Category Archives: Pest Group

Early Harvest Reports – Soybean Seed Quality is Not Good

David Holshouser, Soybean Agronomist & Hillary Mehl, Plant Pathologist

We appreciated the rainfall this year.  It should lead to some very good yields in many parts of Virginia.  However, the same weather conditions for high yields has led to some of the worst seed quality that we’ve ever experienced.  This is in addition to the pod splitting and seed sprouting that we began observing last month.

We began seeing extremely poor seed quality in our May-planter maturity group (MG) 3 soybean in our Orange variety test last week.  The early MG 4 soybean did not look much better, but they were not yet mature.

The photo on the left is the worst that has been called to my attention.  These are April–planted early MG 3 soybean from Madison County.  To use the farmer’s words, “A real kick in the stomach.”  Soybean in this shape are pretty much a total loss.  It is especially hard when the yield potential was outstanding.

Below are photos from Westmoreland and Dinwiddie counties showing similar results.  We are not seeing the same problems in our May-planted MG 4 soybean in Suffolk, but are seeing quite a bit of purple seed stain in some varieties.

So, what caused this?  From these photos, we are seeing signs of several diseases, including Phomosis/Diaporthe, Cercospora, and anthracnose just to name a few.  However, we have not yet confirmed the diseases – samples are on the way to the Tidewater AREC’s disease lab.

But that is just the possible diseases.  What caused the disease to be so bad?

I think that we can blame it largely on excessive rainfall (and many rainfall events) and a very warm September.  Most of the seed diseases come on strong during maturation; hence, our early MG soybean, especially those planted in April and early-May experienced those conditions.

Below is a weather summary from the month of September in Orange County, showing 2018 and long term average temperatures and accumulated precipitation.  Notice the high rainfall, especially during the latter part of the Month, when our MG 3 and early-4 soybean were maturing.  Secondly, notice the temperatures during that same time period.  This set up nearly perfect conditions for many seed diseases to form.

  

What can we do about it?  Unfortunately, we cannot control the weather.  And what we have done this year cannot be undone.  But, here are a few pointers for that may help this and in future years.

  • Harvest as soon as possible after soybean are mature. The diseases will only continue to grow and develop.  If you have drying capability, harvest at a little higher moisture and dry it down to 13%.
  • Plant varieties best adapted to your farm(s). While you have discovered (and heard me – David – say) that early-planted early-maturing varieties have very good yield potential if we have a cool and wet July and early-August (not the normal), there is always the risk of poor seed quality.  Only if September is cooler than average and rainfall is not excessive will this system give us good quality seed.  Year in and year out, May-planted late-MG 4 and MG 5 soybean are our best overall choice.  Also, it is rare that double-crop soybean have poor seed quality.
  • Select disease resistant varieties. Most companies do not list resistance to many of our seed diseases, but we have seen differences in varieties.  Look over our final soybean variety test results for more information on seed quality scores and purple seed stain ratings.
  • Select disease-free varieties. Next year, notice your seed quality.  While most companies will not bag disease-ridden seed, some could sneak through.  Seed treatments may be in order.
  • Minimize insect damage to pods. Though not necessary for infection, insect damaged pods are more likely to be colonized by fungal pathogens.
  • What about foliar fungicides? We have not found that foliar fungicides can overcome warm and wet conditions during seed maturation, especially when applied to the R3 stage.  Will a later application help?  That is hard to say. In some cases, applying a fungicide at R5 will improve seed quality, but this is not a sure thing and a yield is unlikely to be improved with later applications.

Corn earworm moth black light trap report for August 23, 2018

Average nightly corn earworm moth captures for local black light traps this week were as follows: Chesapeake=8.0; Dinwiddie=42.6; Hanover=3.4; Isle of Wight=0.9; Prince George (Templeton)=1.3; Prince George (Disputanta)=3.3; Warsaw=6.6; Southampton=0.9; Suffolk=33.0; and Sussex=1.9

Here is the link to the data table:  BLT_23_Aug_2018

Thanks to Watson Lawrence, Mike Parrish, Laura Maxey Nay, Livvy Preisser, Scott Reiter, Mary Beahm, Neil Clark, and Dwayne Sanders for their reports!

Is it too late to apply foliar fungicides in soybean?

Hillary L. Mehl, Extension Plant Pathologist & David Holshouser, Extension Agronomist

The wet weather conditions, along with relatively cool temperatures over the last few weeks have been nearly perfect for disease development in soybean.  As always, keep in mind the disease triangle – all 3 conditions must be met before a disease can form.

In most cases, one or more soybean pathogens are out there.  Refer to previous blogs for more information on common soybean diseases: Now is a Good Time to Evaluate Your Varieties for Foliar Diseases and Foliar Fungicides for Soybeans.  You can also access the Mid-Atlantic Soybean Disease Scouting Guide here. For a positive identification of the disease, send leaf samples to our diagnostic lab.

We also need a susceptible soybean variety (host).  Some varieties are resistant to specific pathogens; others have some tolerance; many have neither.

Of the 3 conditions, the environment is the most difficult to assess.  In general, we need high relative humidity (RH) for an extended period of time, usually over several days, and cool to moderate (e.g. not hot) temperatures.

Our research indicates that about 1/3 of the time, a foliar fungicide will result in yield increase.  The probability and amount of response will of course depend on the disease.  Frogeye leaf spot can be a quite devastating disease in susceptible soybean that have not been rotated.  Cercospora leaf blight (sometimes called late blight) is less devastating, but is common in almost all of our soybean when conditions are right.

We have attempted to create a foliar fungicide decision aid that would help decide on whether or not to spray.  The decision aid is based on the total number of favorable days for disease development, based on the data shown below.

We think that during the period of 1 week before and 1 week after mid-R3, yield-robbing disease levels may develop if we have 3 favorable days with 10 or more hours of relative humidity at or above 95%  and temperatures in the optimum range (77 to 86 F).  When these conditions are met, we would suggest a fungicide application.

However, after 3 years of on-farm testing we only improved the predictability of a yield increase slightly (approximately 50% versus the 33% we see on average).  Why?  We have not been able to predict the weather after the fungicide application.  If things turn dry, the disease will decline and a yield response is not likely.  If conditions remain favorable for disease, then a yield response to a fungicide application is more likely.

So, back to our original question, is it too late to apply a fungicide?

Most of our full-season soybean are past the R3 stage (though many acres were planted in June due to wet weather); so a yield response is not likely if fungicides are applied now.  Generally, it takes about 65 to 80 days after planting to reach the R3 stage, depending on planting date and relative maturity.

Double-crop soybean will usually take 40 (MG 4 planted in July) to 60 days (mid- to late-5 planted in June) to reach R3.  Since most double-crop soybeans recently entered or will enter the R3 stage, a yield response to a fungicide application is more likely.

What about seed quality?  We’ve seen little relationship between an R3 fungicide application and improved seed quality.  To insure good seed quality, we would suggest 2 applications (R3 and R5).  The R5 application might help with our biggest seed quality issue, Phomopsis seed decay, which tends to develop later in the season.  For seed growers, a late application is a good insurance treatment, but keep in mind that if long periods of wet weather delay harvest, seed quality will deteriorate even if a late fungicide was applied.  Be aware that most fungicide labels restrict applications once soybean enter the R6 stage.  Always follow label instructions.

In summary, there are several things that you need to keep in mind that will affect whether or not you will see a yield response to fungicides:

  • Non-rotated soybean will generally have more disease.
  • Variety Resistance. Many varieties have very good FLS resistance; some have only moderate resistance – this may work pretty well if soybean are rotated, but don’t depend on it if soybean were grown last year (or many of the previous years).  Generally, soybean are not resistant to Cercospora blight, but we have seen differences in varieties.  We measure % purple seed stain in our variety tests; however, this is not always a good indicator for resistance to the leaf spot and blight.
  • Timing & Soybean Stage. R3 applications are usually best; not always, but most of the time.  Still, we occasionally see a response with R5 applications.  I (David) have only seen a yield benefit from 2 applications (R3 and R5) once – this was in 80-bushel double-crop soybean in a very wet and cool year.
  • Historically, strobilurin fungicides were our most effective on most soybean disease.  However, FLS is now largely resistant to that class of fungicides, and control of our other diseases have declined.  However, the strobilurins still have utility.  We suggest a fungicide containing a strobilurin along with a good triazole. The 2018 soybean fungicide efficacy table can be downloaded here.
  • Spray Volume & Droplet Size. Good coverage of fungicide throughout the canopy is necessary.  Use at least 15 gallons per acre spray volume and make sure that your nozzles will deliver medium-sized droplets.  Small droplets will not penetrate the canopy to the lower leaves, where the fungicide is most needed.  Large droplets will not provide uniform coverage.  For more detail on this, see Application Equipment for Effective Insect Pests and Foliar Disease Control.
  • Seed Quality. Although, we don’t always see a seed quality benefit from an R3 application, two applications should improve seed quality.  Furthermore, a late-season (R5) application should help with certain seed diseases such as Phomopsis seed decay. However, keep in mind that if cool, wet conditions delay harvest seed quality will deteriorate even with a late fungicide application. Controlling late season insects such as stink bugs is also critical for preventing fungal infection and maintaining seed quality.

 

 

 

Plant Bug Update – 02 Aug 18

Tarnished plant bugs have continued to be a problem in flowering cotton in some Virginia cotton fields. This week, scouts found 10 out of 30 fields above the bloom threshold of 2-3 plant bugs per drop cloth sample across Virginia’s cotton-growing region. Six of these fields averaged above the bloom threshold for the past two weeks (see map below). Dirty blooms may indicate plant bugs are present but should not be used as a threshold for spray decisions. Cracking bolls and observing internal boll injury in small dime and quarter-sized bolls is also a great indicator bugs may be present and causing significant damage. Internal boll injury includes raised warts or outgrowth on the carpel wall, small black punctures that usually match an external lesion, and stained lint. We recommend observing at least 25 bolls per field for internal injury in addition to drop cloth sampling to make informed spray decisions. Plant bug populations have peaked in mid-August over the past few years in Virginia so we recommend continued scouting during the coming weeks. Check out this video if you would like to see a short tutorial on drop cloth sampling.

Plant bug density averaged over two weeks from 24 July to 02 August 2018.

Plant bug internal boll injury symptoms.

Plant bug nymph feeding on a dirty bloom.

 

Black light trap and vial test update for August 2, 2018

Corn earworm moth captures in black light traps continued to climb this week–please see the attached table (pdf document) for more details: BLT_2_Aug_2018

We have tested 246 corn earworm moths in our vial tests this season, with an average of 15% surviving a 24-hour exposure to the pyrethroid insecticide cypermethrin at 5 micrograms per vial. The line graph is available here: vial_tests_2_Aug_2018

At our research farm in Suffolk this week, we have been finding bollworm eggs in cotton terminals with some small larvae on squares and bolls. Please be sure to scout your fields!

2018 survey of field corn for corn earworm larvae

Statewide, approximately 18% of ears were infested with corn earworm larvae. Corn is considered a nursery crop for corn earworm, allowing the pest to complete a lifecycle and then move on to other crops such as soybean, cotton, and peanut in August. There is a linear correlation between the infestation level in corn and the amount of soybean acreage that gets treated with insecticide for this pest. Please see the attached pdf for more details: CEW_survey_results_2018