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13
Apr
2016
Foliar disease and scab update and 2016 fungicide efficacy table for wheat
Author: Terry Spurlock, Extension Plant Pathologist

By Terry Spurlock, Extension Plant Pathologist

Leaf rust, stripe rust, and Septoria leaf blotch were found on wheat prior to flag leaf emergence.  While severity in fields has been variable, some fields were sprayed with 4oz/A of propiconazole (Tilt or equivalent generic).  This application should provide residual control of stripe rust long enough for many varieties to exhibit adult plant resistance to specific races of the fungus (after Feekes 6 or first node).  Varieties that are susceptible to stripe rust, leaf rust, and Septoria leaf blotch may require a fungicide application at Feekes 8 or 9 (flag leaf) to properly manage the disease if it is present and conditions favor the disease spreading.  If no disease is present or levels are low due to varietal resistance or weather that does not favor disease, evidence suggests a fungicide application will not be an economic benefit.  Bacterial streak has also been observed on some varieties but I have not seen a field where it was severe.  It is important to remember a fungicide application will not be effective as this disease is caused by a bacterium rather than a fungus and does not necessarily predispose the plant to infection by other fungal pathogens.

Figure 1. Flaking dry exudate from bacterial streak (left) and lesions of Septoria leaf blotch with pycnidia (right) shown under magnification. Spores and exudate are two telling signs of the type of pathogens (fungus or bacterium) that are causing disease on the wheat plant.

Figure 1. Flaking dry exudate from bacterial streak (left) and lesions of Septoria leaf blotch with pycnidia (right) shown under magnification. Spores and exudate are two telling signs of the type of pathogens (fungus or bacterium) that are causing disease on the wheat plant.

Figure 2. Leaf rust (left) and stripe rust (right) on susceptible varieties. Leaf rust pustules are darker orange to brown. Stripe rust pustules are brighter orange to yellow and tend to follow the parallel venation of the wheat leaf later in the season.

Figure 2. Leaf rust (left) and stripe rust (right) on susceptible varieties. Leaf rust pustules are darker orange to brown. Stripe rust pustules are brighter orange to yellow and tend to follow the parallel venation of the wheat leaf later in the season.

Fusarium head blight update

Based on the experiences of many in 2015, I have received numerous calls concerning Fusarium head blight (scab) in 2016.  Heads or individual spikelets infected with Fusarium head blight appear light pink or white (bleached) and contain shriveled scabby seed.  Scabby grain may contain one or more vomitoxins such as deoxynivalenol (DON).   The fungi causing Fusarium head blight inhabits most fields in Arkansas.  These fungi survive on residue from previous crops including wheat and corn and produce spores that can infect wheat from heading to maturity.  Evidence suggests infection of the wheat head at flowering results in the most severe disease.  Because of this, a tool that utilizes weather patterns to predict Fusarium head blight incidence seven days prior to flowering is available at www.wheatscab.psu.edu.  When using the tool, the assessment date should be set to coincide with flowering of fields.  Red and yellow indicate conditions are favorable for Fusarium head blight while green indicates that conditions are unfavorable for development of the disease.   As a reminder, an integrated approach (IPM) to disease management is always preferred and use of the prediction tool is one component that should be relied on as a regional guide.  Other sound IPM practice prior to planting includes choosing varieties that are moderately resistant to scab rather than susceptible and/or staggering planting dates and avoiding planting multiple varieties of the same maturity group.  If a variety known to be susceptible has been planted, be sure to select the proper susceptibility in the model. An example of the model predication is shown in the figure below.  Evidence suggests the best application timing for scab is at flowering and applications of Prosaro at 6.5 fl oz/A or Caramba at 13.5 fl oz/A have been the most effective products to suppress scab and DON.  If the model indicates a high likelihood of occurrence for Fusarium head blight in your area during the time of infection, a fungicide application should be made if the yield potential of the field is good.  For updates on the model predication follow @SpurlockLab on Twitter.

Figure 3. Last year’s Fusarium head blight prediction for 4/20/15. There was an occurrence of scab for varieties flowering around this time. Images represent predictions of susceptible, moderately susceptible, and moderately resistant varieties from left to right. The FHB prediction tool can be accessed at http://www.wheatscab.psu.edu/riskTool.html.

Figure 3. Last year’s Fusarium head blight prediction for 4/20/15. There was an occurrence of scab for varieties flowering around this time. Images represent predictions of susceptible, moderately susceptible, and moderately resistant varieties from left to right. The FHB prediction tool can be accessed at http://www.wheatscab.psu.edu/riskTool.html.

Updated fungicide efficacy table

Linked is the updated wheat fungicide efficacy table (pdf). Changes from the 2015 update are highlighted in yellow.  Note that the highlighted areas are different than the current published MP154 wheat efficacy table and include the addition of “VG” ratings for Aproach and Aproach Prima for Stagonospora leaf/glume blotch, the addition of Absolute Maxx (triazole), Fortix (strobilurin), and Trivapro A+B (succinate dehydrogenase inhibitor or SDHI, strobilurin, and triazole).  Additionally, if 4 oz/A of propiconazole was applied earlier in the season and a second fungicide application is required, 10.5 oz/A of Trivapro B or 14 oz/A of Quilt Xcel will not exceed the maximum label rate of 8 oz/A per year of propiconazole.

 


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