2nd-half agronomics

By: Gil Gullickson 07/06/2012 @ 9:44am

Midsummer used to be the time when you thought the whole game was over for your corn and soybeans. Just plant, spray, and go away until harvest.

No more.

Instead, more maladies make this period the halftime in your fields. Whether you're in your shop, office, or walking your fields, you're trying to make sense of what lies ahead. Maladies like soybean aphids and white mold may plague your soybeans later this year. Goss's wilt is becoming more of a threat to your corn. And you now have to eye your rootworm-traited corn for signs of failure.

Just like a coach making halftime adjustments, Successful Farming magazine offers 12 pep talk points to consider as you head into your crop's second half. Addressing these points can aid you through the rest of 2012 and beyond.

If you haven't been out in your fields for a few weeks, get out there. If you can't, hire a scout or consultant to do it for you. Curbing problems now can make for more bountiful future harvests.

“Growers need to be following principles of integrated pest management and scouting for disease and insects to get out in front of them,” says Ty Vaughn, U.S. corn product management lead for Monsanto.

1. Make sure fungicides work

Scientists agree that fungicides have high odds of payoff under disease-inducing factors such as disease-susceptible varieties or residue-laden fields. Opinions are split between university and industry officials and plant pathologists whether yield hikes due to plant physiology benefits will occur.

If you do apply fungicides, check your fields later to make sure they worked. Strains of frogeye leaf spot that resist strobilurin fungicides were documented in Tennessee, Illinois, Kentucky, and Missouri in 2010 and 2011.

One step to forestall development of resistant strains of frogeye leaf spot is to plant soybean varieties that resist it. Mixing different modes of action when fungicide applications are made can also forestall resistance, says Nick Fassler, BASF technical market manager for fungicides.

Thus far, frogeye leaf spot is most prevalent in Southern soybean-growing regions like the Mississippi River Delta. But it's important to watch for expansion in more Northern soybean-growing regions, Fassler adds.

2. Fix n-deficient corn

The funeral parlor pallor of pale-green corn or striped corn that's short of nitrogen (N) is certainly a stomach sinker.

“Generally, we like to make a supplemental N trip before corn gets to the V6 stage. Anytime after that in Minnesota is unpredictable,” says Jeff Vetsch, soil scientist at the University of Minnesota of Minnesota (U of M) Southern Research and Outreach Center at Waseca. “If you can't incorporate it with a decent rain, you risk volatilization. But if you have lots of N-deficient corn, you may not have a choice.”

In other states, you may have a better chance of response beyond V6 (when the third root whorl elongates).

Base a rescue N application rate on the yield potential you have, advises Vetsch. “Normally, 30 to 45 pounds per acre is an appropriate amount,” he says.

Consider root-system condition before you head out with more N, advises Emerson Nafziger, University of Illinois (U of I) Extension agronomist. Under conditions of excess water or severe insect injury, root systems can be compromised. “If roots can't take up enough water, they also won't be able to take up N,” he says.

3. Watch that wily rootworm

Like many farm kids at the time he grew up, Ken Ostlie had a sandbox made out of an old tractor tire.

“We sifted sand all the time,” recalls the U of M Extension entomologist. “One problem was our farm had lots of cats. Which meant if you sifted lots of sand, inevitably you found a surprise.”

Flash forward several decades to 2011, when a handful of fields planted to Monsanto's YieldGard VT Triple and Genuity VT Triple Pro corn products exhibited a similar surprise. Rootworm resisted the trait's Cry3Bb1 protein.

The majority of these fields experienced repeated planting of the trait on corn on corn.

“Lack of a refuge was also common,” adds Ostlie. “All these factors create excessive selection issues. When you use the same trait over and over again, we sift for resistance pretty fast.”

Bear in mind that just because your corn lodges, it doesn't mean something is awry with your rootworm trait. The trait still works in the vast majority of cases. This summer, though, look for tip-offs like abundant adult rootworm beetles. Ditto for downed corn. If you suspect trait failure, take root samples and contact your seed dealer to send in root samples for verification.

If you think your trait isn't working, Mike Gray, U of I Extension entomologist, advises you to consider these actions:

High crop prices have brought many products out of the woodwork that promise you yield increases.

• Rotate to soybeans or another nonhost crop.

• Use a soil-applied insecticide at planting. This does not mean you should routinely apply a soil insecticide at planting to rootworm Bt corn hybrids. Soil insecticides, if applied properly, should provide adequate root protection alone in most situations.

• Use a Bt hybrid that expresses a different corn rootworm Cry protein than the one that previously performed poorly.

• Use a pyramided Bt trait that expresses multiple Cry protein targeted against corn rootworm.

• Use long-term integrated approaches to corn rootworm management that includes multiple tactics.

“Above all, don't plant the crop and drive away,” says Ostlie. “The goal is to catch the problem before it clobbers you.”

4. Find frightening freckles

Freckles on a fair-skinned person are often feckless and do no harm. That's not the case with freckles in your corn. It may indicate Goss's wilt, a bacterial corn disease that's spreading east and north from Nebraska and Colorado.

Symptoms include large leaf lesions containing dark round spots – freckles – bordered by water-soaked margins. Goss's wilt can easily be confused with other diseases and disorders.

“If you don't see freckles, you're likely not seeing Goss's wilt,” says Carl Bradley, U of I Extension plant pathologist.

At its worst, Goss's wilt can clip yields by 50%. It's most likely in corn-on-corn, since the bacteria overwinters in prolific residue. Bacterial cells are disseminated onto corn leaves by wind or by splashing rain, and they enter plant tissue through wounds or natural openings.

Fungicides don't work on this bacterial disease. Tillage following harvest can break up residue that harbors the inoculum.

After that, two choices exist:

• Rotate field to a nonhost crop like soybeans next year.

• Plant a hybrid that resists Goss's wilt the next time you plant corn in that field.

“Resistance may not be complete,” says Bradley. “Even in a moderately resistant hybrid, you will have leaf lesions on a plant. It is still better than planting a susceptible hybrid.”

5. Learn from lodged corn

The good news about lodged corn is there are ways to harvest it. Just bank on it taking up to five times longer than a normal field.

If you're unfortunate enough to get hit by high winds, though, taking the time to walk the field can help determine its cause and provide valuable insight in the future.

Rootworm trait failure may first come to mind. Still, Roger Elmore, Iowa State University (ISU) Extension agronomist, says it could also be due to:

• Poor root development due primarily to cold, wet soil conditions.

• Poor seed placement at planting resulting in too shallow a root mass.

• Stalk rot late in the season.

Bear in mind that hybrids vary in lodging susceptibility. Thus, picking hybrids that withstand root lodging can prevent future problems.

6. Whip white mold

For John Quandt, white mold has been the hardest thing for him and his family to fight while growing soybeans.

“No variety is consistent with resistance,” says the Oakes, North Dakota, farmer. “And fungicides haven't worked well for us, either.”

Once white mold establishes itself in a field, it can surface at any time in subsequent years.

This fungal disease overwinters in hardened bodies called sclerotia. When cool and damp conditions occur, fungal mushrooms can launch spores under soybean canopies during flowering. Because this occurs under closed canopies, white mold is more prone to occur under narrow rows than rows 30 inches or wider. In severe cases, this fungal disease can slice yields up to 60%.

“You want to check what kind of soybean variety you have,” says Randy Myers, fungicide portfolio manager for Bayer CropScience. “Some are more vulnerable than others. No-till has less disease incidence of white mold than conventional tillage.”

Several fungicides can curb white mold if applied before infestations result.

“One problem with waiting until R3 (beginning pod) or R4 (full pod) is it is difficult to push the fungicide through a closed canopy,” says Myers. “R1 (beginning bloom) is when we (Bayer) recommend applying a fungicide for this disease. Petals are a perfect medium for infection of the plant. Applying a fungicide at that stage can suppress fungal growth on the vulnerable petals.”

7. Watch for weeds

Midsummer scouting trips are a great time to assess your weed-control program. Findings can help you forestall any herbicide-resistant weeds that may be festering in your fields.

“If you don't have a train wreck yet, you need to ask yourself what you need to do to avoid it,” says Aaron Hager, U of I Extension agronomist. “If you want to rotate away from a post-only program and use residual herbicides, that's a good thing. Just don't assume it's going to fix the problem, because it won't. In the central Corn Belt, you're not going to have a product that's going to last the whole season.”

Fall is particularly a good time to scout for winter annuals like henbit, marestail, and chickweed.

“Removing them with fall residual herbicide applications prevents winter annuals from shading the ground,” says Jeff Carpenter, DuPont U.S. corn portfolio manager. “The next spring, this can lead to better seed and soil contact, and better stands.”

8. Pummel pigweed's dastardly duo

Rouging a few stray waterhemp or Palmer amaranth plants during summer scouting can save future headaches. Both pigweed family members are prolific seed producers. In crops, female waterhemp plants can produce up to 5 million seeds per plant, while Palmer amaranth females can produce up to 460,000 seeds per plant.

“Scout before and after herbicide applications,” says Jeff Stachler, a U of M and North Dakota State University Extension weed specialist. Look particularly for dead plants next to plants appearing normal but stunted. If this pattern continues, the stunted plants are likely resistant. (To learn more, visit www.ag.ndsu.edu/glyphosateresistance.)

“Removing these plants reduces the risk of creating a resistant population,” he says.

Scout field perimeters. “Edges of fields sometimes often receive partial herbicide rates,” says Stachler. “This can create selection pressure and add to the seed bank of resistant weeds.”

9. Target future tiling

High crop prices have brought many products out of the woodwork that promise you yield increases. Evaluate them carefully. “Don't waste money on unnecessary products,” says Seth Naeve, U of M Extension agronomist. “Instead, invest money on tile in poorly drained soils.”

Summer scouting enables you to target those wet spots for tiling. Tiling can snap a soggy bog teaming with pathogens into a well-drained, vibrant field.

That's what Rick Ryan learned after buying a farm in 1993 that produced half a crop due to that year's flooding.

“When I bought it, I thought I couldn't afford to tile it,” says the Malcom, Iowa, farmer. “If I had tiled it in 1993, it would have paid for the tiling in one year.” In the fall of 1994, Ryan did tile the farm.

To measure payback, the U of I developed an application in the Illinois Drainage Guide to help determine the economic analysis of drainage systems. (To learn more, visit www.wq.illinois.edu/DG/DrainageGuide.html).

“In some cases, the return on investment can be much better than returns from Wall Street,” says Richard Cooke, a U of I Extension drainage specialist.

10. Be alert for aphids

For the most part, soybean aphids have spared soybeans in recent years. Still, this feisty sap-sucking insect should be on your radar later this summer. U of I researchers report good soybean aphid survival in buckthorn, a plant upon which the insects overwinter as eggs.

“This, combined with a mild winter, could set the stage for soybean aphid infestations,” says the U of I's Gray.

Tips to manage aphids from the U of M's Ostlie include the following:

● Start scouting heavily infested fields.

● Scout one to two times per week through pod set.

● Look for aphids on the undersides of the upper three leaves in vegetative and flowering soybeans.

● Estimate aphid density per plant in at least 10 locations throughout the field.

● Consider treating with insecticide if aphids average more than 250 per plant.

Ostlie adds that insecticides may not be economical if soybeans are setting pods, if infestations are spotty, or if most nymphs (immature aphids) are developing wings or diseased nymphs are common.

He advises leaving untreated yield strips for comparison if you use an insecticide. Follow precautions to minimize bee kills and communicate treatment plans to beekeepers, Ostlie adds.

11. Nix corn nematodes

Although nematodes seem like newcomers to the corn-pest scene, they perused the prairie long before European settlers broke it.

Suspect are corn-on-corn acres, since corn – a grass – mimics the prairie's native grasses. Ditto for no-till, which resembles the undisturbed prairie. A move away from organophosphate and carbamate soil-applied corn rootworm insecticides also has possibly encouraged their buildup.

Most nematodes are not damaging at low populations. At high levels, though, that's not the case.

“You'll see yellow leaves, stunted plants, mid-day wilt, and small, poorly filled ears,” says Greg Tylka, ISU Extension nematologist. “Roots will be stunted, discolored, and swollen. You'll also see dead spots in fields if numbers are really high.”

Mid-June through harvest is a good time to soil-sample for corn nematodes. That's normally within the recommended sampling time of V6 (when the third root whorl elongates) through R3 (milk stage). Sampling 12 inches deep of 20 cores in a field will give a good idea of any nematodes that are present.

Needle and sting nematodes in corn are exceptions. Spring sampling is best with these two species since they dive too deep into the soil during summer to be sampled.

If soil samples show high nematode levels, two treatment options exist. Avicta Complete Corn from Syngenta and Poncho/Votivo from Bayer CropScience are seed treatments that you can use to manage corn nematodes.

12. Harvest the weak first

Target those fields prone to losing ears first, such as lodged or hail-damaged fields. Then, check out canopy and stalk quality during your summer scouting trips. Drought-stressed crops can be particularly vulnerable to poor standibility, especially if they have tried to fill ears before drought catches up with them.

“When the canopy is green and stalks are healthy, you can expect it to stand until Christmas,” says agronomist Nafziger. “Healthy stalks retain some sugar. If you have a case where the plants went through dry conditions and were stressed, sugar in the stalk goes into the ear and isn't replaced because leaves shut down. Those are the stalks that will fall over very quickly.”