Asian rust was recently identified in south Georgia on volunteer soybeans. As of April 29 we had completed our fifth survey of Mississippi sentinel plots, and as of May 3 we had no reports of rust.
The process of labeling fungicides continues, but some materials will not be approved until next season. We have a fairly good arsenal but not all materials are equal. What you use will depend on whether or not rust is present when you spray, availability, efficacy and cost.
I hope several co-packs/premixes will be available. They will be good options, but the time of application and what is in the field will determine what is used.
The cost of the mixes has gotten my attention. I am hearing $12 to $14. That is better than earlier reports. One concern we had earlier was that the use rates of the copacks are lower than recommended for either product alone. Data from other countries show these rates are adequate when applied as a tank mix. And the copacks/mixes have exhibited increased levels of control.
We have a control guide available in county Extension offices. It is available also on the Web at www.soyrust.org.
Three approaches can be used this growing season: (1) spray at bloom and 20 days later without looking, (2) wait until rust is found in your vicinity (treat as needed), and (3) ignore it.
Since rust was first found, it has had minimal effects on yield the first year it inhabited any new region. The United States should be no different in 2005. Three years from now, that may change, but it is doubtful this year. In other words, widespread spraying without rust being detected is probably premature.
How much risk are you willing to take? I feel previous experience in other countries is a good indicator.
When we find rust will determine what to spray. Over the last several years an R3 application of a Quadris-based program has increased yield 5.9 bushels in Mississippi field trials. That has been predominately at reduced rates, but we have seen that increase in the absence of rust.
Similar results have been observed in Arkansas, Tennessee and Louisiana. Throw rust into the picture, and I expect the number to increase.
Spray volume should be 5 gallons by air. We may find that less will work, but that is the volume currently listed on the various product labels. Ground applications may be applied at 10 to 20 gallons per acre. Based on work done in Mississippi, we recommend 15 gallons per acre.
Spray tip selection is also important. Some of the best time you could spend would be letting some tip suppliers highlight their recommended tips and expected performance.
As you consider equipping a spray rig, keep in mind anything you might have done to minimize drift of Roundup is bad for fungicides. You want to create medium to fine droplets, not coarse droplets (droplet size ranging from 285 to 335 microms). Increasing pressure too high to increase volume may create too many fines. Select a tip and work in the mid-range regarding volume and pressure; do not work on the extremes.
Drift must be considered. Most fungicide applications need to be kept on target. A major concern is toxicity to catfish ponds, primarily plankton. Consider leaving spray borders with both air and ground rigs. Pay attention to wind direction.
Ground rigs and field scouts will not be a factor in the spread of rust spores. If we have enough spores in the atmosphere to cause concern, a tractor driving through a field or someone going from field to field scouting is not going to be a contributing factor. Wind will be the major dispersal agent.
Spraying early in the day and late in the afternoon will help limit the problem of materials not reaching the canopy during hot, humid conditions. Spraying when the canopy is wet will aid in spreading a fungicide over the leaf's surface. The use of a surfactant or oil will aid in penetration/coverage and ultimately will improve control.
It will take several years to get a complete handle on rust. It will get easier to deal with as we gain more experience. In the future, preventative control will most likely become the norm for dealing with rust, but that option is most likely premature this first year.
Alan Blaine is the Mississippi Extension soybean specialist. e-mail: [email protected]