 |
Early rust detection
critical to minimize losses
Early identification of Asian soybean rust is critical so chemicals can be sprayed in time to prevent or reduce yield loss.
However, identifying the disease is not easy. Even rust experts have a hard time recognizing the fungus.
To identify the infection in soybeans, Greg Tylka, Iowa State University plant pathologist, says the key is to look at low-lying areas and down in the soybean canopy.
The logic of scouting low areas in fields is Asian soybean rust likes cool, wet environments for its development, he notes.
The infection could be further along in those environments compared to other field areas.
The entire soybean field should be scouted in a standard pattern — for example, a "W." Areas of a field that have distinct yellowing or browning of leaves should be scouted because that is sign of infection.
When you are in a good location within a field, the next step should be to look low in the soybean canopy.
Asian rust generally develops there first and works its way to the top of the plant. That is because the lower part of the canopy is more humid and cooler, providing an ideal environment for infection.
Leaves with yellow spots is a first sign of infection.
"Sometimes, if you hold the soybean leaf up, the yellow areas become more transparent," Tylka says.
While the spotted yellowing is visible on the top of leaves, the underside likely will have tules or pimples.
Further in its development, rust develops tan or brown lesions or spots, starting along the leaf veins.
As the pustules develop on the underside of the leaf, they will have a round opening, Tylka notes. The round opening is key to identifying rust. Other diseases also will produce pustules that will crack open and not have round opening.
To see the pustules, it is generally recommended to use at least a 10X magnifying lens.
With the scouting that will likely occur this summer for Asian rust, Tylka fears many farmers will mistake other soybean leaf diseases for it.
Other soybean leaf spot diseases generally do not cause economic harm.
"There are a lot of leaf spot diseases in soybeans," Tylka says.
Some diseases that look similar to Asian soybean rust are bacterial pustule, brown spot, bacterial blight, frogeye leaf spot, cercoporea leaf blight and downy mildew.
However, there are some key differences that can be observed to tell them apart from Asian rust:
= Bacterial pustules do not contain spores, and the pustule openings are cracks, instead of round openings. The pustules from this disease are not associated with lesions on the top of the leaf.
= The key differences between brown spot and Asian rust are the spots are more angular, and there no pustules on the underside of the leaf.
= Bacterial blight does not have pustules on the underside of the leaf, the spots are more angular, and the leaves become ragged.
= With frogeye leaf spot, the spots are larger and have dark, defined edges. Frogeye leaf spot does not produce pustules on the leaf underside.
= Cercoporea leaf blight discolors only the upper leaves and creates few spots and no pustules on the underside of the leaf.
= Downy mildew has larger spots than Asian rust. That disease also does not produce pustules on the leaf underside. Some fuzzy fungal growth can be seen with downy mildew.