Weather and herbicides – what to do (or not) this week

Source: Dr. Mark Loux, OSU

Current forecast is for fairly warm temperatures through late evening Tuesday evening, followed by a substantial drop in temperatures and chance of snow, followed by cold/cool temperatures through the weekend.  Primary question concerning this scenario seems to be whether it is okay to apply wheat or burndown herbicides prior to this cold snap.  Some things we know about herbicides and cold weather:

– Herbicides applied to an emerged crop just prior to or during cold weather may be more injurious compared with favorable weather conditions.  During cold weather when plants are not actively growing or growing slowly, the rate of translocation and metabolism of herbicide by the plant slows down, which can mean an accumulation of herbicide that is not being metabolized.  This can increase the risk of crop injury since metabolism of herbicide by the crop, or conversion to an inactive form, is what allows that herbicide to be safely used on the crop in the first place.  For some herbicides, there is such a large margin of safety with regard to crop safety that this is all inconsequential.  For others the margin is narrower and issues such as cold weather and sprayer overlaps are more important.  The inclusion of safeners in herbicide formulations reduces the risk of injury, usually be increasing the rate of metabolism, but may not completely solve issues that arise because of adverse weather or too high a dose.  So with regard to this week and risk of injury to wheat, we would recommend avoid applying herbicide once the cold weather starts (from Wednesday on), until warm weather resumes.

– There is less certainty in making a recommendation about whether to treat wheat on Tuesday prior to the cold weather.  We have seen instances in corn where application just prior to cold weather has resulted in greater injury.  Wheat is actively growing now under favorable weather, and should readily translocate and metabolize herbicides.  Much of this process occurs within the first few hours of application.  Temperatures do not really start to plunge until early Wednesday morning per the forecast.  While it’s somewhat of a guess, it seems that application during the first part of Tuesday would be possibly safer to the crop than later in the day.  Past experience has shown us that some wheat herbicides are just generally safer than others, so one option would be to omit the ones that have stricter growth stage guidelines or have more of a history of causing injury.  Having said this, in our research we have really not experienced injury from small grain herbicides applied per label.

– With regard to efficacy of burndown herbicides and cold weather, some of the same principles apply.  Applying herbicide from Wednesday through the weekend, when weeds are not actively growing, is not recommended due to the likely loss of activity.  Susceptible weeds metabolize herbicide slowly anyway, so the issue is a lack of translocation within the plant and the inability of herbicide to do it’s thing at the active site when plant processes are shut down.  This is the type of cold weather we referenced in the recent article about dandelions, when we have observed control of this weed to plummet.  We have also observed extremely slow control of overwintered annual weeds during cold weather.

– We would recommend going ahead with burndown herbicide applications on Tuesday, prior to the cold.  As with wheat, weeds are actively growing under favorable weather so we assume herbicides will work.  It’s still a bit of a guess, but it could be a while before field conditions and weather are suitable for application again.

– This is the type of scenario that makes us want to remind everyone again that a few dollars of herbicide in the fall can help avoid some of the nasty burndown issues that develop when spring conditions are less than optimum.  Just saying.

 

OSU Extension Virtual Programming: Week of March 8

The following virtual programs are available next week.

 

MONDAY, MARCH 8

Farm Bill Webinar: 2021 Corn and Soybean Crop Insurance Considerations

10:00 am to 12:00 pm

2021 Virtual Ohio AgritourismReady Conference

6:30 pm to 8:30 pm

 

TUESDAY, MARCH 9

Virtual Conservation Tillage Conference

8:00 am to 3:00 pm

 

WEDNESDAY, MARCH 10

Virtual Conservation Tillage Conference

8:00 am to 3:00 pm

Southern Ohio Farm Show (Virtual)

10:00 am to 11:00 am

Beef Sire Selection for the Dairy Herd (Virtual)

12:00 pm to 1:00 pm

Farm Office Live

7:00 pm to 8:30 pm

 

THURSDAY, MARCH 11

Virtual Conservation Tillage Conference

8:00 am to 3:00 pm

The Dirt on Soil Health: Investing Below the Surface (Virtual)

8:00 am to 8:30 am

 

Wood Destroying Insect Inspection Training Webinar

8:30 am to 3:30 pm

Midwest Women in Ag Community Education Series

9:00 am to 11:00 am

County Outlook Meeting (Virtual)

10:00 am to 11:30 am

East Ohio Women In Agriculture Program Series

12:00 pm to 1:00 pm

Butler Innovative Farm Forum (Virtual)

7:00 pm to 8:30 pm

 

FRIDAY, MARCH 12

Virtual Conservation Tillage Conference

8:00 am to 3:00 pm

A DAY in the WOODS (Virtual)

10:00 am to 11:30 am

Escape to the Forest Webinar

10:00 am to 12:00 pm

Farm Office Live

 

Herbicide Resistance in Ohio Waterhemp Populations

Source:  Mark Loux, OSU Extension

Waterhemp populations across the Midwest continue to develop more complex variations of herbicide resistance.  Multiple resistance to an increasing number of herbicide sites of action is the norm in many populations in states west of Ohio.  Waterhemp has on the whole developed resistance to seven sites of action, including the following:

Group 2 – ALS inhibitors – chlorimuron, imazethapyr, etc

Group 4 – Synthetic auxins – 2,4-D, dicamba, etc

Group 5 – Photosystem II inhibitors – atrazine, metribuzin, etc

Group 9 – EPSP synthase inhibitor – glyphosate

Group 14 – PPO inhibitors – fomesafen, flumioxazin, sulfentrazone, etc

Group 15 – long chain fatty acid inhibitors – metolachlor, pyroxasulfone, etc

Group 27 – HPPD inhibitors – mesotrione, isoxaflutole, topramezone, etc

Individual populations with resistance to three or more sites of action are common.  Mutations are occurring that confer resistance to several of these sites of action simultaneously, through a resistance mechanism that enhances the metabolism and inactivation of the herbicides by the plant.  For example, there appears to be a linkage in the resistance to mesotrione and atrazine, where resistance to one means it’s likely that resistance to the other occurs also.  Weed scientists have concluded that this weed is capable of developing resistance to any herbicide site of action used against it.  We aren’t actually sure what the correct recommendation is for stewardship of herbicides once a single mutation can confer resistance to multiple sites of action.  Which is the reason we stress the need to take steps in mid to late season to prevent seed from plants that survive management strategies.

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Spot the Spot – Efforts Continue to Look For Spotted Lanternfly (SLF) in Ohio

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Spotted Lanternfly Adult
Recently, an Ohioan returned from a road trip to Pennsylvania. In addition to all the memories made, this traveler unintentionally brought back a hitch-hiker – a spotted lanternfly (Lycorma delicatula) (SLF). The individual quickly captured and ended the insect’s life before reaching out to his local Extension Educator. The suspect sample was submitted to the Ohio Department of Agriculture (ODA) for confirmation based on the USDA protocol established to confirm non-native pests not currently established, or with limited presence in the case of Asian Longhorned beetle, in the state.
SLF is well-known for its ability to hitch-hike into a new area within an already infested state, or in the case of Ohio, a state that is currently considered uninfested. If you are traveling to, or through, and infested area, you are encouraged to check your vehicle and any items that may have been outdoors during the trip (i.e., tents, camping supplies, recreational equipment). It is important to know that states have quarantines in place to limit the unnatural spread of SLF both within their state and other states and includes both any stage of the actual insect and any item that could move the insect (i.e., plant material, firewood, logs, outdoor furniture).

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Leafhoppers, Grasshoppers, and Beetles, Oh My!

Source: Kelley Tilmon, Andy Michel, OSU

As the summer progresses we are receiving reports of insect problems often encouraged by hot, dry weather.  Last week we reported on spider mites and especially if you are in an area of continued dry weather we recommend scouting your soybeans and corn  https://agcrops.osu.edu/newsletter/corn-newsletter/2020-22/watch-spider-mites-dry-areas .

Some areas are also reporting increases in young grasshoppers in soybeans, another insect favored by dry weather.  Grasshoppers of often start on field edges so early scouting may allow for an edge treatment.  Japanese beetles are another common defoliator of soybean that are starting to appear.  Both of these pests fall into a general defoliation measurement, and we recommend treatment if defoliation is approaching 20% on the majority of plants in post-flowering beans.  Download our guide to estimating defoliation in soybean at https://aginsects.osu.edu/sites/aginsects/files/imce/Leaf%20Defoliators%20PDF_0.pdf

A weird problem being reported not just in Ohio but in parts of the Midwest as far-flung as Minnesota is the red headed flea beetle, which is being found in corn and soybean.  This is a small, narrow, shiny black beetle with a red head which springs like a flea when disturbed.  Feeding in soybean creates small round holes and in corn longer narrow strips of damage.  This feeding is seldom economic.  In soybean follow the general defoliation threshold of 20%.  Leaf feeding in corn is almost never economic, but be on the watch for silk-clipping, which is rare but possible.  There are no thresholds in corn, but our Minnesota colleague Bruce Potter suggest this guideline: “flea beetles are very numerous (it is likely more than 5-10/plant), pollination is less than 50% complete, and numerous plants have silks clipped to within 1/2 inch, you might consider an insecticide.”

Finally, earlier in the season we reported higher than usual numbers of potato leafhopper in alfalfa and encouraged stepping up scouting.  In some fields third-cut alfalfa is being heavily impacted by this insect.  You can review our scouting advice for this insect at https://agcrops.osu.edu/newsletter/corn-newsletter/2020-17/time-start-scouting-potato-leafhoppers-alfalfa

Ready to manage spider mites?

With the current hot and dry weather conditions in Ohio, we expect to hear reports of spider mite outbreaks on specialty crops. Because mites are tiny, they are often overlooked or misdiagnosed as a disease. Infested leaves have fine webbing on the leaf undersides. Tomato leaves damaged by spider mites usually have yellow blotches, while bean leaves show white stipples or pin-prick markings from mite feeding. Pumpkins can tolerate moderate levels of mites, but watermelons are more sensitive to injury from mite feeding. A simple method of diagnosing spider mites is to shake leaves over a piece of paper and look for moving specks that are visible to the naked eye. A closer look with a magnifier can show the tiny mites that are white, marked with two large dark spots on the middle of the body.

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Drought Projections Do Not Go Well With Fungicide Applications

Source: Anne Dorrance, Pierce Paul, OSU

Several calls this past week for fungicide applications on corn and soybean at all different growth stages.  So let’s review what might be at stake here.

Soybeans.  Frogeye leaf spot and white mold on susceptible varieties when the environment is favorable for disease easily pay the cost of application plus save yield losses.  Let’s dig a bit deeper.  Both of these diseases are caused by fungi but frogeye leaf spot is a polycyclic disease, meaning that multiple infections occur on new leaves through the season while white mold is monocyclic and the plant is really only susceptible during the flowering stage.  Both of these diseases are also limited geographically in the state.  White mold is favored in North East Ohio and down through the central region where fields are smaller and air flow can be an issue.  Frogeye has been found on highly susceptible varieties south of 70, but it is moving a bit north so it is one that I am watching.

White mold is also favored by closed canopy, cool nights and high relative humidity.  So farmers in these areas should double check their variety ratings first.  If it is moderate to low score for resistance (read the fine print) then this year a spray may be warranted.  We have gotten consistent control of white mold with Endura at R1.  Herbicides that are labeled for white mold suppression have also knocked back this disease, but if a drought occurs or no disease develops, losses of 10% or greater can occur due to the spray alone.  For these purposes R1 is a flower on the bottom of 1/3 of the plants in the field.

Frogeye leaf spot –There also must be some inoculum or low level of disease present in the field for this disease to cause substantial and measurable yield losses.   This disease will only move in the canopy when there is regular rainfall.  And again only on susceptible varieties. With dry weather, this will sit and hold. Time to scout for this will be at the end of flowering if it can be found in the field.  With drought conditions, the disease will not impact the crop.

The story is very similar from a corn pathology standpoint. Most of our major diseases (gray leaf spot, northern corn leaf blight, eye spot) are driven by wet, humid conditions, consequently, the dry weather we have experienced over the last several days will keep most diseases in check. Fungicides are not warranted under these conditions; it just does not pay. Although some product labels may mention yield responses under drought-like condition, our data do not support such a benefit. We see the highest yield responses when fungicides are applied to susceptible hybrids at VT-R1 under disease-favorable conditions. These conditions would include extended periods of dew and high relative humidity, especially during the early- to mid-morning hours.

For a disease like southern rust that usually blows up from the south, and tar spot, an emerging disease of increasing concern in the state, fields should be scouted before making an application. Both diseases develop well under warm conditions, but they also need moisture and high relative humidity to spread. In the case of tar spot, based on what we have seen in 2018 and 2019, it usually develops well into grain fill (R4-R5), and as such, may have little effect on grain yield. Data from some states in the western half of the corn belt show that when tar spot develops early, yield loss may be substantial. The same is true for early southern rust development. So, scout fields to see what is out there and at what level before investing in fungicide application.

True Armyworm Infestations

Source: Andy Michel, Curtis Young, CCA, Kelley Tilmon, OSU

 

As you scout your fields this week be on the lookout for this pest!

 

We received many reports of true armyworm infestations in wheat, barley, and corn. These are black or green caterpillars with stripes along the side and orange heads.  In the spring, true armyworm moths migrate from the south and lay eggs in grasses such as forage and weed grasses, winter wheat and barley, and rye cover crops.  When the eggs hatch, the larvae can significantly damage wheat and barley before then moving to young corn. Usually, moth flights occur in April, but we may have had a second peak the first or second week of May—it’s likely the caterpillars feeding now are from this later flight. Right now, wheat, barley, and corn should be inspected for true armyworm populations. Armyworms like to hide during the day and feed at night, so scouting should occur at dusk or dawn, and/or on cloudy days.

Corn: True armyworm in corn cause the most damage when planted in no-till grassy fields, such as a rye cover crop.  In this case, after feeding on the cover crop, the caterpillars shift onto the emerging corn.  The name armyworm comes from the caterpillars’ behavior of migrating en masse from one location to another. Thus, one should pay particular attention to cornfields adjacent to wheat fields that may have supported a high armyworm population, especially the first several rows into the cornfield. As the wheat matures and dries down, it could stimulate the caterpillars to move.

One may only need to treat the edge of the field closest to the wheat field from which the caterpillars are marching. If armyworms are found in a cornfield, check for the percentage of plants damaged in 5 sets of 20 plants.  If more than 10% of the stand has feeding damage, it may indicate a large infestation, and the field should be re-checked in a few days to see if defoliation is increasing. If defoliation has increased and plants have two or more caterpillars per corn seedling, an insecticide application may be necessary. However, if most larvae are longer than 1 inch, then much of the feeding is complete as the caterpillars will begin to pupate. Also, look for the presence of diseased (black and shriveled) or parasitized caterpillars (having a few or several small, white egg cases on their body)—if found, do not include them in your counting.

If defoliation exceeds 50%, even a rescue treatment may not recover the field without a significant impact on yield.  According to the Handy Bt Trait Table (https://agrilife.org/lubbock/files/2020/02/BtTraitTable_FEB_2020.pdf), only the Vip3A (e.g., Viptera) Bt trait is effective against true armyworm.  Insecticidal seed treatments may offer some control but can be overwhelmed with high populations. Plus, insecticidal seed treatments last only about 4-6 weeks after planting.

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