posted on April 26, 2006 15:07
Daniel S. Egel, Extension Plant Pathologist,
Southwest Purdue Agricultural Center
As organic soybean growers go about the business of getting ready for the 2006 season, some thought should be given to the possibility of soybean rust (SBR) causing problems for Midwest growers. While worrying is not advised, there are a few items that growers should accomplish prior to the arrival of SBR season:
- Arrange to have on hand an organically labeled fungicide, just in case.
- Know the details for applying fungicides to soybeans, i.e., the appropriate spray pressure, nozzle type, etc.
- Be prepared to scout fields, i.e., know what the symptoms look like and how to scout for SBR.
- Find a way to keep abreast of where SBR is in relation to the Midwest . One might read newsletters or watch the Internet (some likely websites will be discussed below).
You will find more details on these items below. First, let's review the 2005 season briefly. Our story really starts with November 2004, when SBR was found for the first time in the continental U.S. By January 2005, SBR had been found in nine states. Since SBR cannot survive without green plant tissue of soybean or a close relative, winter freezes caused SBR to die out except for a few kudzu patches in Florida and in protected areas elsewhere in the southeast.
Above, Kudzu vines cover a ravine and many nearby trees. The fungus that causes soybean rust will only survive in green tissue of soybean, kudzu or a few other related plants. Therefore, kudzu is important in survival and spread of soybean rust. Except for in parts of Florida and in protected areas in the southeastern United States , Kudzu will freeze back to woody parts and loose all the green tissue in the winter months. A kudzu leaf is shown in detail in the inset.
As the 2005 season progressed, SBR finds increased until a total of 138 counties and nine states were confirmed with SBR (Figure 2). (To watch this spread on an animated map, f ol low this link: http://www.ceal.psu.edu/sbr05.htm ). Note that the general direction of spread was north through the Car ol inas and west toward Texas . The number of both counties and states that have been confirmed with soybean rust in 2006 is considerably higher than at the same time in 2005 (Figure 2). This may be because SBR spread sufficiently during the 2005 season so there are many more places from which SBR can begin to spread in the 2006 season.
The increased rate of SBR finds in the 2006 season compared to the 2005 season should convince growers in the Midwest to be vigilant. The final SBR map (Figure 3) shows that SBR did come as far north as Caldwell County ( Princeton , Ky ). SBR has also been reported in Mexico . So there will be no lack of SBR spores to blow around should the winds turn toward the Midwest.
Let's go over the four points listed above with more detail.
There are no varieties of soybeans resistant to SBR. Thus, if SBR shows up, it will be necessary to apply fungicides to avoid severe losses. To my knowledge, the only organic fungicide labeled for SBR is Ballad (AgraQuest, Inc.). Soybean growers may choose to purchase a fungicide prior to the arrival of SBR or make arrangements to purchase the fungicide from a dealer when and if SBR shows up. In either case, don't wait until the arrival of SBR before making arrangements.
Most soybean producers do not apply fungicides regularly. Therefore, growers may need to acquire the proper equipment and learn about application methods before SBR arrives. A brief description of proper fungicide application methods f ol lows. An effective fungicide application will deposit spray onto the lower leaves of the canopy where the disease is most likely to develop first. Spray droplets should be in the medium range (220 microns). Ground applications should use 15 to 20 gallons per acre. The boom should remain about one foot above the canopy of the soybeans. More detailed information can be found at:
Riding past one's soybean fields at 40 mph with the windows down will not be sufficient to scout for SBR. The initial symptoms of soybean rust will occur on the lower leaves where dew will remain the longest (several hours of dew at temperatures from 59 to 82 degrees F are sufficient to allow SBR to get started). So, to observe SBR one must wade out into a field and, using a yardstick, move the plants apart, bend over and look at the lower leaves. A 15-20X ocular will be helpful. More on symptoms and scouting tips can be found at:
No SBR management program will be complete without a plan for keeping up on the movements of SBR. We know that SBR overwinters only on green tissue. Therefore, SBR spends the winter lurking in kudzu patches in Florida . As the weather warms up, SBR migrates north with the greening of kudzu patches and the planting of soybean acres.
This movement will be tracked by plant path ol ogists in states from Florida to Michigan , from North Car ol ina to Iowa . In each state, so called sentinel plots of soybeans will be planted and observed by specialists trained to recognize SBR. If SBR is found and confirmed, this information will be posted on this USDA website: http://www.sbrusa.net/ . Most states will issue advisories on when fungicides should be applied. As a general rule, it will be wise to begin fungicide applications when SBR is observed in a state south of the producer.
The timing of the fungicide application is critical. If the fungicide is applied too soon, additional applications may be necessary before the disease actually shows up in one's field (most data show that fungicide applications in the absence of SBR do not pay for themselves). If the application is made too late, no amount of fungicide will provide economic contr ol . The best application is made when the disease is in close proximity, yet not actually in one's own field. Remember that if you find one lesion in your soybean field, more lesions are present at the microscopic stage and will be visible soon.
SBR is not likely to be a pest of Midwest soybean producers every year. However, SBR will arrive in Midwest soybean fields eventually. It will pay dividends to learn more about this important disease.