Soybean disease and management

Written by Steve Fairchild on .

Scout and treat when it makes sense

Making yield and making profit is a balancing act. You get out what you put in—sometimes. And weather is always around to mess up the mix. But some research from the United Soybean Board is a reminder that some practices pay off more than others.

You probably read about the Kitchen Sink study funded by USB last year when it made headlines for recommending tightening row spacings. It showed significant yield gain in row spaces closer than 30 inches. 15-inch rows were the yield winner across multiple sites in multiple states.

The study also compared early-season versus late-season input system timing. In the study’s  early/late plots, researchers split a high-input program into preplant products versus postemergence applications, applying plots with only preemergence and only post programs.

For the early season trials, researchers used  preplant fertilizer, seed inoculant and seed treatment, but no postemergence applications; the late-season program started with bare seed and included only foliar fertilizer and foliar fungicide.

The idea was to gauge what happens in fields where producers take a low management view, putting on all the field’s inputs early and waiting for harvest. Those were compared to more hands-on in-season management.

A report from the study said, “Late-season programs yielded significantly less than the high-input/narrow-row plots. Though at a difference of 1.5 bushels per acre, the late-season approach was pretty close to the full high-input result.”

Researchers saw benefit from foliar fungicide, which obviously benefits plants under attack from fungal diseases as well as the “health effect” that can accompany the right timing of foliar fungicides.

What diseases warrant worry and treatment?
MFA’s trade territory will have different diseases from year to year, some of which have much more potential to reduce yield. Septoria brown spot and bacterial blight can usually be counted on, and foliar diseases like frogeye leaf spot and downy mildew will show up when weather conditions are right.  You can’t control bacterial blight with fungicide. Bacterial blight is favored by cool, rainy weather. You’ll see symptoms most often a few days after strong thunderstorms with wind or hailstorms. Hotter, dryer weather will slow its development.

Foliar fungicides can control frogeye leaf spot and Septoria brown spot.

Of these two diseases, frogeye leaf spot typically figures to do more yield damage to soybeans. Septoria brown spot will also reduce yield, but tends to occur in excessively wet growing seasons.  Soybean rust, another damaging foliar disease in more southern states, has shown up sporadically in MFA’s trade territory but usually late in the season. Rust blows in on wind currents.

A mild winter in Louisiana made for an early appearance of Asian soybean rust in south Louisiana this year.  
“Finding the rust is not really new following mild winters, but finding it in eight parishes mainly along the coast, and also finding it in 6-inch-high volunteer soybeans, is new,” said LSU AgCenter plant pathologist Clayton Hollier.

Hollier said no need to panic elsewhere in the country, and that local growers will just need to be vigilant.

In the past, freezing temperatures have delayed the emergence of the disease to the point that the soybean crop was far enough along in development to not be affected much by it.

University of Missouri plant pathologist Dr. Alan Wrather said that he has seen rust in Missouri five out of 10 years since the disease was first discovered in the United States.

“Every time I’ve seen it, it has arrived late in the season. The crop has almost always been mature by the time rust arrives. I think you’ll continue to see that trend.

We’ll see rust in this growing region, but it will arrive late in the season. States to the south have the right conditions for soybean rust to develop. The spores needs about 12 hours of wetness to germinate, and it gets those conditions farther south during the soybean growing season. But as you move north, we don’t get those heavy dews that keep leaves wet that long until later in the season, when the crop is nearly mature.”

Wrather said that unlike other fungus, soybean rust isn’t affected by previous crops or cultural factors; rust infection is a matter of how much rust multiplies in southern states and blows into the Midwest. There is no varietal resistance.  Check out the IPM PIPE website (http://sbr.ipmpipe.org/), which monitors sentinel plots throughout the Mexico, the United States and Canada. It gives weekly rust reports during the growing season along with forecast maps.
Will my beans get fungal disease?

Soybean varieties have differing levels of resistance to many foliar diseases. Most growers get disease resistance information straight from the seed producer, but if you have questions about disease packages and particular soybean lines’ susceptibility to foliar diseases, check out the Varietal Information Program for Soybeans at www.vipsoybeans.org. Knowing your varieties’ susceptibility can help during foliar disease outbreaks. If your variety is more susceptible to disease, you might consider a lower threshold for fungicide application compared to seed lines that have strong resistance traits for particular diseases. Scouting and consultation with your agronomists are good investments as soybean foliar disease season approaches. MU’s Wrather said that maintaining the proper level of soil nutrients is one defense against fungal diseases. “It’s something you always hear, but you should be testing your soil and making sure it is up to the recommended fertility rates for your crop. Soybeans deficient in some nutrients are more susceptible to disease.”

Does your crop history contribute?
Soybean pathogens often survive in field residue. So your risk for foliar disease increases in fields planted in multiple years of soybean crops. And, the more residue left from previous years, the more habitat for pathogens. Heavy residue increases the chance for fungal foliar diseases. Crop rotation helps break the cycle, but be vigilant regardless of the previous crop. In fields of soybeans after soybeans, scouting is even more important.

Midwest rain and humidity counts
The good news about a Midwest summer is that when the rain hits right, we grow excellent soybeans. The bad news is that Midwest humidity is a contributor to foliar disease outbreaks. Like most fungi, soybean foliar fungus pathogens thrive when soybean plant leaves stay wet for extended periods of time. Fortunately, breaks in those conditions tend to slow foliar diseases. “Either way,” said Wrather, “scouting pays. If producers don’t have time to scout their fields, hiring a scout is a good idea. Our ability to just predict whether fungicides will be needed isn’t well enough refined.”

Scouting is reaction and planning
When soybean start their reproductive stage, scouting should intensify. As  bloom starts, watch closely for symptoms of foliar fungal diseases by scouting  in random patterns throughout the field.  Watch low as these diseases often begin to show on lower leaves first. It’s difficult to tell some of these foliar diseases apart from others. Grab a magnifying device or consider buying a macro lens band for your phone camera. You can send close-up pictures of the fungus to your agronomist for further identification.  

Wrather said that scouting is always a good practice. Even if you find diseases that you can’t treat for this year, you’ll have better information to plan for next year, including the option of altering rotations, planting times, fertility adjustment or pre-planned disease treatments.

When to treat
When scouting reveals foliar fungal disease pressure sufficient to treat, make sure you have properly identified the disease to get the most effective fungicide for treatment. Like all rescue treatments, there is a balance between cost and return. Also consider the possibility that some diseases can grow resistant to fungicides—follow the label for rates.

Your local MFA agronomist can help you with information about availability and rates for the most effective fungicide treatment for your situation.


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