Asian Soybean Rust Profiled
Soybeans are big business in the U. S. According to the United States Department of Agriculture (USDA)’s Prospective Plantings report, 77 million acres of soybeans will be planted in 2011. If realized, this would be the third largest planted area on record. The top producing states are Iowa, Illinois, and Minnesota. Naturally, anything that is likely to put a significant dent in soybean yield is worrisome. Asian soybean rust has the capacity to do just that.
What is soybean rust?
Soybean rust is a fungus that targets the foliage of the soybean plant. Previously prevalent in Taiwan, Thailand, Japan and India, the disease has since found its way to South Africa, Paraguay, Brazil and Argentina. There are two types of fungi that cause soybean rust. Phakopsora pachyrhiz is the most aggressive form of soybean rust and was first identified in the U.S. in 2004 in Louisiana.
How does soybean rust spread?
The two fungi that cause soybean rust have a wide spectrum of host plants. Of these hosts, Kudzu has been identified as the most important for over wintering the disease. The fungal spores from the parasites are freely distributed over long distance by the wind; rain splashing up into the leaves will also spread the spores more locally. The spores generally require more than eight hours of continuous wet leaves or humidity in the 75% to 80% range. Temperatures over 86°F will slow the progress of the disease. In ideal conditions, new spores will be released around 10 days after the plant has been infected.
How can I identify soybean rust?
Focus your attention on:
- Early maturing varieties of soybeans planted early in the season
- Sheltered or low lying fields with longer periods of dew
- Fields that have early canopy closure
Look under the lower leaves deep in the canopy with a hand lens, as this is where the disease first manifests itself. You are looking for approximately one eighth inch diameter irregular spots, which may be grey to tan initially. Later in the development of the disease these spots will increase in size and turn into reddish brown or black lesions.
Mature lesions consist of small pustules surrounded by a slightly discolored deadened area with masses of spores on the lower leaf surface. Lesions are scattered within yellow areas that appear see-through (translucent) if the affected leaves are held up to the sun.
Once pod set begins on soybean, infection can spread rapidly to the middle and upper leaves of the plant. After infection, pimple-like structures or pustules can be seen after about 10-14 days. The pustules produce masses of spores and spore production may continue for weeks. Spores are easily spread by the wind. Soybean plants are susceptible to soybean rust at any stage of development, but symptoms are most common during and after flowering. The disease usually starts within the low to mid canopy and moves up the plant.
In the early stages of soybean rust diseases it is easy to know of spores on your feet or clothing. It is wise to wear disposable spray suits and change and wash the clothing thoroughly before entering another field.
What are the effects of soybean rust?
Soybean rust attacks the leaves of the plant leading to early defoliation, early maturation and resultantly lower yields. The soybean rust infection is known to affect pod filling, seed numbers and the weight of the seeds. Yield loss is difficult to predict in the U.S., but other countries have reported yield losses of anything from 10% to 90%.
How can I prevent or treat soybean rust?
Until there are any soybean rust disease resistant plant varieties the only real effective weapon against the soybean rust disease is the application of fungicides. Products such as, Folicur, Proline and Stratego are all potential weapons in your arsenal for lessening the effects of soybean rust.
The effectiveness of the application of a fungicide, whether it is a protectant variety or a curative variety, is largely down to timing. The fungicide will be most effective if it is applied before the soybean rust spores arrive on the leaves. However, spray too early and the fungicide protection may dissipate prior to spore infection. Spray too late and again the fungicide protectant will not be effective and the curative fungicide will be limited in its activity. Early detection of soybean rust is therefore vital if you intend to apply any fungicide.