R. Keith Collins | 3/20/2012 11:59:02 PM
Rusts are fungal diseases that can negatively impact wheat yields in Louisiana. Leaf and stripe rust are the most common forms in the state with occasional incidences of stem rust, which is not as prevalent in the state.
Factors that can increase the incidence of rust include environmental conditions, planting in poorly drained fields, planting susceptible varieties and cultural practices. Wheat varieties with resistance to these pathogens are available and should be a consideration when selecting varieties. Cultural practices that promote lush growth and extend leaf wetness periods should be avoided. Excessive seeding and nitrogen rates are common practices that lead to these conditions.
Stripe rust develops best in late winter and early spring when nighttime temperatures range from 40 to 60 degrees F and 6 to 8 hours of leaf wetness, but can continue to develop when temperatures are near freezing. There is also research showing evidence that some strains of the stripe rust pathogen can continue to develop when temperatures exceed 60 degrees F. If stripe rust infects early in the season before flag leaf emergence, an early fungicide may be warranted. Early infections on seedlings often occur as small clusters, but as the plant grows, the yellow-orange pustules appear along leaf veins that form lines or stripes.
Leaf rust develops in the spring with warmer temperatures. It develops best when nighttime temperatures range from 60 to 80 degrees F and plants are exposed to 6 to 8 hours of leaf wetness. Leaf rust is characterized by small pin-point pustules on the upper leaf surface usually found on the lower foliage. Pustules are deep orange to dark red that occur randomly on the leaf.
Scouting should begin in early winter through late boot stage/early head emergence, especially with varieties susceptible to rusts. It is critical to protect the flag leaf once it has emerged. Even when planting resistant varieties, it is prudent to perform random scouting to ensure the flag leaf is protected. History has proven that resistant varieties eventually become susceptible to new strains of fungal pathogens.
Fungicides are available that adequately control rusts with proper selection, timing and rates. LSU AgCenter research has shown fungicides effective for managing stripe rust include the strobilurins (Quadris, Headline), propiconazoles (Tilt, Propimax) or combination products (Quilt, Xcel, Stratego Yld, Twinline). To optimize effective control with strobilurins, a preventative application should be made before stripe rust is evident. This would normally occur between jointing and flag leaf emergence, especially with susceptible varieties.
Leaf rust can be controlled with a strobilurin, propiconazole or combination product. Applications (susceptible varieties) would normally occur with flag leaf emergence through heading. Check fungicide labels to ensure compliance with cut-off dates for late applications.
This table is not an exclusive list of fungicides but indicates some that should be commercially available. Remember to read and follow labels regarding rates, time of application and pre-harvest interval.
|Stripe rust||Leaf rust||Rate/acre|
|Quilt Xcel||E||E||10.5-14 oz|
|Stratego Yld||E||E||4 oz|
Winter wheat vernalization is the accumulated exposure to cold temperatures of 32 degrees F to 50 degrees F. Vernalization begins when water is absorbed by the seed in the germination process. Without adequate vernalization, plants will remain vegetative and will not produce grain resulting in wheat that exert heads later than normal and will be very erratic. If wheat has not received adequate chilling to vernalize, the developing seed head and stem joints will not be present within the stem. At Feekes 6.0, (jointing), the first node is swollen and appears above the soil surface. Above this node is the seed head which is being pushed upward that will emerge as the harvestable seed head. This stage will not occur without adequate vernalization and is easy to identify by splitting stems. At this point in the growing season (March 15) some wheat heads are emerging and much more of the crop is in the boot or pre-boot stage. Maturity and vernalization vary among wheat varieties but indications are that we have received adequate vernalization. Later maturing varieties planted late may be the exception.
The table below indicates total row feet per acre for various row configurations that may be helpful when calibrating row planters or drills for various crops.
|Row width||Row feet/acre|
|(38" twin) 19"||27,570|