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

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

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

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

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

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

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

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

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.

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

Corn
Some corn may have been sprayed as early as V5 but now we are approaching tasseling and some fields have already begun to tassel. With the corn prices being what they are more growers are inclined to apply a fungicide. Multiple studies have shown that a single application of a DMI (FRAC Group 3) + QoI (FRAC Group 11) fungicide at VT-R1 provide the best chance of return on investment. The Corn Disease Working Group of the Crop Protection Network updates the corn fungicide efficacy table in the following link.

https://crop-protection-network.s3.amazonaws.com/publications/fungicide-efficacy-for-control-of-corn-diseases-filename-2021-03-09-163332.pdf

The last few years we’ve seen more Gray leaf spot on corn in Virginia than other foliar disease. Be aware that if you used Xyway in-furrow you may not be protected against Southern corn rust that comes in later in the season some years. Typically, by the time Southern corn rust is observed in Virginia it will not impact yield greatly. However, for late-planted corn fungicide application must be made by R3 (milk stage) to limit yield loss due to Southern corn rust.

Soybeans
In Virginia, our primary foliar disease of soybean are frogeye leaf spot, Cercospora leaf blight and Septoria brown spot. Scouting for these diseases at or prior to beginning flower (R1) will give an estimation on the amount of disease pressure in a given field if diseases are present. Fields with disease pressure early are more likely to benefit from a fungicide application. Fungicides have the best chance of potentially reducing yield loss to foliar pathogens when sprayed beginning at pod initiation (R3) through seed initiation (R5). If dry conditions are prevalent during this spray window a fungicide spray is not advised. Be aware that foliar fungicides do not offer much protection against seed decay organisms or soilborne pathogens of soybeans. Below is the link for the Crop Protection Network’s soybean fungicide efficacy table.

https://crop-protection-network.s3.amazonaws.com/publications/fungicide-efficacy-for-control-of-soybean-foliar-diseases-filename-2021-03-12-182833.pdf

It is not advisable to spray QoI (FRAC Group 11) fungicides alone as fungicide resistance to this group has been observed in the fungal pathogen that causes frogeye leaf spot.

If you have questions or concerns please don’t hesitate to contact me.

David B. Langston, Jr.
Professor and Extension Plant Pathologist
Tidewater Agricultural Research and Extension Center
6321 Holland Rd., Suffolk, VA 23437
Office (757) 807-6536
Cell (757) 870-8498
FAX (757) 657-9333
e-mail dblangston@vt.edu

After looking at some wheat fields this morning I believe most of Virginia’s wheat is between head emergence (Feekes 10.3) and flowering (10.5.1). Wheat disease pressure has been low but with rain chances increasing by the middle of next week that could change. I have seen some physiological damage caused by weather or hypersensitive responses (resistant responses) to disease but no real disease outbreaks. With the flowering stage here we typically begin spraying for wheat scab or Fusarium head blight (FHB). Right now risk of FHB is low. The risk of FHB can be monitored by the FHB Risk Assessment Tool provided by Penn State (http://www.wheatscab.psu.edu/). Fungicides such as Caramba, Prosaro, and Miravis Ace are recommended for FHB and DON control. Caramba and Prosaro will provide better control when applied at flowering while Miravis Ace has a wider window of activity from heading to 6 days after flowering. Remember to avoid using strobilurin (FRAC Group 11) fungicides after flag leaf as these fungicides can increase DON levels in grain. Fungicides applied for FHB will control the other major foliar wheat diseases. The 2019 Fungicide Efficacy for Wheat Diseases can be found at the following link. NCERA 184 Wheat fungicide table 2019_Final

If you have questions about wheat diseases you can reach me, David Langston, via e-mail or phone.

e-mail dblangston@vt.edu
office phone (757) 807-6536
cell phone (757) 870-8498