Arkansas Rice: Midseason Nitrogen Timing, Scouting for Leaf Blast

    Mid-season rice field. ©Debra L Ferguson

    Who needs a little relief from this heat wave?  That was a rhetorical question.  The rice is loving it, so long as you can keep the water to it.  It does look like we’re in for a short break in the extreme temps for a few days next week, but the rain chances are still minimal.  Fingers crossed that we catch a good one.

    With more rice entering reproductive growth, it’s time to be mindful of the cut-off timings for herbicides (included in last week’s update).  Be mindful about herbicides we’re applying to other crops nearby too.  Let’s keep a good-looking rice crop healthy.

    Also, with midseason comes the time to be on the lookout for diseases.  While little has happened so far with the extreme heat, a small decline in temperatures could put us directly in the crosshairs of disease development.  Scout!

    Next week we’ll have info on the June Acreage report which will give us a new look at rice acre expectations.  It’s the last survey estimate before we start seeing FSA acreage data in August.

    Be safe in the heat, keep yourself and your rice watered up.

    Let us know if we can help.

    NOAA 7 day precipitation forecast

    Fig. 1.  NOAA 7-day precipitation forecast. Click Image to Enlarge

    Midseason Nitrogen (N) Timing for Varieties

    The majority of rice acres are entering or have already entered reproductive growth.  We want to continue emphasizing that our recommendations for midseason nitrogen (N) timing for varieties have changed in recent years.  Remember we only recommend midseason N applications for varieties, NOT for hybrids.

    Rice News on AgFax

    Data from 2012-2018 across a range of popular varieties during that time has shown how we can improve our midseason N applications compared to previous recommendations.

    We recommend to apply midseason N after beginning internode elongation AND 4 weeks after preflood N was incorporated by the flood.  You must meet both conditions before applying midseason N to maximize your benefit.

    Fig. 2 shows the percent of optimum yield based on timing of midseason N after the flood was established and indicates that 4-5 weeks after preflood N is incorporated is the optimum time to apply midseason N.

    Fig. 3 is another way of looking at the same data but based on days after beginning internode elongation (BIE).  In Fig. 3, ½” IE corresponds with 7 days after BIE.

    Again, the absolute earliest to ever apply midseason N is BIE, but only if it’s been 4 weeks since the flood was established to incorporate the preflood N.

    Percent of optimum yield for midseason N timing based on days after preflood N incorporated

    Fig. 2.  Percent of optimum yield for midseason N timing based on days after preflood N incorporated (flood establishment date). Click Image to Enlarge

    Percent of optimum yield for midseason N timing based on days after beginning internode elongation

    Fig. 3.  Percent of optimum yield for midseason N timing based on days after beginning internode elongation (BIE; green ring). Click Image to Enlarge

    Start Scouting for Leaf Blast

    Rice blast is unpredictable. It may be lurking in fields with a history if planted with susceptible or moderately susceptible varieties. Infection may go fast when conditions are favorable for the spores to germinate.

    What are favorable conditions for blast fungus spore germination?

    • Susceptible or moderately susceptible varieties
    • 9 hours + leaf wetness
    • Dew, fog, shaded areas (trees), frequent light rains
    • Cooler temperatures than needed for rice sheath blight
    • Late planting
    • Excessive nitrogen (N) rate
    • Sandier soils that do not hold water
    • Low grounds or river valleys
    • Dry field edges, levees, and fields not adequately flooded

    Where should you scout for rice leaf blast?

    Look at tree lines, dry field edges, levees, and spots in the field with a greener canopy due to excessive N.

    What are the symptoms of rice blast?

    Early Symptoms

    Early symptoms may look like greyish-black spots, as seen in Fig. 4. You may find the typical blast symptoms if you open the canopy and look at the lower leaves (Fig. 5).

    Close up of a rice plant with greyish-black spots, indicative of rice leaf blast disease

    Fig. 4. Early symptoms may look like greyish-black spots.

    A closeup view of the lower leaves of a rice plant with typical symptoms of blast, with the canopy of the plant open

    Fig. 5. Often typical blast symptoms are clearly seen when you open the canopy and look at lower leaves in susceptible varieties planted in fields with a history.

    Later Symptoms

    Leaf lesions are spindle-shaped and elongated with brown borders and grayish centers. A brownish lesion on the internode at the base of the panicle causes “blasting” of heads followed by breaking over of the head, later producing the “rotten neck” symptoms after heading.

    Learn the Source and Spread

    Source of Inoculum

    The source of inoculum for early infection has not been satisfactorily determined. The fungus may overwinter on diseased straw and stubble or, in some cases, carried on infested or infected seeds.

    Means of Spread

    The spores can easily be carried by wind and hence, blown from a long distance across the field and neighboring fields.

    What are management practices for rice leaf blast?

    For leaf stages of the disease, maintain proper flood level. Levels of infection tend to be less severe where maintained flood water is at adequate but not excessive depths. Avoid excessive nitrogen rates. (Nitrogen amounts vary with cropping history, soil type, varieties, etc.)

    For later stages of the blast disease, using fungicides will be helpful. Fungicide timing is critical for effective control. Early scouting aids in protective fungicide decisions, particularly for neck and panicle blast. 

    Note: When scouting for blast, you may need to scout for other diseases such as sheath blight. They can be found together as in Fig. 6.

    Close up of a rice plant infected with both blast and sheath blight diseases

    Fig. 6.  Blast and sheath blight on the same variety of rice.

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