Asian Soybean Rust rust_field_view

Grower Alert: Getting Ahead of Asian Soybean Rust

As the fine article on the UW Extension website observes “Asian soybean rust is caused by the fungus Phakopsora pachyrhizi. The soybean rust pathogen has been moving progressively westward from its center of origin in China and has now reach North America. There is much speculation and apprehension on how soybean rust will develop and impact soybean production in the U.S.”

Asian Soybean Rust could have a serious impact on Wisconsin soybean crops. That’s why it’s essential to know the signs of the disease—especially since some symptoms of the blight are common to other infestations—and to keep a close eyes on your crop.

To help look for the disease, Wisconsin uses sentinel plots as an early warning system. This is part of a national sentinel plot monitoring effort made up of over 30 states. The sentinel plot network uses trained observers to look for signs and symptoms of rust. Most Wisconsin soybean growers will have a sentinel plot within their county or in a border county. These sentinel plots are monitored through a combined force of UW county extension agents, agriculture research station staff and UW-Madison campus research staff. You can see the sentinel plot observations for yourself here on the USDA Pest Information Platform for Extension and Education (PIPE) website.

Will this blight show up in Wisconsin? That’s the million dollar question and it depends on three key factors:

  1. Whether soybean rust occurs during early spring and summer in the Gulf coast region. This determines the amount of spores available to blow northward.
  2. Climate conditions in July—August. This determines whether conditions are favorable for rust to develop
  3. The northward movement of soybean rust carried by the weather and by “green bridging.”

(Unlike with Tospovirus, the soybean rust pathogen has not been shown to be transmitted by seed. So you needn’t be reluctant to purchased seed grown in areas where rust was found in the previous growing season).

While the sentinel plots are a great precautionary step, there is an active measure you can take as well and it doesn’t cost a dime: scout your crops.

As the UW Extension points out “Accurate and timely diagnosis of soybean rust is critical to achieve control of soybean rust, especially if fungicides are involved in the management plan. If national monitoring efforts indicate that rust spores are moving northward, soybean growers and crop advisors should plan to scout frequently for rust. Scout these areas first:

  • Early-planted fields
  • Early-maturing varieties
  • Low-lying or protected fields with prolonged dew periods
  • Fields with early canopy closure”

There’s a two-step process for scouting.

  • Check the lower leaves from the main stem in the lower canopy first. These leaves are most likely to show symptoms first. Why? These leaves are likely to stay wetter longer, conditions the rust prefers. Use a 10-20X hand lands for viewing and do this on site to avoid spreading spores. Backlighting might help you see.
    • Initial symptoms include small, gray spots, often on the underside of leaves or along the veins. You might also see them on petioles, stems and pods. These spots can grow and change in color from gray to tan, reddish brown or black. It’s vital to know that these symptoms are NOT exclusive to rust (You might be seeing brown spot blight or downy mildew. If you observe symptoms, you need to verify the cause. See Common Soybean Leaf Diseases and Asian Soybean Rust (pdf) for a good overview of look-alike diseases.
  • If you see symptoms, look for signs (sporulation) of the virus pathogen. You know the gray/tan lesions you were looking for, they mature to to form pimple-like structures called pustules on the lower leaf surface. Active pustules contain the the powdery tan spores of the rust fungus.
    • In the early stages, these pustules look like miniature volcanoes topped by with a pore, but there is no yellow halo around the pustule. In later stages these pustules burst and release spores to spread the disease. Use your 10-20x hand lens to spot the pustules. It is critical to observe sporulation in order to diagnose rust. This is the only definitive means for diagnosis.
    • For an excellent printable online diagnostic guide of Asian Soybean Rust, look here.

Should it reach our state, Asian Soybean Rust can certainly negatively affect yields. So stay vigilant, scout your fields and ask questions if you see something that bothers you.