group. Always read and follow label instructions. Bayer, the Bayer Cross, Delaro and Stratego are registered trademarks of Bayer Group. Not all products are registered in all states. For additional product information, call toll-free 1-866-99-BAYER (1-866-992-2937) or visit our website at Bayer CropScience LP, 800 North Lindbergh Blvd., St. Louis, MO 63167. SE0418MULTIPB347S00R0

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