Tag Archives: Frogeye leaf spot

How Late Can You Spray for Frogeye Leaf Spot?

Frogeye leaf spot is easily recognized by the circular to angular spots on the upper and lower leaf surfaces.  These spots are usually surrounded by dark, reddish-brown margins and can be found throughout the canopy. Frogeye Leaf Spot - Painter 2013 If these spots are numerous enough to cover 30% of the leaf area, leaves will quickly wither and fall off of the plant prematurely.  This is a serious disease and can cause substantial yield loss, so keep a close eye on your fields.

Many, if not most of our soybean varieties have some resistance to the disease.  If the variety is labeled as “resistant”, you’ll usually not experience the disease.  However, I have seen and continue to see significant disease on varieties will “moderate resistance”, especially in non-rotated fields.

The main question that I’m getting now is “How late is too late to spray for frogeye leaf spot?”  First, check the fungicide label that you intend to use.  Applications with some fungicides must cease when the soybean reach the R6 stage.  Others may have a 14 to 21 day pre-harvest interval.  The R6 stage will last nearly 3 weeks and the R7 stage will last another week to 10 days; so the pre-harvest interval is not my major concern.  Regardless, make sure that you follow the label.

Assuming that there are no label concerns with fungicides, my inclination is to spray the field, especially if yield potential is good.  My concern with not treating is that if weather conditions are favorable, the infected areas will produce more spores and cause additional infection.  Keep in mind that at R5, only about 25% of the yield has been made; at the start of R6, only 50% has been made.  So, from this standpoint, there’s still lots of yield to lose if the problem were to get worse.

Seed Formation through MaturitySoybean Yield AchievedIf you decide to spray, I highly recommend a combination product such as Priaxor, Quilt XL, Quadris Pro, or Stratego XL.  There are reports of failures to control frogeye with only strobilurin fungicides (Headline, Quadris, Approach).  We have not documented this, but using a combination product may help prevent.  In addition, a strobilurin + a triazole (or carboxamide in the case of Priaxor) should provide better control.

Frogeye Leaf Spot Increasing in Virginia Soybean

Ed Seymore, TAREC Ag Technician who is scouting fields for brown marmorated stinkbug (BMSB), aphids, kudzu bug, etc., reported to me today that every soybean field that he is checking west of I95 in VirginiaFrogeye Leaf Spot - Painter 2013 (He’s in the Shenandoah Valley today) has frogeye leaf spot.  Some fields are heavily infested (all leaves; up to 20-25 spots per leaflet).

I called this to your attention earlier this year, as I was seeing frogeye leaf spot symptoms in several variety tests.  Many of our varieties have resistance to the disease, but some do not.  In addition, the level of resistance varies with variety.  The disease will also be worse in non-rotated fields (continuous soybean).  In the past, I’ve found that varieties with good resistance truly resist the disease.  Varieties with moderate resistance tend to hold up pretty well if soybean are in rotation.  In rotated fields containing a susceptible variety, the disease can be severe but not devastating.  But, the disease can devastate soybean varieties with no resistance when these soybean are following soybean (see photo below from non-rotated field planted to susceptible variety).Frogeye Brunswick Co - 2004 5

So, be sure to scout your soybean fields for this disease.  Symptoms are round spots with tan/grey centers and reddish halos around the spot.  I have no good threshold for treatment, but if you have a susceptible variety and/or are growing soybean after soybean, a fungicide application is in order.  If you have not already applied a fungicide, I’d also suggest using a fungicide that combines a strobilurin with a “curative” fungicide.  Combination products that have performed well on other disease, but not necessarily on frogeye leaf spot (frogeye has not been a big problem in the recent past) in Dr. Pat Phipps tests include: Priaxor, Quilt XL, Stratego YLD, and Quadris Top.

Foliar Fungicides May Pay in 2013

It seems that it’s been raining constantly in many places this year.  In addition, soybean growth is generally very good, creating a canopy that will maintain high relative humidity through much of the day.  Although the long-term forecast has temperatures getting into the 90’s on some days, it looks as if the 80’s will be the norm for the next 10 days or so.  These high relative humidity, rainfall events, and favorable temperatures will favor foliar disease in soybean.  Therefore, fungicide applications to R3 (beginning pod) to R5 (beginning seed fill) may pay off in 2013.

This past week, I’ve looked at several variety trials, both on-farm and the Official Variety Tests (OVT).  I’m seeing a significant amount of frogeye leaf spot.  This photo was taken in our OVT in Painter.  Although most of our Frogeye Leaf Spot - Painter 2013varieties have resistance to this disease, some do not.  You should check the seed catalog or with your seedsman to determine whether the varieties that you’re using has resistance to this disease.  If not, a fungicide will be in order if you see leaf spots forming.
Although frogeye leaf spot can be quite devastating, I’m not overly concerned since most varieties are not showing symptoms.  However, I consider this disease an indication that conditions are right for disease formation, sort of a “canary in the coalmine”.  If frogeye is prevalent, then other diseases such as Cercospora leaf blight will likely be raising its head as well.

Dr. Pat Phipps is developing a model to help us predict whether or not a fungicide will be needed.  He has much experience with and has developed effective prediction models in peanut; therefore, we think that such a model may be effective in soybean.  He will present his research at next week’s Virginia Soybean Field Day, so be sure to attend to hear his latest update.  In the meantime, listed below are the conditions that will favor disease development.  We seem to be meeting these criteria.Disease Risk Model

Many of you have likely already applied a fungicide to your full-season soybean.  The most consistent yield response has been when the fungicide has been applied at the R3 development stage.  However, we’ve seen responses as late as R5 (see article in this newsletter for an example of this).  So, with the weather conditions that we’re experiencing, I think that we could see a response even with late applications.

Still, keep in mind that fungicides are preventative; therefore, the weather conditions after the fungicides have been applied are most relevant.  Unfortunately, predicting temperatures and rainfall events is not always accurate.

Finally, it is very important to select the proper fungicide.  Our research has proven that strobilurin fungicides or pre-mixes that contain a strobilurin fungicide are most effective again the most common soybean diseases that we experience in Virginia.  The triazole fungicides are not as effective.  However, strobilurin/triazole tank mixes or pre-mixes have tended to give us the best control.  Products that have proven effective in our tests and that we would recommend using in soybean include:

Headline (pyraclostrobin)

Quadris (azoxystrobin)

Stratego YLD (trifloxystrobin + prothioconazole)

Priaxor (pyraclostrobin + fluxapyroxad)

Quilt Xcel (azoxystrobin + propiconazole)

Quadris Top (azoxystrobin + difenconazole)

Note that all contain a strobilurin, which we think is necessary.

Keep in mind that if soybean rust were to come into Virginia, we will likely need a triazole as these fungicides are most effective against that disease.  Although soybean rust is still far from Virginia (see map below), it is on the move.  I suspect that we’ll see the disease in Virginia this year.  But, I hope that it comes late as it has done in past years.Soybean Rust 081313

Now is a Good Time to Evaluate Your Varieties for Foliar Diseases

September is a great time to evaluate your crop and the performance of varieties that you chose.  In addition to general growth and health of the crop, take some time to determine if you have any of the below diseases.  If so, you could be losing some yield.  If you sprayed with a fungicide and still have disease, reconsider the product and rate used and the time that the fungicide was applied.  Keep in mind the weather conditions when the application was made and the conditions 2 to 3 weeks after or before the product was applied.  Cool temperatures (70’s) and high relative humidity (>95% for 12 hours or more) will usually increase disease incidence.

Another caution is to never diagnose a specific disease on the plant without verifying it with a person trained to identify plant pathogens.  Only when the reproductive structures are found on the leaf can a disease be confirmed.  Many things will cause look-alike symptoms.  Be sure before you cast the blame.  There are more diseases than just the ones shown below, but these are the most common.  Brown spot is normally found in the lower part of the crop canopy (the lower leaves), Cercospora blight and leaf spot will be found throughout the canopy, and the frogeye leaf spot and downy mildew tend to be found in the upper part of the canopy.