Instructions for rice blast prevention

Pyricularia oryzae attacks rice in two stages — leaf blast at tillering and panicle collar blast at flowering. This article clearly distinguishes two types, how to forecast outbreaks based on weather and prevention and treatment regimens for the winter-spring crop in the Mekong Delta.

Summary: Rice blast is the most dangerous disease for rice in the Mekong Delta in the winter-spring crop, which can cause a loss of 30-70% if a second outbreak occurs (collar blast). Unlike the general guide, this article separates the 2 stages of the disease + provides weather-based forecasts to proactively handle before symptoms become apparent in the field.

Applies to: OM, IR, ST, Dai Thom rice in the Mekong Delta, part of the Red River Delta. Highest pressure in winter-spring season.
Duration: Monitor the whole crop, prioritize 2 periods: 25-40 days after sowing (branching) and 5-7 days before flowering.
Difficulty level: Medium. Need to look at the leaves + guess the weather.
Estimated additional costs: Spraying at the right time is 300-500 thousand/acre (1000 m²). Spraying incorrectly = wasting money + resistant plants.

Blast fungus and two fronts

Blast is caused by the fungus *Pyricularia oryzae* (synonym *Magnaporthe oryzae*). Features:

Spores spread through air + dew drops. One field with a disease outbreak can spread to 2-3 neighboring fields in one foggy night.
Develops strongly when temperature 20-28°C + humidity > 90% + night dew lasting 6-8 hours. This is the condition of the winter-spring crop in the Mekong Delta in December-February.
There are 2 attack phases with completely different mechanisms and consequences. Many growers only prevent leaf blast but ignore cotton neck blast — the most expensive mistake.

Leaf blast

Attack when the tree branches (20-40 days after sowing). Symptoms: diamond-shaped brown spots on leaves, dark brown edges, gray-white center. Count any 10 leaves, if you see > 2 leaves with spots = outbreak begins.

Consequences if not treated: weak plants, poor tillering, yield reduced by 15-30%.

Cotton neck blast

Attack at flowering time (3-7 days after flowering). The cotton neck is black, dry, the cotton does not form seeds or is flat. Three times more dangerous leaf blast because:

At this stage, it is difficult for the treatment to get into the cotton neck.
The infected cotton yield is lost.
Consequences: 30-70% loss of revenue if widespread outbreak occurs.

Cotton neck blast often breaks out from orchards with uncontrolled leaf blast. Prevention of leaf blast = 80% reduction in risk of cotton blast.

Forecast outbreaks through weather — be more proactive than looking at leaves

Waiting until you see spots on the leaves before spraying is 5-7 days late. Proactive way: monitor weather conditions "characteristic of blast outbreaks":

Element	Danger threshold
Night temperature	20-25°C for 3 consecutive nights
Air humidity	> 90% from 10pm to 6am
Early morning fog	Appears 2-3 days in a row
Light rain / no heavy rain	3-5 days
The field has disease outbreaks nearby	Within 1-2 km

When 3/5 of the above factors appear within 5 days → prioritize preventive spraying, do not wait for symptoms. This rule costs 1 spray but saves 2-3 sprays when the disease has flared up.

The Hau Giang, Soc Trang, Bac Lieu regions have this typical weather pattern from mid-December to the end of February — be on high alert.

Room — four layers of protection

Choose resistant varieties

Priority is given to varieties with average — good blast resistance: OM5451, OM6976, OM7347, ST24, ST25. Avoid sensitive varieties: IR50404 (although high yield, rice blast eats quickly).

Refer to the list of recommended varieties of the Provincial Department of Agriculture and Rural Development — updated by season.

Amount of seeds + moderate density

Sparse sowing: 80-120 kg of seed/ha instead of 150-180 kg as in old practice.
Low density = good light + low humidity = difficult to explode.

Thick sowing is one of the biggest causes of rice blast outbreaks in the Mekong Delta.

Fertilize with nitrogen in moderation

Reduce protein by 10-20% compared to the old formula. High nitrogen = thick soft leaves = easy fungus invasion.
Absolutely do not fertilize with nitrogen when there is a risk of rice blast outbreak. Rice blast + high protein = outbreak 3 times faster.
Increase potassium in the tillering + tillering stage. Potassium makes cell walls thick, making it difficult for fungi to penetrate.

Spray the room at the right time

Stage 1 — leaf blast: 25-35 days after sowing. Spray when there are signs of adverse weather (item 2) OR see 1-2 spots on 10 inspected leaves.

Active ingredients:

Tricyclazole (250-500 g/ha depending on label).
Isoprothiolane (300-500 g/ha).
Fenoxanil + tricyclazole combination.

Spray in the cool afternoon from 4-6pm. Avoid spraying early in the morning when there is dew or < 4 hours before rain.

Stage 2 — panicle collar blast: 5-7 days BEFORE rice emerges. This is a prevention spray, not a treatment. Waiting until it blooms and then spraying is too late.

Active ingredients are similar to phase 1, can be combined with chlorothalonil if there is an epidemic.

Treat when it breaks out

Spray correctly as soon as you see it:

Leaf blast: > 5 spots/average leaf or > 30% of leaves showing symptoms.
Cotton neck blast: cotton neck is black and dry — it's late but the spread needs to be limited.

Regimen:

Spray immediately at the highest dose according to the label (DO NOT exceed the label).
Repeat spraying after 5-7 days, change active ingredients to avoid drug resistance.
Spray evenly on both sides of the leaves. The sprayer pressure must be enough to reach the lower canopy.

If you have reached the cotton collar blast stage + the infected area is > 30% copper: accept partial damage, prioritize saving the uninfected part. Can't save the whole field at the same time.

Follow up every 5-7 days

[ ] Count 30 random cards in 3 coins. Calculate the percentage of leaves with spots.
[ ] Observe the lower floor + middle floor. Blasts often start downstairs.
[ ] Check the leaf collar and flower collar before flowering.
[ ] Monitor night dew + temperature. See forecast for the next 5 days.
[ ] Compared to neighboring copper. Has the same surname been translated yet? Is the wind direction towards you or away?

Five common mistakes

Mass spraying the entire field even though you only see 1-2 small nests: wastes money + causes fungus to become drug resistant. Hitting the nest is enough.
Agent's "special treatment to treat rice blast" news: many drugs have the same active ingredient but different packaging. Read the active ingredient + dose, not the trade name.
Spray the active ingredient Tricyclazole continuously 3-4 times/crop: fungus is resistant to the drug. Active ingredients must be rotated.
Using nitrogen to "bring plants back to green" when you first see the disease: is seriously counterproductive.
Skip spraying for leaf blast prevention because after spraying for leaf blast, you can see everything: 2 different stages, need to prevent 2 times.

Case notes

Sowing date + seed.
Actual amount of seed (kg/ha).
Nitrogen fertilizer schedule (time + amount).
Preventive spray date + active ingredient + dose.
Symptoms detected: date, location, rate.
Weather during critical periods (dew, night temperature).
Yield + flat rate at the end of the season.

After 2 recorded incidents, you will see your own community's rice blast outbreak pattern — making forecasting much more accurate than the province's general monitoring.

References

*Technical process of winter-spring rice production in the Mekong Delta* — Department of Crop Production + Mekong Delta Rice Institute, 2023.
*Integrated pest management (IPM) for rice* — Plant Protection Department, 2022.
*Blaze disease prevention — technical handbook* — Mekong Delta Rice Institute, 2021.