SW MN IPM STUFF 2018 Issue 11

Volume 21 number 11

8/17/2018

This newsletter and the advice herein are free. You usually get what you pay for.

A printer-friendly pdf version of this newsletter is available to download here: SW MN IPM Stuff 2018-11

Crop weather

Rainfall, air and soil temperatures, degree-days, soil moistures, and other current and historical weather data for the University of Minnesota Southwest Research and Outreach Center (SWROC), a little spot about two miles west of Lamberton, MN, can be found at SWROC Weather

From May 1 to August 13, we are 213 GDDs (more than a week) ahead of the long-term average in corn growing degree-days. 2023 GDDs vs. 1810 GDDs average. At 20.28 inches of rain since May 1, we are 7.86 inches ahead of average. Some areas of the state have started to see moisture stress but not right here. This weather has helped shape this season’s crop weed, disease and insect pests.

Corn

We have corn at dent in some fields. While I am sure you all like to see yellow corn kernels this time of year, seeing fields with yellow and stunted plants is not pleasurable. The severity of this wet season’s yield impacts is becoming more apparent as harvest approaches. Silage harvest will be early this year.

I suspect poor root development and nutrient stress will make this another good year for stalk rots. Some signs of anthracnose are now showing up.  

On the bright side, areas with extremely wet soils in May and June seem to have reduced western corn rootworm populations by 50% or more, particularly where planting was delayed until late May. On the other hand, areas with late-silking plants could have recruited more beetles.

While driving around the state, I am now starting to see an occasional symptom of univoltine and second generation European corn borer (ECB). Look for individual plants with the tassel and upper stalk red or dead. I have had reports of corn borer in some non-Bt fields in the west central part of the state. They are not extinct. Thanks for the heads up Luke. If you have fields without a Bt corn borer trait that you would like to volunteer for a fall corn borer and disease survey, let me know.

soybean plant showing foliar symptoms of SDSFigure 1. Foliar symptoms of SDS. Brown Co., MN 2018.

Soybeans

Some soybeans are at now at R6 and many more will reach that stage of development next week.

Late season diseases are becoming visible. Given the wet spring, it was not too surprising to start seeing sudden death syndrome (SDS) symptoms but the proportion of fields with symptoms is greater than previous year. Symptoms (Fig.1) are usually associated with headlands and other areas where compaction is likely. These same fields may have issues with soybean cyst nematode (SCN). 

As soybeans reach the R6 stage, the stem, and often the foliar symptoms of brown stem rot (BSR) will become apparent. Foliar symptoms can resemble SDS. Use the stem pith browning of BSR to distinguish the two diseases. This disease can be associated with low pH soil conditions. Brown stem rot Type A produces foliar symptoms, Type B does not. 

A soybean plant can be infected with more than one disease, and often is!

Some things to consider right about now

Many insects and some diseases are often worse on field edges. Keep this in mind when evaluating crops.

ear of corn with corn earworm and damageFigure 2. Late season corn earworm and damage in tip of dent corn ear.

Corn

  • Some folks will be starting yield estimates. 
  • If you are out in cornfields now, note foliar and stalk diseases for future hybrid decisions.
  • Increase the efforts on stalk rot as the milk line moves down the kernel. Schedule fields for early harvest.
  • Make a note of fields with ECB and corn rootworm damage for 2019 and 2020 management.
  • Some large corn earworm flights mean some dent corn fields, particularly late planted, may see some ears with corn earworm damage (Fig. 2).

Soybean

  • Yes, scouting wet, tangled soybean fields is not easy or comfortable.
  • Soybean development and pesticide pre-harvest intervals (PHI) will mean pest intervention will be soon be over for 2018.
  • Scout for soybean aphids. Scouting should soon focus only on the latest maturing fields.
  • Note defoliating (e.g. caterpillars) and pod-feeding (e.g. stink bugs) insects and assess where damage warrants.
  • Note diseases. Frogeye leafspot symptoms are much more widespread than typical. Stem canker and pod and stem blight are also making a strong showing this year.
  • Where you see areas with poor growth, check the roots for disease and soybean cyst nematode (SCN). After harvest, soil sample suspicious areas for SCN. Unfortunately, we are seeing more fields where SCN can attack and reproduce on resistant varieties.
soyban plants showing dieback symptomsFigure 3. Top dieback symptoms on the upper leaves. Tehse plants had soybean aphids aboveground and SCN below.

Yellow Beans

Some of you may be noticing the tops of soybeans turning yellow.

In some cases, these upper leaves die. The symptoms could be top dieback, a disease associated with Diapothae/Phomopsis and SCN.

Where these late-season upper leaf symptoms resemble IDC, also suspect SCN.

soybean plants infected with phomopsisFigure 4. Phomopisis was isolated from these plants. This field also had issues with both soybean aphid and SCN.

Potassium deficiency? 

In some fields, areas of fields are showing potassium deficiency symptoms on the upper leaves. Is the yellowing along the margins of upper leaves a sign that the plant is deficient in potassium? Perhaps, within the symptomatic leaves.

However, several soybean pests and diseases can exacerbate these symptoms, most often in soils testing low to moderate for potassium. Infestations of SCN, soybean aphids, clover root mealybug often trigger K deficiency symptoms. Pod and stem blight is often found in these same areas.

And now for a few late-season soybean defoliators

two color morphs of the yellow woolly bear caterpillarFigure 5. Two of the color morphs of yellow woolly bear caterpillar from the same field.

A number of insect defoliators are present now. In some SE MN fields, Japanese beetles are active and the dominant defoliator.

wolly bear feeding on a soybean leafFigure 6. Woolly bear and feeding damage. Larger caterpillars will feed through all but the largest leaf veins.

In the Lamberton area, yellow woolly bears (Fig. 6) are unusually abundant this year. It’s the first time in 21 seasons here that I have seen these larvae abundant enough to cause noticeable defoliation. Still, defoliation levels are less than 5% on upper leaves, well below a whole plant 20% defoliation threshold. There are several color morphs ranging from white to deep orange. For some reason, the large number of furry caterpillars remind me of the 1967 Star Trek episode, “The Trouble with Tribbles”.

yellow striped armyworm on soybean plantFigure 7. A yellow striped armyworm. The moth is much less gaudy but common in the light trap this summer. Like most caterpillars, the armyworm has five sets of prolegs.

There are also a few grasshoppers, green cloverworms, loopers (Fig. 7), and yellow-striped armyworms (Fig. 8) present. At this time, I have not seen or heard of anything economic.

looper caterpillar on soybean plantFigure 8. A looper caterpillar. Note the three sets of true legs and three sets of prolegs on the abdomen. Green cloverworms have four sets of prolegs.

Soybean aphid watch 2018

More reports of poor control with pyrethroid insecticides and re-infestation of early treated fields. If you sprayed aphids more than 10 days ago or if you have used an insecticide that contains only a synthetic pyrethroid(s) (e.g. Warrior, Brigade, and generics) check control. Seriously! 

As fields reach the beginning of R6 or full seed development stage (seed fills pod cavity in one of the top four nodes), they come off the aphid scouting and aphid-treating schedule. Scouting until then will prevent any late-season yield loss.

Sometimes, you miss things scouting or insecticide applications go wrong. Yield loss can still occur during R6 so plants with thousands of aphids per plant may still benefit from aphid control. However, as the soybean plants fill seed, the chances of an economic return decline and PHIs will limit insecticide options. Once you see any pods turn yellow (the R6.5 stage) or leaves begin to turn, walk away from aphid infestations.

Any potential economic risks from other insects and mites should also have been determined and dealt with by this time. On the bright side, late season soybean aphid populations are prone rapid decline for several reasons:

soybean aphids on soybean leafFigure 9. A fungal disease has killed the tan aphids on this soybean leaf. It is likely that many of the live aphids are also affected.
  1. Movement to buckthorn can begin any time after mid-August. Cool temperatures and soybean maturity helps encourage this movement.
  2. Predator populations often peak in August and early September.
  3. In late summer, cooler temperatures, dew and high humidity inside dense canopies make disease epidemics from entomopathogenic (insect disease) fungi more likely. This is occurring in soybeans at the SWROC now. Once an epidemic starts, populations can collapse very quickly. Infected winged aphids can move the disease to new plants, new fields, or to buckthorn.

Can these factors influence an insecticide application? Yes. Remember that increasing aphid populations are part of the 250/plant threshold.

Will movement to buckthorn, predators and disease eliminate aphids in your soybean fields?  Maybe, maybe not.  Unfortunately, there is only one way to tell.

Happy trails,

Bruce