Arkansas Rice: Pairing Fungicides to Manage Diseases

    Rice sheath blight. Photo: University of Arkansas

    Fungicides are meant to kill or suppress the development of fungi. However, fungicides have different modes of action. Consequently, a single fungicide is not a cure for all. After all, the fungicides that we use for rice are not strictly curative.

    Tissues once killed are dead already, but new emerging tissues can be rescued. The fungicides we have are either suppressive or protective. Fungicide application can be carried out after the disease is seen for the purpose of suppressing further development of the fungus as in sheath blight or “severe early-season leaf blast” Fig.1.

    On the other hand, fungicides can be applied as protection as in smuts, late season neck/panicle blast and Cercospora narrow brown leaf spot.  Fungicides are not recommended in Arkansas for other minor or infrequent diseases. Yet, it is possible the minor fungal diseases to get suppressed by the fungicides we apply for the major diseases.

    Fungicide application cost can sometimes be reduced by pairing fungicides targeting more than one disease as long as the timings align to do so. Triazole fungicides containing Propiconazole or difenoconazole are recommended to be applied from early to mid-boot as protective fungicides to suppress the kernel and false smut fungi.

    While strobulurin fungicides containing either Azoxystrobin or Trifloxystrobin are recommended to suppress sheath blight and blast. Propiconazoles are also used to suppress Cercospora narrow brown leaf spot (Cercospora leaf spot) Fig. 2. 

    The timing is generally at boot if your field and variety has a history.

    Pair fungicides only as long as the timing aligns:

    If sheath blight is at threshold under favorable weather before mid or full boot and your field planted with a susceptible cultivar has a history of either kernel smut or false smut, pre-mix fungicides containing Triazoles and strobulurin can be used. In doing so, sheath blight and the smuts will be addressed at one go.

    You have to use the high end of the recommended rate since the crop at this level is far from maturity. Again, you need to make sure to use at least 6 fl oz of Tilt or Tilt equivalent in the mix to address adequate smut suppression. Alternative to pre-mix, you may compose the mix yourself.

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    For instance you can make 21 oz of QuiltXcel by mixing 12 oz of Quadris with 6 oz of Tilt per acre rate. Look at Table 1.

    If sheath blight is progressing slowly and happens to reach at threshold towards late boot, fungicide application can be paired with the 1st application for neck/panicle blast which is usually applied in the window of late boot to 10% head out.

    In such a situation, a fungicide containing Azoxystrobin or Trifloxystrobin alone can be applied since both diseases can be suppressed by strobulurin fungicides. The 2nd application for blast need to be done all by itself. Using a pre-mix or combining fungicides may not be needed unless you need to address Cercospora narrow brown leaf spot which is known to be suppressed by Propiconazole.

    Waiting for stink bug threshold to apply the 2nd fungicide to suppress secondary tillers for blast could be a disaster.

    Note: Fungicide timing is the major factor here. Unless timings align, delaying a fungicide application for another disease or insect to come is waiting for a tragedy.

    Table 1. Examples of fungicide compositions in different rates of Quilt Xcel

    10.5 oz QuiltXcel =   6.0 oz Quadris + 3.0 oz Tilt

    14.0 oz QuiltXcel =   8.0 oz Quadris + 4.0 oz Tilt

    15.75 oz QuiltXcel = 9.0 oz Quadris + 4.5 oz Tilt

    17.5 oz QuiltXcel = 10.0 oz Quadris + 5.0 oz Tilt

    21.0 oz QuiltXcel = 12.0 oz Quadris + 6.0 oz Tilt

    Fig.1. Such leaf burndown by blast pathogen early in the season may require separate fungicide application. 

    Fig. 2. Cercospora narrow brown leaf spot affects leaves, sheath and head often late in the season. You require protective fungicides if your field or variety has a history in recent years.

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