Instructions for preventing sheath spot disease on rice

Rhizoctonia solani attacks rice from the tillering to flowering stage, causing leaf burn and stem rot. Instructions on how to follow the nitrogen fertilization schedule and spray at the right threshold for rice in the Mekong Delta.

Summary: Sheath spot (also known as sheath dry disease) caused by the fungus *Rhizoctonia solani*, is the second most common disease on rice in the Mekong Delta after rice blast. Characteristics — the fungus exists in the soil, spreads through contact between rice stems, and explodes when sowing thickly and with high nitrogen fertilization. This article focuses on spray thresholds, appropriate active ingredients, and measures to reduce outbreaks through field management.

Applies to: Winter-spring and summer-autumn rice crops in the Mekong Delta. High pressure in the summer-autumn crop due to high temperature and humidity.
Duration: Room from 25-30 days after sowing to 7-10 days before harvest.
Difficulty level: Medium. It is necessary to count the percentage of infected bodies to decide on spraying.
Estimated additional costs: 200-400 thousand VND per acres (1,000 square meters) for prevention + treatment.

Characteristics and harmful effects

The fungus *Rhizoctonia solani* differs from other rice diseases in several ways:

Exists in soil as sclerotia for 3-5 years. If this case is not resolved = the next case will flare up again.
Transmitted through contact, not transmitted through the air as far as rice blast. Wet leaves touching each other = orchids.
Grows strongly when the temperature is 28-32 degrees Celsius and the humidity is above 90 percent — exactly the characteristics of the summer-autumn crop.
Attack from the bottom up — starting at the leaf sheaths close to the water surface, gradually moving up to the leaves.

Harmful effects:

Fungal-affected leaf sheaths show characteristic streak-shaped spots — gray-white with dark brown edges. The sheaths are rotten, the leaves are dry.
Yields decrease by 10-30 percent if outbreak occurs early in the season.
Flat seeds increase because the plants are weak during the seed raising stage.
Stripe spot does not cause complete death of rice like cotton collar blast, but clearly reduces yield and causes poor rice quality.

Outbreak conditions

The zebra spots flare up strongly when:

Thick sowing over 150 kilograms per hectare: crowded stems, leaves touching, fungus spreads quickly.
High nitrogen fertilization: dark green plants, soft leaves, easy fungus invasion.
Prolonged high water level in fields: fungus spreads from leaf sheaths close to the water surface.
The field had an epidemic last year and was not treated: sclerotia remained in the soil.
Summer-autumn crop: high temperature + humidity is perfect for mushrooms to grow.

Recognition and level

Observe the lower layer of leaf sheaths (about 1/3 of the rice stem from the water surface):

Early spots: oval spots 1-2 centimeters long, dark brown border, gray-white center.
Growth spot: spreads along the leaf sheath to form characteristic mottled spots — this is the name "stripe spot".
Severe spots: rotten sheaths, dry leaves from the bottom layer up, small brown fungal sclerotia can be seen on the sheaths.

How to count to decide to spray:

Count 30 random rice stalks at 5 points in the field.
Count stems with striped sheaths.
Calculate percentage.

Spray threshold:

Rice stage	Spray threshold (percentage of infected stems)
30-45 days after sowing	Over 5 percent
45-65 days (cultivation)	Over 10 percent
65 days to flowering	Over 15 percent
After flowering	Spray only when more than 25 percent — close to harvest, pay attention to drug isolation

Room — four layers of protection

Managing sowing and density

Sow 80-120 kilograms per hectare — more sparsely than traditionally. The body is not crowded.
Sowing in rows or by machine can be more even than sowing by hand.

Balanced nitrogen fertilization

Reduced protein by 10-20 percent compared to the old formula.
Divide into 3-4 small fertilizations instead of 2 heavy fertilizers.
In the final phase (preparation for planting), only fertilize 20-25 percent of the total amount — do not fertilize much at this stage.
Increased potassium — especially during active tillering and flowering periods. Potassium makes cell walls thick, making it difficult for fungi to penetrate.

Water Management

Drain water 5-7 days during the maximum tillering period (25-35 days after sowing). "Alternatively wet and dry" technique. Reduce leaf sheath moisture, making it difficult for fungus to grow.
Do not keep the water level above 10 centimeters for extended periods except for necessary periods.
Drain water 10-14 days before harvest — the field is dry, the fungus does not spread further.

Cleaning the field after the crop

Plowing and drying fields after harvest — mushroom sclerotia die in the sun.
Burning straw in fields that had severe epidemics. Frequent burning is not recommended because it is harmful to the soil, but it is recommended during epidemics.
Rotate crops with other crops (beans, corn) once if possible.

Spray correctly

Suitable active ingredients

Choose 1 active ingredient by group:

Benzimidazole-based fungicides: *Carbendazim*, *Thiophanate methyl*. Quite effective, cheap. Easily resistant if used a lot.
Triazole fungal drug group: *Hexaconazole*, *Difenoconazole*, *Propiconazole*. Highly effective, long lasting treatment.
Strobilurin-based fungicides: *Azoxystrobin*, *Trifloxystrobin*. Good prevention and treatment.
Combination of 2 active ingredients: *Hexaconazole + Carbendazim*, *Azoxystrobin + Tebuconazole*. More effective than single agent, less drug resistance.

Spray technique

Spray in the cool afternoon from 16-18 hours.
Spray low, spray on the lower leaf sheaths — where the fungus is growing. Spraying the tops has no effect.
The tank pressure is strong enough to reach the lower floor.
Amount of water 250-400 liters per hectare — enough to cover the plant evenly.

Repeat spray

First time: right after exceeding the threshold.
Second time: after 10-14 days if pressure is still high. Change active ingredient to another group to avoid resistance.
Maximum of 3 spraying times per crop. It's too uneconomical and increases drug resistance.

When the disease has broken out

If detected late and many stems have been affected:

Inject immediately the highest dose allowed by the label.
Combining 2 active ingredients from different groups.
Drain water for 3-5 days to reduce infection.
Add potassium (50-70 kilograms *K2O* per hectare) to increase plant strength.
DO NOT add nitrogen.

If the disease has reached the flowering and spreading stage — accept partial damage, focus on spraying against cotton blast and brown planthoppers to protect the rest of the crop.

Follow up every 7-10 days

[ ] Count the infection rate at 5 points.
[ ] Watch for low-lying areas, near the shore — striping often starts in this area.
[ ] Field water level and humidity.
[ ] Weather — daytime temperature, nighttime temperature, rain.
[ ] The condition of field grass — dense grass increases moisture and spreads disease.

Common mistakes

Spraying on an uncounted schedule: wasting treatment, creating resistance.

Spray the tops of leaves instead of the lower sheaths: fungus is on the lower sheaths, spraying the tops does not reach.

High nitrogen fertilization "for green plants": makes the disease worse.

Keeping the water level high for a long time: increases leaf sheath moisture, fungus thrives.

Uncleaning the field after the crop: residual fungus sclerotia will flare up again in the next crop.

Take notes

Amount of sowing seeds and density.
Nitrogen fertilization schedule — date, dose.
Stripes appear — date, infection rate.
Spray — active ingredient, dose, day.
Yield at the end of the season + flat rate.

References

*Managing sheath spot disease in rice* — Mekong Delta Rice Institute, 2023.
*Handbook for rice disease prevention* — Plant Protection Department, 2022.