March 14, 2011
The battle against wheat diseases has begun in Louisiana.
Stripe rust has been reported from several wheat-producing regions in Louisiana. Therefore, producers, consultants, agents, and others involved in Louisiana wheat production are encouraged to begin scouting their fields.
For more, see Stripe rust found in susceptible Louisiana wheat.
Stripe rust development is most aggressive when temperatures are 50 to 65 degrees in the presence of intermittent rain or dews. However, development can occur when temperatures are near-freezing up to 70degrees.
Initial infections on seedling wheat may not have the characteristic striping pattern that occurs on more mature 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 in 7 to 10 days when conditions are optimum for development.
Leaf rust is usually evident later in the season than stripe rust. This is because the leaf rust pathogen requires warmer temperatures for development than stripe rust.
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 70degrees and leaves remain wet for six to eight hours. Similar conditions will favor the development of leaf and glume blotch caused by Stagonospora and Septoria, respectively.
Disease management begins with planting high-yielding varieties with good 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 has saved producers more than $20 per acre by eliminating the need for a fungicide application.
We encourage producers and consultants to check the disease package of their varieties before applying a fungicide. Data can be accessed at: http://www.lsuagcenter.com/en/crops_livestock/crops/WheatOats/Variety+Trials++Recommendations/
Genetic resistance isn’t bulletproof. This resistance can break down over time. Pathogen populations can evolve 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 consultants 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. Catching early infections will allow you to plan for the spring.
Efforts should be taken to utilize genetic resistance; however, once the disease(s) 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 managing leaf and stripe rust are Quilt, Stratego, Twinline or tank mixes of propiconazole (Bumper, Tilt, Propimax) and a strobilurin (Quadris or Headline). Propiconazole or Prosaro applied alone is efficacious against rust as well. Strobilurins applied alone are another option for managing stripe rust; however, to optimize the effectiveness of these products, they must be applied before infection by the stripe rust pathogen.
(Note: an earlier version of this story included Stratego Yld as a fungicide option for leaf and stripe rust in wheat. However, the product is not labeled for such use.)
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. 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 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 present, choose high-yielding, disease-resistant varieties, and make timely applications of efficacious fungicides when necessary.
For more information concerning disease management in wheat, contact your local LSU AgCenter county agent/specialist or agricultural consultant.
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