Author Archives: David Langston

Nematodes in Atlantic Soybean Production Webinar

Nematodes consistently rank as a top yield robber of soybeans. Please plan to join us virtually on December 5 from 1:00-3:00 PM for the Nematode in Atlantic Soybean Production Webinar. This webinar will include results from Soybean Cyst Nematode seed treatment trials, Root-Knot Nematode on-farm trials, emerging management strategies, and feature a panel discussion on needs to improve nematode management in the region. The webinar is free, but registration is needed for access to the zoom link. Registration information can be found athttps://www.pcsreg.com/2022-nematodes-in-atlantic-soybean-production-webinar.  Please email Alyssa Koehler akoehler@udel.edu with any questions. 

Thoughts about late season peanut disease this year…

I have received a lot of questions regarding leaf spot fungicide treatments in the past two weeks. A lot of this is driven by either: 1) fungicide failures in fields with high disease pressure; or 2) fields under heavy drought stress. In both cases I have advised growers and consultants to look at low cost options. I’ll cover both situations below.

Fields with high late leaf spot pressure Late leaf spot outbreaks have shown up in fields that have been sprayed with Miravis that has traditionally performed well in past years. In each case the fields were in a 4 year or less rotation, most of these fields were irrigated, and growers were using the 4 week extended spray interval to take advantage of the residual activity provided by Miravis. What growers and consultants are interested in now is what to do to hold the leaves on until digging. I try to reduce fungicide costs this close to digging while providing late leaf spot management that reduces excessive defoliation. My “go to” has been Microthiol Disperss (dry sulfur formulation) mixed with a Group 3, DMI fungicide. University researchers, including myself, have observed favorable results with this combination in field trials. The hesitancy in using this program is the added complexity of using a dry formulation and sourcing Microthiol Disperss. Some grower opt for liquid formulations of sulfur because of the two reasons just mentioned, but most of the positive results reported have been with Microthiol Disperss and the amount of sulfur provided by liquid formulations is variable depending on each product, and often the amount of sulfur used with liquid formulations is less than what is provided with Microthiol Disperss. I prefer Microthiol Disperss because it has proven effective in managing high levels of late-season late leaf spot and because it is inexpensive (about $1 per pound which is $5/acre at the recommended 5 lb rate). The sulfur provides instant reduction in leaf spot inoculum but little to no residual activity. That’s why I recommend mixing it with a DMI fungicide like Provost Silver or less expensive Alto. DMI’s provide some curative activity, but most importantly residual activity. Group 7 (SDHIs) and Group 11 (strobilurins) don’t provide curative activity and are most active when applied preventively when little-to-no leaf spot is present.

Fungicide decisions on drought stressed peanuts In many areas peanuts are under severe drought stress with many fields exhibiting wilting 24 hours a day with growers being reluctant to apply leaf spot fungicides due to dry conditions. In many cases these fields are 10-14 days away from digging. This is really a situation where I tend to favor no fungicide applications or at least one application of a very inexpensive fungicide. During dry conditions we often avoid using chlorothalonil (Bravo) due to it’s propensity for flaring spider mites. In my opinion the fungus that parasitizes spider mites to keep them in check is already absent due to the severe drought which is why many growers are seeing outbreaks of spider mites in certain areas. Since the fungus that keeps spider mites at bay (and that chlorothalonil reduces) is already absent, if chlorothalonil is chosen I see that as a low-risk application. Another less expensive option would be Alto which shouldn’t flare spider mites. My personal favorite is to not apply a fungicide at all because: 1) there is little to no leaf spot in these wilted peanuts; and 2) leaf spot outbreaks take a good bit of time to get going and by the time you see any leaf spot in these fields it will be too late for the disease to cause yield damage by the time they are dug. In Suffolk, VA the extended forecast shows no rain chances over 20% for 9 days, at least with the weather app I use. So leaf spot pressure will be low until digging in most cases which further decreases the chance of outbreaks occurring. Deciding not to spray a fungicide is a low risk option in my opinion.

Aflatoxin potential It’s been many years since growers have had to think about dry conditions leading to aflatoxin-contaminated peanuts, which is a good thing. This year is different due to the drought stress near harvest. I recommend keeping peanut field harvests separate between fields with low and high risks of aflatoxin contamination. In other words, avoid mixing peanuts from severely drought-stressed and fields that have not had as much drought stress (possibly irrigated). If peanuts that have had drought stress are to be used for seed, they should receive a quality seed treatment to reduce poor stand due to the aflatoxin fungus in peanuts planted next spring.

Be on the lookout for Sclerotinia blight in peanuts.

The system that brought rain on August 10th seems to have broken our streak of hot weather for Virginia’s peanut growing region. Dailey high’s in the 80s with lows in the 60s will continue until August 23rd according to my weather app. The temperature Saturday the 13th may not get above 80o F and the low may be in the upper 50s, which is very favorable for the development of Sclerotinia blight. I anticipate the risk of Sclerotinia blight to be high the next 2 weeks, so fungicide sprays may be warranted in fields with a history of that disease. If growers have sprayed the second spray of Miravis + Elatus fungicides within the past 2 weeks they should be covered. I do not recommend spraying the Miravis + Elatus fungicide tank-mix now if you haven’t sprayed it already due to fungicide resistance concerns with late leaf spot and the lack of curative or “kick back” activity if leaf spot already exists. If the Miravis + Elatus tank-mix has not been sprayed twice already I recommend using Omega 500 at the labeled rates for Sclerotinia blight sooner than later. Typically we recommend growers scout for Sclerotinia blight when the risk of that disease is high according to the Virginia Sclerotinia Blight Advisory. Omega 500 will have greater efficacy if applied prior to or at the initial onset of disease. Follow-up sprays may be made using Omega 500 21 days after the first application if favorable disease weather persists.

As always, if you have questions or concerns about diseases of agronomic crops please feel free to call or text me at (757) 870-8498 or e-mail me at dblangston@vt.edu.

Corn, Peanut and Soybean Disease Update

Corn

Virginia growers are over the hump so far as corn disease management is concerned.  Late season diseases such as tar spot and southern corn rust have not been observed in Virginia at this time and we are past the R3 (milk) stage which is the cutoff for making effective fungicide applications for each disease.  Click the link for updated maps showing where each disease is currently in the U.S. https://corn.ipmpipe.org/diseases/

Peanuts

As of August 1st, peanut growth and development is a little behind in areas that have experienced periods of severe drought.  Heat units for peanuts planted on May 1st so far are on par with 2021, which was a record-breaking year so far as yield is concerned.  Growers in Virginia have made at least one fungicide application for leaf spot by August 1st, and many have made 2 applications.  The risk of Sclerotinia blight has been variable based on the differences in vine growth due to drought in certain areas, however, recent rainfall events have been more numerous in the region and I expect disease pressure will continue to increase.  Increased rainfall and relative humidity will shorten the LESD (last effective spray date) for leaf spot for most fungicide chemistries and growers using Miravis should consider spraying at 21 days after the first application if disease risk for leaf spot is continually high since the last spray of that product.  Growers that have already made applications of Miravis tank-mixed with Elatus will have some protection against Sclerotinia blight and growers that are opting for Omega 500 should be scouting fields with histories of that disease, especially in those fields where peanut canopies are dense, to make applications at disease onset.  If Miravis or the Miravis/Elatus combo was not applied as the first or second fungicide application I do not recommend using it in August or later due to concerns with fungicide resistance and the lack of curative or “kick back” activity associated with those chemistries.    If peanut leaf spot severity becomes high due to weather and the inability to get equipment in fields to make timely fungicide applications, rescue treatments using Microthiol Disperss or chlorothalonil tank-mixed with Group 3 fungicides, such as Provost Silver, to reduce defoliation prior to digging.

Soybean

Fungicides have recently been going out for foliar diseases recently as most soybeans are in the R1 – R5 growth stages.  Recent weather patterns have brought and continue to bring rain events and high relative humidity which favor the development of frogeye leaf spot and other foliar diseases.  My main concern for fungicide applications is to avoid using Group 11 (strobilurin) fungicide chemistries alone as fungicide resistance to those has been previously documented in the frogeye leaf spot pathogen in Virginia.  There are plenty of Group 3 (triazoles), Group 7 (SDHI) and Group 11 fungicide combinations available to manage foliar soybean diseases.  Also, I’m thinking of naming August as “SDS (sudden death syndrome) month” as that is when SDS usually starts showing up.  The first clinic sample with SDS came in Tuesday on soybeans planted April 15th.  Typically, SDS symptoms occur in early planted and early maturing soybeans first and continue as soybeans mature.  Soybeans planted early in cool (at or below 60°F), moist soils are more at risk of infection by the pathogen that causes SDS, Fusarium virguliformeThe most visible symptoms occur at the late R5 growth stage and into R6.  Managing this disease is strictly preventive through the use of crop rotation, delayed planting, resistant varieties and the fungicide seed treatments ILeVO and Saltro.  We are gathering data on SDS outbreaks in Virginia so please contact us if you suspect this disease in your fields.

As always, feel free to contact me if you have questions regarding disease and nematode issues in your fields. e-mail: dblangston@vt.edu, cell: 757-870-8498

When to start spraying peanuts for leaf spot and other diseases.

I have had a few questions about starting a peanut fungicide program on slow growing or late planted peanuts recently. The rule of thumb date I typically use is if you haven’t sprayed for leaf spot by July 10th, go ahead and start making applications by that date. I know many growers that may spray once or twice before July 10th or R3 (early pod), but this year may be a good year to start spraying later. My thought process here is that: 1) leaf spot fungicide applications made on young peanuts in the early, vegetative stages are less susceptible to leaf spot; 2) much of the fungicide applied ends up on the soil surface and can’t protect peanuts or be taken up by foliage; and 3) since it seems peanut growth is delayed digging may occur later than normal so fungicide applications may be needed later than normal. If you start spraying now you may increase the number of fungicide applications you end up making, driving up production costs. In fields on rotations of 3 years or less (higher risk of leaf spot), spraying earlier than July 10th may be warranted. The best way to reduce fungicide inputs is by using a spray advisory such as the Virginia Peanut Leaf Spot Advisory available on the Peanut-Cotton InfoNet (https://webipm.ento.vt.edu/cgi-bin/infonet1.cgi) or the Virginia Peanut Hotline at 1-800-795-0700.

Speaking of disease advisories, don’t forget the VSBA (Virginia Sclerotinia Blight Advisory). One thing to make note of with the VSBA is that much of the risk calculated for Sclerotinia blight is based on the vine growth and canopy development of peanuts. So if peanuts are behind in growth the risk of developing Sclerotinia blight is also delayed. Therefore fungicide applications made to prevent Sclerotinia blight should be made later depending on vine growth. I would at least wait until vines are less than 6-inches from meeting in row middles before making a fungicide application specifically for Sclerotinia blight. The Miravis + Elatus tank-mix targets both leaf spot and Sclerotinia blight, so the July 10th start date still applies to the first application of that combination. Keep in mind that if it looks like we may be digging peanuts late that an additional application of Omega 500 may be needed up to 30 days prior to harvest to manage Sclerotinia blight if the second Miravis + Elatus tank-mix is applied in early August. Also, I recommend a FRAC Group 3 or DMI fungicide (like Provost Silver) that’s effective against Sclerotinia blight to go out with the last application of Miravis in case some leaf spot is present at the time of application. FRAC Group 3 or DMI fungicides have some curative or “kick-back” activity which can arrest leaf spot that’s present at the time of application.

Wheat Disease Update 4/27/2022

I have had reports of powdery mildew showing up in a few fields lately. Look for white, powdery spots on leaves in the lower canopy. The undersides of leaves can be covered in the powdery growth of the fungus. Cool, humid, dry conditions favor infection, while prolonged periods of cool, humid weather in spring can allow the disease to reach the flag leaf and cause yield losses. If you see powdery mildew in your wheat prior to heading a triazole, such as propiconazole (Tilt and generics) can do the trick, but the earlier the application the better. Fungicides applied at heading for wheat scab are also very effective against powdery mildew.

White, powdery spots. Photo courtesy of Rick Brown.
Powdery growth covering underside of leaf. Photo courtesy of Rick Brown.

Wheat is heading and flowering across wheat growing regions in VA. The risk of wheat scab remains low due to dry, windy conditions. No fungicide application is warranted for scab management to crops that are currently flowering or will flower in the next several days. To assess risk of wheat scab, go to the web site https://www.wheatscab.psu.edu

Wheat Diseases – Diseases prior to heading and flowering.

It’s time to be scouting wheat for diseases and Virginia has had favorable conditions for disease development in wheat.  There are really 2 times we should think about spraying wheat: 1) at Feekes Stage 9 (ligule visible on flag leaf); and 2) heading and early flowering (Feekes Stage 10.1 – 10.5).  The spray at Feekes 9 is based on weather conditions and scouting while the Feekes 10.1 – 10.5 application is based on predicted disease risk and stage of flowering.  Feekes 9 applications target powdery mildew, stripe rust, head and stem rust, Stagonospora leaf and glume botch, Septoria leaf blotch, and tan spot.  The sprays at heading and flowering target wheat scab (Fusarium head blight).  Most fungicides are efficacious against the diseases growers would spray for at Feekes 9 but only certain fungicides are recommended for sprays at heading and flowering as they have been shown to reduce the mycotoxin DON that the wheat scab pathogen produces.  Also, strobilurins or FRAC Group 11 fungicides are not recommended for sprays at heading/flowering as these fungicides can actually increase DON levels.  Fungicides recommended for DON are in the table below.

Fungicide efficacy for wheat diseases can be found in the Fungicide Efficacy Table produced by the Crop Protection Network at the following link: https://cropprotectionnetwork.s3.amazonaws.com/fungicide-efficacy-for-control-of-wheat-diseases-filename-2021-04-21-154024.pdf

Scouting for Wheat Diseases:

Begin scouting for wheat diseases around flag leaf emergence (Feekes 8) through Feekes 9 which is when the ligule of the flag leaf is visible.  Be sure to look in different areas of the field, especially where wheat appears further along.

Pictures and descriptions of early wheat diseases are below.

Septoria tritici blotch: Look for elliptical or diamond-shaped, tan-brown lesions that often have yellowish halos. As lesions enlarge the center of the lesion dies and appears dark with small, dark specks which are fruiting bodies that give the lesion a characteristic speckled appearance. Subsequent lesions on higher leaves generally follow leaf veins, having straight edges and no yellow margin or halo.  The dark lesion center and dark speckling is the key to distinguishing Septoria blotch from tan spot or Stagonospora leaf and glume blotch.

Symptoms of Septoria tritici blotch

Crop Protection Network: Grau and Burrows

Left is a picture of Septoria blotch showing lesions with dark centers and little to no yellow halo except for new lesions.  The right picture illustrates symptoms of larger lesions that have grown together.

Tan spot: Tan spot causes light brown or tan lesion with a light tan center that have a distinctive yellow halo (unlike Septoria that has a dark lesion center). Tan spot lesions are more oval than those of  Septoria tritici blotch, but are more angular than Stagonospora leaf blotch lesions.

Symptoms of tan spot

Crop Protection Network: Burrows
 
Left is a picture of tan spot lesions that are fairly oval and have tan centers.  The right picture demonstrates the characteristic yellow halo caused by tan spot.

Stagonospora leaf and glume blotch: Foliar symptoms generally begin with small yellow flecks or spots. Spots grow to become lens-shaped lesions that are surrounded by yellow halos.  Lesions are typically longer than those of tan spot but have a dark center like those of Septoria tritici blotch.  However, a yellow halo is often present with Stagonospora and absent with Septoria.  Symptoms on glumes appear as purple blotches or streaks.

Stagonospora leaf blotch symptoms

Crop Protection Network: Wise

Left are early lesions while the right picture are coalesced lesions in more advanced infections.  A distinct yellow halo is almost always present with Stagonospora.

Powdery mildew: Powdery mildew produces powdery, often fluffy fungal growth on stem and leaf surfaces.  Leaf tissue can turn yellow under the powdery fungal growth.  Signs of this disease typically appear on the lower leaves and work their way up the canopy.  In advanced infections, little black dots can be seen in the fungal growth.

Signs of powdery mildew

Crop Protection Network: Burrows

Left is fluffy fungal growth of powdery mildew on lower stems and leaves.  On the right can be seen small, black fruiting bodies in the fungal growth.

Stripe rust: Although stripe rust doesn’t appear every year, when it does appear it can cause losses very rapidly.  For that reason, you need to contact your local county agent or myself if you suspect stripe rust so we can quickly recommend control measures and alert other growers in the area of occurrence.  Like most foliar diseases, strip rust starts out as yellowish flecks on leaves.  From lesions appear yellow-orange, rust-like pustules on susceptible varieties. Pustules on mature leaves occur in parallel lines along the length of the leaf which appears as stripes.

Stripe rust symptoms and signs

Crop Protection Network:  Burrows and Grau
 
Very linear sporulation and symptoms of stripe rust on left.  Larger, coalesced lesions of stripe rust on right.

Most products in the aforementioned efficacy table are effective against these foliar diseases if applied just before or soon after symptoms appear.

Nematode in Atlantic Soybean Production Webinar

Nematodes consistently rank as a top yield robber of soybeans. Join us November 16 from 9-12:00 for the Nematode in Atlantic Soybean Production Webinar to discuss nematode distribution and management strategies for the region. The webinar is free, but registration is needed for access to the zoom link. Registration information can be found at https://www.pcsreg.com/nematodes-in-atlantic-soybean-production-webinar.  Topics of emphasis will include updates from regional nematode distribution surveys, assessment of soybean cyst nematode seed treatments, root knot nematode on-farm field trials, general management strategies, and how to collect and submit soil samples. Please email Alyssa Koehler akoehler@udel.edu with any questions. 

David Langston, Extension Plant Pathologist, Virginia Tech Tidewater AREC

Peanut leaf spot waxes as Sclerotinia blight wanes.

This week I have noticed a significant uptick in the amount of peanut leaf spot. It’s mainly in peanuts that have been sprayed once or twice with fungicides and the spray interval is longer than the recommendation on the label. I recommend using Microthiol Disperss (sulfur) tank-mixed with Headline, Lucento, or Provost Silver. I do not recommend stand-alone group 7 fungicides like Miravis and Fontelis. Normally I’d want to repeat the sulfur/fungicide mix in 10 days but this close to digging we may just go with a chorothalonil spray before harvest for leaf spot.

It seems the heat wave we’ve had has arrested Sclerotinia blight to some degree. I just finished rating my plots this morning and the Sclerotinia in the untreated checks hasn’t advanced in about 2 weeks. That’s a good thing. According to the weather forecast this heat will continue until the end of August until it drops significantly to highs in the lower 80’s on September 1. This temperature drop will likely be accompanied with rain. At this point for Sclerotinia blight I’d go with labeled rates of Omega 500 if you spray soon due to the 30 day PHI. Once again, I really want to see a chlorothalonil application as the last fungicide spray of the season for resistance management for leaf spot.

David Langston, VT Tidewater AREC, e-mail: dblangston@vt.edu office phone (757)807-6536; cell (757)870-8498

Wet weather Brings Disease Pressure for Peanuts

Recent rains and cooler temperatures have maxed out out the risk for leaf spot and Sclerotinia blight of peanuts. In fungicide trials at the Tidewater AREC in Suffolk, VA I counted 9 hits of Sclerotinia blight in 70 row-ft in one untreated plot. Late leaf spot was evident at low levels in the lower canopy as well. Hopefully 2 preventive sprays have been made for leaf spot, but many growers may not have made an application for Sclerotinia blight at this time.

I know my above comments sound like doom and gloom, but growers typically have less disease pressure than we do at our experiment station as they use longer crop rotations. Most of our plots are on a 2-3 year rotation where most growers in Virginia rotate land to peanuts every 4 or more years. Crop rotation remains our best tool to reduce risk of disease losses in peanuts in the Commonwealth.

Peanut leaf spot can occur in all peanut fields, but soilborne diseases such as stem rot and Sclerotinia blight are problems in less than half of production fields, which is a good thing. Most growers know which fields they have problems with these diseases which allows them to treat them differently than most fields. With Sclerotinia blight risk high and already observed, growers need to make preventive fungicide applications of fungicides specific to this disease. if an application of Miravis/Elatus has been made in the last 10-14 days you should be good for another 10-14 days depending on weather conditions. If you haven’t applied a fungicide that effectively controls Sclerotinia blight, I recommend Omega 500 at 1 – 1.5 pts tank-mixed with a leaf spot fungicide in fields with histories of the disease. I also advise growers to scout these fields to determine if disease is present and to assess the effectiveness of fungicides already applied. The best place to look for Sclerotinia blight is in the largest, deepest vines in the field and in low areas that tend to hold moisture longer. The disease may cause leaves to wilt on individual stems (flagging) and tan, necrotic tissue that has white, fully fungal growth can be observed on these stems near the stem base beneath the canopy. See picture of typical Sclerotinia blight on peanut stems in the image below.

Tan lesions and white fungal growth on peanut vines affected by Sclerotinia blight. At the top of the picture just left of center you can see the small, black sclerotia on one of the stems.

You may see flagging from Tomato Spotted Wilt Virus (TSWV) but no lesions on stems or fungal growth is present. I have had to take a closer look at several plants that had flagging leaves only to find that it was TSWV causing symptoms.

Typically a follow-up spray for Sclerotinia blight is warranted 21 days after the initial application of Omega 500. If you are using the 2-spray program of Miravis/Elatus there is no need to make an application of Omega 500 unless you are more than 30 days prior to harvest and disease pressure and weather conditions pose a high risk of severe damage.

If you have questions related to peanut disease management please feel free to contact me.

David Langston – e-mail dblangston@vt.edu, cell (757) 870-8498, office (757)807-3536