Arkansas Rice: Scalding Temps Continue; When to Apply Fungicides

    Crop Progress

    Dry and scalding hot. Water use will be at a fever pitch in the coming week.  Next week is expected to bring temperatures in the upper 90s with some 100 degree days next weekend.

    Last week’s comments about nighttime temperatures held true. Depending on exact location in the state, areas have experienced 5-7 consecutive nights where temperatures were 75 or above. Where rice is heading this could have serious negative effects on grain quality.

    In 2010 and 2011 when grain quality complaints were high, we experienced prolonged periods of elevated overnight temperatures. In 2010 from July 18-Aug 16 we had only one night with a low temp below 75 degrees. In 2011 from July 7-Aug 8 we never dropped below 75 for an overnight low.

    While we have yet to face that stretch this year, the extended forecast points to a 10-day run of 75+ overnight temps before falling back into the low 70s. We can only hope that the timing isn’t just right to hurt us.

    According to DD50 enrollment, the crop should be approaching 44% heading which should be in line with NASS estimates on Monday (Table 1). Also included for reference are the projected dates for harvest – the time at which we will reach 20% grain moisture, which hasn’t changed over the past week (Table 2).

    2015-21 Table 1 50 Heading

    2015-21 Table 2 Harvest

    Rice Stink Bug Update

    The first heading fields in particular areas are still reaching treatment level for rice stink bug. However, as surrounding fields head out numbers seem to be spreading out. One interesting thing to watch for is the amount of rice stink bug seen in grain sorghum.

    With a major increase in grain sorghum acres this year, the crop may provide a breeding ground for stink bugs to build up and move out into rice – particularly later planted rice that is always at greater risk. Continue to scout and stay on top the situation as stink bugs are strong fliers and adults can move into fields quickly.

    Recommended Timing for Rice Fungicide Applications in Arkansas

    Hot and dry weather conditions with highs in the 90s and lows in the 70s may slow threats from sheath blight and blast. However, these diseases may still progress to some extent under the warm nighttime temperatures in heavy dew fields. Fields in low lying regions surrounded by trees and at river bottoms tend to have longer dew periods that can favor blast when dew stays for longer than 9 hours.

    Sheath blight can progress slowly by warm nights with dew in fields over-fertilized with nitrogen and with thick canopies. Therefore, the hot and dry weather does not fully cancel the “alert” to scout for both sheath blight and blast.

    Sheath Blight:  Continue scouting from green ring until pre-heading and make sure at least the upper three (3) leaves including the flag leaf are clean from the disease at heading.  A single fungicide application from early- to mid-boot is recommended to suppress sheath blight development unless it previously became severe enough to reach threshold levels. To read more on sheath blight fungicides go here.

    Blast:  If the current hot and dry weather conditions hold, leaf blast will be suppressed and disease pressure at heading should be low to moderate. But this does not mean we should not plan for at least one fungicide application to suppress neck rot and panicle blast. The disease may still progress in isolated conditions as mentioned above on warm and dewy nights in certain fields. Therefore, continue scouting and maintain a 4+ inch flood depth.

    Timing for Protective Fungicide Application for Blast: Two applications are usually recommended to prevent damage from neck blast. If the current hot and dry weather holds, one application may be good enough. However, this is a judgment call, the best and safest approach is two fungicide applications. History of the field, cultivar susceptibility, soil type, flood depth, conditions that favor dew, and your overall field management need to be considered before deciding on the use of a single application instead of two.

    The optimum timing for a one-time application is at 30-50% heading. Higher rates are usually preferred (i.e. 12 oz. Quadris, 21 oz. Quilt Xcel, 19 oz. Stratego). If you wait on the fungicide application so you can tank-mix with an insecticide, the application will be too late for neck blast. Side note: if you spray a fungicide on time and throw in an insecticide prior to heading, you’ll waste the insecticide.

    For two applications, late boot to 10% heading (Fig. 1) and 50-75% heading (Fig. 2) are recommended. The whole point here is to protect the delicate neck tissues as they come out of the boot from the blast spores that may be lurking at the collar of the flag leaf. The 1st application is to protect the primary tillers, and the 2nd application is mainly for younger tillers.

    However, if a large proportion of the necks are out of the boot (Fig. 3), forget about fungicides because it’s too late. Failure to detect leaf blast early on does not guarantee the absence of neck blast. The blast fungus is wind borne and can be blown from anywhere.

    2015-21 Fig 1 Boot to 10 heading

    Fig. 1. Boot to 10% head out: recommended timing for 1st blast application.

    2015-21 Fig 2 50-75 Heading

    Fig. 2. 50-75% head out: recommended timing for 2nd blast application. Note that necks are still in boot.

    2015-21 Fig 3 Neck out of Boot

    Fig. 3. Panicle necks completely out of boot. Wrong time to apply for blast, fungicides will not help.

    Kernel Smut and False Smut:  Situations to consider a fungicide application for smut prevention include: 1) susceptible cultivar, 2) field history of smut, and 3) excessive nitrogen rates applied. If a field meets an two of these three criteria, then plan for protective fungicide and apply at the right time and rate.

    Timing for Protective Fungicides for Kernel Smut and False Smut Suppression: Early- to mid-boot (Fig. 4) application is recommended as the best timing – if heads are emerging (Fig. 5) it is too late.

    Use a minimum of 6 oz. per acre of Tilt (propiconazole) or equivalent if possible. High rates are recommended such as 19 oz. Stratego, 21 oz. Quilt Xcel, or 21 oz. Quilt.  Four (4) oz. is not as effective particularly to suppress false smut. Use as much water as possible for good coverage, no less than 5 gallons per acre. If done properly with correct timing and rate, kernel smut can be suppressed 90-95% but false smut only 50-75%.

    If coordinated application to target multiple diseases is desired (for sheath blight, 1st application for blast, kernel smut, and false smut), an application at mid-boot but before boot-split may work. However, it all depends on which disease is actually the worst in your field. It’s a judgment call and knowing the history of the field helps to make this decision.  Note that late-planted, susceptible rice fields are more prone to diseases than early-planted fields.

    2015-21 Fig 4 Early to mid boot

    Fig. 4. Early- to mid-boot: recommended timing for kernel smut and false smut fungicide application.

    2015-21 Fig 5 Boot Split

    Fig. 5. Boot split to any percentage head out is too late for a fungicide application for smut suppression.

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