Soybean rust fungicides don’t do well in ‘mixed’ company

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WEST LAFAYETTE, Ind. – Farmers can control Asian soybean rust and soybean aphid by spraying fungicides and insecticides from the same tank mix.
However, if they mix fungicides and herbicides, rust or weed control is likely to go in the tank, Purdue University Extension specialists said.
Can’t mix and match. Fungicides and insecticides are similar enough to be used simultaneously, said Greg Shaner, Extension plant pathologist. That cannot be said about fungicides and herbicides.
Differences in application timing and spraying methods make it difficult to combine fungicides and herbicides, Shaner said.
Timing, application differ. There’s two reasons why he’s recommending producers not mix fungicides with herbicides. One is the timing.
“Unless rust would come in very, very early in the crop season, if you waited to apply herbicide until you needed rust control, it would be too late to get effective weed control,” Shaner said.
The other reason is that herbicide application – especially glyphosate products – is focused on spray drift reduction.
Producers use low carrier volumes, spray nozzles that create larger droplets and low tank pressures when applying herbicides.
“If you set up your spraying system to do a good job with a herbicide, you’re going to be doing a poor job with a fungicide,” Shaner said.
“Conversely, if you set up to do a good job with a fungicide, you’re probably going to have some problems with herbicide drift.”
Tackling rust. Fungicides are the only control options available for soybean rust, a devastating crop disease that was first detected in the continental United States this past November.
Soybean aphid is a tiny sap-sucking insect that has plagued Midwestern producers off and on since 2000. Entomologists expect greater aphid numbers this year, after the insects were mysteriously absent in 2004.
Coming in June? While it is unknown when rust might arrive in Midwest soybean producing states, many pathologists believe it could make its first appearance in June or July, when soybean plants are flowering.
That timeframe coincides with previous soybean aphid infestations, but is weeks behind the emergence of winter annual weeds, said Bill Johnson, Purdue extension weed specialist.
“If we’re in a total post-weed management system in Roundup Ready soybeans, our optimal herbicide application timing is somewhere around the V2 to V3 soybean growth stage to minimize yield losses due to weed competition,” Johnson said.
The V2 – vegetative stage 2 – and V3 growth stages usually occur in late May.
It’s all about the drops. Droplet size differences between fungicides and herbicides also make using the products together problematic, Johnson said.
“With glyphosate applications, we typically want a droplet size between 300 and 600 microns,” Johnson said. “With an insecticide or a fungicide application, we typically want smaller droplets of around 200 microns.”
He said experts also want higher carrier volumes of 20 gallons per acre or better, “so we can fog those products into the soybean canopy and get good penetration.”
By comparison, 300 microns is about as big around as a toothbrush bristle, while 200 microns is slightly larger in diameter than sewing thread, Johnson said.
Finding space in a spray tank for fungicide, insecticide and water should not be an issue, Shaner said.
“With fungicides you might be using from 4 to 10 fluid ounces per acre of product, and that’s going to be diluted in water. So there’s plenty of room for insecticide, too,” he said.
Scouting tips. Scouting soybean crops for rust and aphids can be done on the same walk through the field, said Shaner and John Obermeyer, Extension entomologist. But farmers should be careful not to apply fungicide or insecticide unless the applications are necessary, they said.
“Fungicide labels talk about applying the fungicide when no more than about three to five leaves out of a hundred are showing any sign of rust,” Shaner said.
When to spray. “In practical terms this means you’re probably looking at 150 to 200 plants at the very least, and if you see just one or two rust pustules on two to five leaves out of a hundred that would be an economic threshold. But I would say if you see any rust – if only on one leaf – you still want to spray.”
“In the few years we’ve have experience with soybean aphid, the later July to early August time period is when we’re going to be most concerned about this insect pest,” Obermeyer said.
“We would advocate scouting before that time, to determine whether populations are building to economic thresholds.”
When scouting, “turn over the undersides of leaves and you will see the aphids,” Obermeyer said. “Once you get to 250 aphids per plant then you’ve reached a treatable level.
“If aphids are not present, we would not recommend the insecticide. It’s not needed and, more than likely, it will do more harm than good because beneficial insects are going to be wiped out.”
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