Wheat is affected by several diseases that negatively impact yield if not properly managed. The major wheat diseases are leaf and stripe rust. In LSU AgCenter tests, rust-infested plots yielded more than 50 percent less than non-infested plots. 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.
To minimize the impact of these diseases, an effective disease management strategy is critical. Therefore, LSU AgCenter scientists conduct research in an effort to develop effective management strategies. This is accomplished in part through assessing genetic disease resistance in varieties entered in the LSU AgCenter ‘Official Variety Tests’ (OVT). Standardized OVTs are conducted on several research stations every year. Each location represents a unique production area in the state.
In addition to variety testing, experimental and commercial fungicides, as well as application timings are evaluated for efficacy against the major diseases affecting Louisiana wheat. Data generated from this research is used to develop extension service recommendations. This information is also used in LSU AgCenter extension programming ef-forts across the state.
The first step toward an effective disease management strategy begins with proper disease identification. Diseases should be correctly identified in individual fields prior to applying a fungicide. It is important to know what symptoms are associated with each disease and the environmental conditions that favor disease establishment and spread.
Stripe rust development is most aggressive when temperatures are 50 to 65oF in the presence of intermittent rain or dews (6 to 8 hours). However, development can occur when temperatures range from near freezing to 70oF. 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 repro-duction) can be completed within 7 to 10 days under optimum conditions.
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 70oF and leaves remain wet for 6 to 8 hours. Similar conditions favor the development of leaf and glume blotch diseases caused by Stagonospora and Septoria.
Effective disease management begins by selecting and planting high-yielding varieties with genetic resistance. Genetic resistance to wheat pathogens is extremely effective. In studies conducted by LSU AgCenter scientists over the past several years, fungicides were not beneficial when applied to resistant varieties. Therefore, planting resistant varieties can saved producers more than $20/A by eliminating the need for a fungicide application. Producers and consultants should check the disease package of their varieties before applying a fungicide. Data can be accessed here.
Genetic resistance is not bulletproof. This resistance can break down over time with pathogen populations evolving to overcome resistance. This was the case in 2010 when stripe rust was seen in AGS2060 (a stripe rust resistant variety).
Therefore, agents, producers, and consult-ants should always scout their crops beginning no later than early spring. In some cases, leaf and stripe rust can develop to very low levels in the fall. Detecting early infections will allow producers to plan for the spring.
When genetic resistance breaks down and disease is identified, a fungicide application may be needed. Typically, a single application at flag leaf emergence (F8) is adequate for managing most foliar diseases of wheat. Based on LSU AgCenter research, fungicides effective for man-aging leaf and stripe rust are Quilt, Stratego, Twinline or tank mixes of propiconazole (Bumper, Tilt, Propimax) and a strobilurin (Quadris or Headline). Propiconazole, tebuconazole (Folicur, Orius) or Prosaro are also efficacious against rust as well. Strobilurins may be applied alone; however, to optimize the effectiveness of these products, they must be applied before infection by the stripe rust pathogen.
Realize fungicides are effective against fungal diseases, but NOT effective against bacteria (black chaff) or viral diseases. Application timing and sprayer set up 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 af-fected 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 in-crease 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 applica-tions of an efficacious fungicide when necessary. For more information concerning disease management in wheat, contact your local LSU AgCenter county agent/specialist.