Arkansas Rice: Determining 50% Heading; Diseases to Scout

Crop Progress

The general rule is that you should wait to drain until 25 days after 50% heading for long grain cultivars and 30 days after 50% heading for medium-grain cultivars. There’s a reason that it’s a general rule temperature, rainfall, humidity, etc. play major roles in the relative drying time of grain in the field. These factors cannot be predicted for every field and cultivar.

To improve on the decision-making process, you should combine the general rules of days past 50% heading with a field-based observation of the relative maturity of grain in your field. At left – nearly all kernels are straw-colored and the field is well past the point that it is safe to drain regardless of soil type.

At center – nearly 2/3 of kernels are straw colored and it is safe to drain on a silt loam soil. At right – 1/3 of kernels are straw-colored and would be close to the point where it is safe to drain on a clay soil. Always use caution when deciding to drain – pulling the water too early can sacrifice grain fill of the lower kernels and impact yield.

Determining 50% Heading

Based on questions received, there needs to be some better explanation of the “50% heading” timing referred to so often in our recommendations and the DD50 program. Agronomically, “heading date” or “50% heading” is defined as the time when 50 percent of the panicles have at least partially emerged from the boot.

This is in contrast to “headed”which refers to the time when 100 percent of the panicles have completely emerged from the boot (some panicles may never completely emerge from the boot). At left, the plant is at boot-split; at center, the plant is at what is commonly referred to as “cracking the boot”; and at right, the plant is heading.

Note that the center plant at cracking is not at heading yet the panicle has not yet begun to extend above the flag leaf collar. So, 50% of your plants should look like the plant at right to say that your field has reached 50% heading.

Disease Update

Sheath Blight: The heat in the past few weeks has not been favorable for sheath blight. However, in fields where N is excessive and the canopy is dense to retain nighttime dew, the disease can slowly progress. We have seen this in our test plots.

Continue scouting and make sure the upper three leaves including the flag leaf are clean from the disease at heading. One time application at early to mid boot is recommended as the best timing and economical to suppress sheath blight progress for Arkansas. However, one-time application may not be viable in poorly managed fields where the disease started early in short, susceptible cultivars under favorable weather conditions.

Blast: We have received a few reports of blast in the past week. The scattered rain showers have been a concern in a few counties in northern Arkansas for blast progress. However, the disease has not progressed much due to the heat. Fields that have difficulty maintaining flood depths had more blast spread across the field.

There have been repeated questions on making only one application for blast instead of the recommended two application approach. One application is not recommended, particularly in late planted blast-prone fields with susceptible cultivars. If you decide to make only one application, use a higher fungicide rate at 30-50% head emergence.

The recommended two-application approach is for the first application at boot to 10% head emergence and the second application at 50-75% head emergence. If the necks are emerged from the boot you are too late and there will be no benefit from a fungicide application.

Bacterial Panicle Blight: The scorching heat in the past three weeks has been a concern for BPB occurrence. None of the suspicious samples received from commercial fields this year have tested positive. Research has shown that the incidence and severity of the disease increases in late planted (late May) than early planted (March and April). Dew has also shown to play a great role in BPB disease development. Continue scouting in late planted fields. Keep us informed if you see any.

Kernel Smut and False Smut: We received repeated questions on timing and rate of fungicide application to suppress these two diseases. Early to mid boot applications have been recommended with a inimum of 6 oz/A Tilt (propiconazole) or equivalent. High rates are recommended such as 19 fl oz Stratego, 21 fl oz Quilt Xcel, or 21 oz Quilt.

These diseases are severe in fields with a history planted to susceptible cultivars. Excessive N fertilization and high seeding rates increase the disease situation. False smut is less sensitive to fungicides and is more prevalent in late-planted fields. Applying fungicides earlier than early boot may not last long enough to offer the desired suppression. Applying after boot split is too late and there will be not benefit for smut management.

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