Asian Soybean Rust: What to Look For

No one is sure where, when or to what extent Asian soybean rust will develop this year – if at all. But there are some things we can watch to foretell the potential and movement of soybean rust and help prepare to protect fields from this potentially yield-devastating fungal disease.

1. Understand the Conditions

Understanding the conditions that are conducive to an Asian soybean rust outbreak in 2006 will help growers protect their crop. The disease triangle outlines three requisite conditions for plant diseases, like rust, to run rampant:

1. A susceptible host – in the case of Asian soybean rust, soybean crops or an alternate host such as kudzu.

2. A disease-causing organism – Asian soybean rust spores.

3. Environmental or weather conditions that foster disease – moisture and temperature.

If any of these three factors are missing, no plant disease will be found. That was the case in 2005, when Asian soybean rust was first discovered in the U.S. soybean crop, but never developed into a widespread disease outbreak. While there were plenty of soybeans serving as susceptible hosts, the initial spore load was low because frost killed kudzu (another host to the spores) from the Gulf Coast to central Florida. Spores found in traps monitoring the disease, may not have been viable.

2. Watch the Weather

Asian soybean rust development is dependent on weather. Weather conditions promote the introduction of rust inoculums from states in the South. Weather conditions that favor development of the disease during the growing season will determine the severity of soybean rust each year.

Long periods of leaf wetness are needed for Asian soybean rust spore germination, as well as temperatures between 60- and 85-degrees Fahrenheit, plus a high relative humidity of 75 -80 percent. Asian soybean rust has been most destructive in the tropical and sub-tropical regions of Asia, Africa, Australia, and more recently South America.

During the 2005 U.S. growing season, there was a severe drought and temperatures were warmer than normal. When temperatures are above 90 degrees Fahrenheit, Asian soybean rust spore growth and germination stop.

3. Patterns to Note

While no one can accurately predict the impact of Asian soybean rust this year in the U.S., certain weather patterns can help determine if it will be an active rust year. Patterns to watch include:

• Warmer winter temperatures – especially in the South where warmer temperatures during the 2006 winter have allowed some rust spores to survive on rust-infected kudzu.

• Spring temperatures and rainfall – we’re currently watching to see if they are favorable for increased spore load and disease development. Several rainy days with high humidity and increased dew could disseminate new infections.

• An active storm season, especially increased hurricane activity – as is anticipated this year. Because spores are carried by the wind, an active storm season could move spores from southern states or Mexico (the disease was identified in Mexico in 2006) into soybean-growing areas.

4. Preventative Measures

If these conditions play out and Asian soybean rust becomes a greater threat, there are some preventative measures growers can take:

• Monitor the movement of rust in your area and fields.

• Apply fungicides in a timely manner.

• Ensure you get good coverage on the entire plant.

See the backgrounder detailing BASF recommendations on the management of Asian soybean rust for specific timing and application recommendations for Headline® fungicide and Headline SBR fungicide.

When used as part of a crop management plan to achieve Plant Health™ in soybeans, a well-timed application of Headline provides the longest lasting, most effective preventative protection for soybean rust while also improving yields through Plant Health benefits.

5. Lessons from South America

When it comes to predicting what could happen in the U.S., most experts look south to Brazil, where in 2003, ASR was present in 95 percent of the country’s soybean-producing areas. Conditions in Brazil, however, differ from those in the U.S. The climate is tropical with more rain and no winter freezing temperatures to kill spores.

BASF Marketing Manager Jose ‘Zeze’ Munhoz-Felippe, believes U.S. Asian soybean rust conditions will more closely follow the disease-infestation pattern of Argentina rather than Brazil. Felippe joined the U.S. BASF Agricultural Products Group in 2005 from Brazil where he was BASF Manager of Research and Development and Technical Services. U.S. weather and geographical conditions more closely resemble Argentina where rust first surfaced in 2002.

Since 2002, Asian soybean rust has been present in Argentina but losses have been directly correlated to weather conditions during crop development that are conducive to Asian soybean rust spread. In particular, temperatures that are not too hot (<90º F) with frequent rainy days. The weather conditions are a clear factor determining the spread and impact of Asian soybean rust in Argentina. This is in contrast to Brazil where Asian soybean rust has been very aggressive every year due to ideal weather conditions.