Trey Price and Boyd Padgett- LSU AgCenter
If not properly managed, diseases can negatively impact yield and quality. Leaf rust and stripe rust are the most troublesome diseases producers have to manage. Based on LSU AgCenter tests, yields may be reduced by 50 percent by rusts. This demonstrates the need to effectively manage these diseases to optimize profits. Other diseases that occur less frequently or are not as widespread are bacterial streak, stem rust, leaf and glume blotch, barley yellow dwarf, and head scab. These diseases can significantly affect yield when conditions favor disease development.
Proper disease identification is the first step toward an effective disease management strategy. Diseases need to be correctly identified in individual fields prior to applying a fungicide. Knowing the environmental conditions that favor disease development and what varieties are susceptible to specific diseases will aid in proper identification. Keep in mind that several symptoms may resemble diseases, but are not.
Stripe rust development is most aggressive when temperatures are 50 to 65 degrees F in the presence of intermittent rain or dews (six to eight hours). However, development can occur when temperatures range from near freezing to 70 degrees F. Initial infections on seedling wheat may not have the characteristic striping pattern that occurs on older plants. Seedling infections often occur in thumb-sized clusters on the leaves, as opposed to a random distribution that occurs with leaf rust. Infections may appear as linear rows of small yellow to light orange pustules (stripes) on the lower leaves during late winter or early spring. Striped patterns are typical of infections in older pants. If conditions remain favorable for development, pustules may cover the entire upper leaf surface, as well as portions of the head. A lifecycle (infection to reproduction) can be completed within seven to 10 days under optimum conditions. Stripe rust has been confirmed at the Macon Ridge Research Station in Winnsboro in F5 stage wheat. Conditions favorable for development are prevalent this time of year. Unless disease becomes severe, applications usually are not needed at this stage, but susceptible wheat varieties should be monitored for disease development. If necessary, fungicide applications are usually most efficacious at flag leaf stage (F8). Strobilurin and triazole fungicides or pre-mixes with both chemistry types are very effective at reducing disease incidence and severity and preserving yield. For a list of susceptible varieties, please refer to ratings from 2013 and 2014 available at: http://www.lsuagcenter.com/en/crops_livestock/crops/WheatOats/Variety+Trials++Recommendations/.
Leaf rust is usually evident later in the season than stripe rust. This is because the leaf rust pathogen requires warmer temperatures for development. Initial symptoms of leaf rust begin as light yellow spots, usually on the lower foliage. As the disease develops, small pin-point pustules form on the upper leaf surface. Pustules are brick or dark red and occur randomly on the leaf. Similar to stripe rust, pustules can cover the entire leaf surface if conditions remain favorable for development. The disease develops optimally when nighttime temperatures are 50 to 70 degrees F and leaves remain wet for six to eight hours. Leaf rust has not yet been found in Louisiana this year. Management recommendations for leaf rust are similar to those for stripe rust, and a list of susceptible varieties is available at the above URL.
Septoria/Stagonospora leaf and glume blotch is caused by the fungi Septoria or Stagonospora. Symptoms are similar for both diseases. Small elliptical lesions initiate on the lower leaves and progress up the plant if conditions remain favorable for disease development. As the lesions mature, the centers are straw-colored with small, raised black fruiting bodies (dots). The fungus can survive on infected wheat debris. Similar conditions favor the development of leaf and glume blotch diseases caused by Stagonospora and Septoria. Most fungicides are efficacious on these diseases; however, applications usually are not necessary unless disease progresses upwards to the flag leaf.
Genetic resistance is not bulletproof. This resistance can break down over time with pathogen populations evolving to overcome resistance. For example, in 2010, stripe rust was observed in AGS2060 (a stripe rust resistant variety). Therefore, agents, producers and consultants should always scout crops beginning no later than early spring. In some cases, leaf and stripe rust can develop to very low levels in the fall. Early detection will allow producers to plan for the spring.
When genetic resistance erodes and disease is identified in resistance varieties, a fungicide application may be needed. Typically, a single application at flag leaf emergence (F8) is adequate for managing most foliar diseases of wheat. Strobilurins may be applied alone; however, to optimize the effectiveness of these products, they must be applied before infection by the stripe rust pathogen. Please refer to the table below for more specific information.
|Class||Active ingredient||Product||Rate/A (fl. oz)||Septoria leaf blotch||Stripe rust||Leaf rust||Harvest Restriction|
|Strobilurin||Picoxystrobin 22.5%||Aproach SC||6.0 – 12||VG||E2||VG||Feekes 10.5 and 45 days|
|Fluoxastrobin 40.3%||Evito 480 SC||2.0 – 4.0||—||—||VG||Feekes 10.5 and 40 days|
|Pyraclostrobin 23.6%||Headline SC||6.0 – 9.0||VG||E2||E||Feekes 10.5|
|Triazole||Metconazole 8.6%||Caramba 0.75 SL||10.0 – 17.0||VG||E||E||30 days|
|Propiconazole 41.8%||Tilt 3.6 EC3||4.0||VG||VG||VG||Feekes 10.5|
|Prothioconazole 41%||Proline 480 SC||5.0 – 5.7||VG||—||VG||30 days|
|Tebuconazole 38.7%||Folicur 3.6 F3||4.0||VG||E||E||30 days|
|Prothioconazole19%Tebuconazole 19%||Prosaro 421 SC||6.5 – 8.2||VG||E||E||30 days|
|Mixed modes of action4||Metconazole 7.4%Pyraclostrobin 12%||TwinLine 1.75 EC||7.0 – 9.0||VG||E||E||Feekes 10.5|
|Fluxapyroxad 14.3%Pyraclostrobin 28.6%||Priaxor||4.0 – 8.0||VG||VG2||VG||Feekes 10.5|
|Propiconazole 11.7% Azoxystrobin 7.0%||Quilt 200 SC3||10.5 – 14.0||VG||E||E||Feekes 10.5|
|Propiconazole 11.7% Azoxystrobin 13.5%||Quilt Xcel 2.2 SE||10.5 – 14.0||VG||E||E||Feekes 10.5|
|Prothioconazole 10.8%Trifloxystrobin 32.3%||Stratego YLD||4.0||VG||VG||VG||Feekes 10.535 days|
|Cyproconazole 7.17%Picoxystrobin 17.94%||Aproach Prima SC||3.4-6.8||VG||E||VG||45 days|
1Efficacy categories: NL=Not Labeled; NR=Not Recommended; P=Poor; F=Fair; G=Good; VG=Very Good; E=Excellent; — = Insufficient data to make statement about efficacy of this product.
2Efficacy may be significantly reduced if solo strobilurin or SDHI products are applied after stripe rust infection has occurred.
3Multiple generic products containing the same active ingredients also may be labeled in some states. Products including tebuconazole include: Embrace, Monsoon, Muscle 3.6 F, Onset, Orius 3.6 F, Tebucon 3.6 F, Tebustar 3.6 F, Tebuzol 3.6 F, Tegrol, and Toledo. Products containing propiconazole include: Bumper 41.8 EC, Fitness, Propiconazole E-AG, and PropiMax 3.6 EC. Products containing propiconazole + azoxystrobin include: Avaris 200 SC.
4Products with mixed modes of action generally combine triazole and strobilurin active ingredients. Priaxor is an exception to this general statement and combines carboxamide and strobilurin active ingredients.
Realize that, in general, fungicides are effective against fungal diseases ONLY, but NOT effective against bacterial (black chaff) or viral diseases. Application timing and sprayer setup are just as important as the fungicide choice. Ideally, fungicides should be applied before disease onset or when disease incidence is very low. The residual activity of the fungicide may be lost too soon if applied too early. Apply too late, and disease severity may be too high to arrest disease development.
Sprayers should be configured to optimize coverage. Poor coverage of a good fungicide could result in poor disease control. Coverage is affected by gallons per acre, pressure, nozzle size, nozzle type, and nozzle spacing. Aerial fungicide applications should deliver fungicides in 4 to 5 gallons of total solution per acre and ground applications should be configured to deliver 10 to 20 gallons per acre.
Nozzles should be selected that deliver small droplets (200 to 300 microns). Nozzles configured to reduce drift potential will usually result in poor coverage. Boom height and nozzle spacing should be adjusted to the manufacturer’s specifications. A boom height too high will increase the potential for drift, and a boom height too low will not provide adequate overlap for the nozzles. Pressure should be adequate to force fungicide down in the canopy.
On a final note, remember that an effective disease management program will only be successful when all of the components are working together. Efforts must be made to correctly identify the diseases. Choose high-yielding, disease-resistant varieties, and make timely applications of an efficacious fungicide when necessary.
For more information concerning disease management in wheat, contact your local LSU AgCenter county agent/specialist or agricultural consultant.