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Delta Farm Press
by Jarrod Hardke, Rice Extension Agronomist - June 23, 2023
“It’s too hot to fish, too hot for golf…”
Now we’re cooking with gas. Welcome rainfall over the past two weeks has certainly
improved the mood of many, both from a crop health standpoint and an overall season-long
We have a great deal of rice reaching or passing into reproductive growth (or midseason)
right now. Conditions are excellent and the crop seems to be progressing steadily
along. As the crop shifts into reproduction, remember that there will be the possibility
of actual nutrient deficiencies show up, but also subtle, short-lived instances that
appear like deficiency but aren’t.
When we shift into reproductive mode, our highly mobile nutrients (N, P, K) get on
the move and can give short flashes of concern before balancing back out. Be careful
not to react too quickly to small changes in crop appearance – when in doubt we can
take a closer look with some tissue testing to be sure we’re still running in the
proper ranges. This is often a time when midseason N on varieties gets applied too
early because we think the crop is already out of N, but if it hasn’t been 4 weeks
since preflood was incorporated it’s doubtful the rice is finished taking up the preflood
and ready for midseason N.
Some calls have started on finding sheath blight. Given the more moderate temperatures
and rainfall events, this was to be expected. Nothing is really on the move yet,
we’re just finding it. Don’t jump the gun – see more below on fungicide timing.
It is possible to outrun and outlast sheath blight without needing to treat.
Let us know if we can help.
Fig. 1. NOAA 7-day precipitation forecast.
In Fig. 2, Notice the short, erect, poorly tillered plants that are darker in color – these
are phosphorus (P) deficient. In the middle of the picture are a couple of plots
where fertilizers were added containing P. There has been a tremendous growth response
and these plants appear to be catching up greatly to plants elsewhere in the field.
Anytime a nutrient deficiency is suspected, it is always best to send off some tissue
samples for analysis (one from a good area and one from a bad for diagnostic comparison).
In addition, you can put out what are commonly referred to as “10x10” plots where
you apply some different fertilizers to 10 ft x 10 ft areas to observe the response.
This helps to serve as added confirmation before spending money on fertilizer you
may not need.
Fig. 2. Phosphorus (P) deficiency in rice (h/t Randy Chlapecka).
Without a doubt, panicle differentiation (PD) is an important growth stage in the
life cycle of rice. In addition to being the crucial stage in determining productivity,
it is also the stage that rice becomes more susceptible to certain diseases. The
combination of the rice growth stage (PD) and the rainfall from the last few weeks
contributed to the development of some diseases in the field.
Brown spot, caused by the fungus Cochliobolus miyabeanus, was the disease found in the rice fields this week at Cross and Woodruff County.
Identifying brown spot in rice plants requires careful observation and knowledge
of their characteristic symptoms. To help with this identification, here are the
principal differences between them: Brown spot (Fig. 3) lesions typically have a distinct oval or spindle shape with a dark brown center
and a yellowish halo. While rice blast (Fig. 4) lesions are typically diamond-shaped or elliptical with a grayish center and a dark
brown or purplish border.
Brown spot has no recommended treatment and is generally not considered to be a problem
in rice in Arkansas – however, it is often an excellent indicator of possible potassium
(K) deficiency. However, if the lesions are from blast, then we have real disease
concerns to manage.
Fig. 3. Brown spot lesions.
Fig. 4. Rice leaf blast lesions.
Along with the rains that occurred last week, some rice fields were affected by lightning.
However, in the absence of this weather information you might think that a severe
Sheath blight, Rhizoctonia solani, infection has occurred in the field if you look at Fig. 5. That was not the case as Fig. 5 is a representation of damage from lightning on rice plants giving them a “fried”
appearance. However, the period of possible Sheath blight infections may be ahead.
The appearance of sheath blight can vary depending on several factors, including the
specific region, weather conditions, and cultivar. Generally, sheath blight tends
to become more prevalent during periods of high humidity and temperatures ranging
from 77 to 86 degrees. However, it's important to note that the timing of sheath
blight onset can vary. In some cases, sheath blight symptoms may start to appear
around the time of PD and continue throughout the crop's growth stages. Typically,
initial symptoms include water-soaked lesions on the leaf sheaths near the waterline.
These lesions may later turn grayish-white or brown and expand along the leaf sheath,
leading to the withering and rotting of the affected tissues. As the disease progresses,
the lesions may also affect the panicles, causing empty or partially filled grains.
Fig. 5. Damage from lightning on rice plants.
It’s that time of year for fungicide timing questions. Depending on the disease,
rice growth stage at time of fungicide applications matter to varying degrees.
Most cultivars take 28-40 days to progress from Beginning Internode Elongation (BIE
/ green ring) to reach 50% Heading. For simplicity, let’s use a 36-day example (but
remember that these intervals will be slightly different for all cultivars).
Consider this generalization:
Day 0 is Green Ring;
Day 9 is ½” Internode Elongation;
Day 18 is Mid-Boot (flag leaf “points”);
Day 27 is Late Boot (flag leaf collar visible);
Day 36 is 50% Heading.
**Remember that this is generalized, the actual windows will vary for all cultivars
and could be shorter or longer to progress through these stages.**
From the generalized version, the optimum timing to apply fungicides for kernel or
false smut suppression is mid-boot into the start of late boot (Day 18 to Day 32 in
example). Waiting until too close to full boot or split boot and 50% heading reduces
fungicide efficacy because time is needed to move the fungicide in the plant where
it can protect the kernels.
Sheath blight scouting and possible treatment should not occur before ½” internode
elongation (Day 9 in example). Once we reach that stage, if we meet our sheath blight
threshold (35% positive stops for VS/S cultivars; 50% positive stops for MS cultivars)
AND sheath blight is aggressively moving up to threaten the upper canopy leaves, THEN
we can consider treatment.
Fungicide rate is dependent on amount of time needed to suppress sheath blight. Generally,
research has shown 8 oz Quadris to provide suppression for 14 days, 10 oz for 21 days,
and 12 oz for 28 days. If you make it to 50% heading and the upper canopy leaves
are still clean, then we have successfully outrun the disease and yield will not be
improved by making a fungicide application.
However, occasionally intense sheath blight pressure at 50% heading could justify
a fungicide application at a low rate to protect rice standability. BUT – note that
you cannot apply propiconazole (Tilt) fungicides after heading.
Fungicide applications to prevent neck and panicle blast should not be made before
late boot (Day 27 in example). If making two fungicide applications to VS/S cultivars,
the first application should be made in the window of late boot to beginning; the
second application should be made when the panicles are about halfway out of the boot
but with necks still in the boot. If making a single fungicide application to MS
cultivars, split the difference with the application made around boot split to when
panicles are about a third out of the boot with necks still in the boot.
Fungicide applications to prevent smuts or neck black must be made proactively based
on cultivar and field experience – no scouting can tell you whether a spray is warranted.
However, for sheath blight we need to scout as we can avoid unnecessary applications.
The DD50 Rice Management Program is live and ready for fields to be enrolled for the
2023 season. All log-in and producer information has been retained from the 2022
season, so if you used the program last year you can log in just as you did last year.
Log in and enroll fields here: https://dd50.uada.edu.
The Arkansas Rice Advisor site https://riceadvisor.uada.edu functions like an app on your mobile device. There you can readily access the DD50
program, rice seeding rate calculator, drill calibration, fertilizer and N rate calculators,
publications, and more.
Arkansas Rice Updates are published periodically to provide timely information and
recommendations for rice production in Arkansas. If you would like to be added to
this email list, please send your request to email@example.com.
This information will also be posted to the Arkansas Row Crops blog (http://www.arkansas-crops.com/) where additional information from Extension specialists can be found.
More information on rice production, including access to all publications and reports,
can be found at http://www.uaex.uada.edu/rice.
We sincerely appreciate the support for this publication provided by the rice farmers
of Arkansas and administered by the Arkansas Rice Research and Promotion Board.
The authors greatly appreciate the feedback and contributions of all growers, county
agents, consultants, and rice industry stakeholders.
Rice Extension Agronomist
Extension Weed Scientist
Rice Verification Coordinator
Extension Rice Pathologist
Extension Soil Fertility