Managing durian orchards during flowering and fruiting stages

In the period from the crab eye to the young fruit, the size of the egg determines 70% of the crop yield. Instructions on how to read tree health, control water, prune fruit and prevent major fungal diseases to keep fruit set rate ≥ 60%.

Summary: The 45-60 day period from bright crab eyes to young fruits the size of chicken eggs is the decisive gate for the entire durian crop. Fertilizing the wrong nitrogen once, letting it dry suddenly overnight, or leaving fruit pruning a week late can all cause 40-70% of young fruit to drop. This article records the way I always advise orchard owners when consulting — not a theoretical framework but a series of decisions that need to be made each week.

Applies to: Durian Ri6, Monthong, Musang King in the Southwest, Southeast, Central Highlands. The tree is 5 years old or older, transitioning from old leaves to crab eyes.
Duration: 45-60 days of continuous monitoring.
Difficulty level: Medium. Requires daily leaf reading and soil moisture reading.
Estimated additional costs: 2-4 million/ha (mainly root fungus treatment + fruit pruning).

What does the tree think during this period

When the crab's eyes brighten and prepare to release the pistil, the durian tree enters a state that Tien Giang growers often call "standing up". The root system is less active because hormones shift to prioritize flowers, old leaves fall prematurely, and 90% of the plant's energy goes into flowers and young fruits.

Consequences of this state:

Sensitive to soil moisture fluctuations: roots cannot rehydrate in time if allowed to dry suddenly and then watered.
Excess nitrogen = bud failure: Fertilizing urea at this stage will push out young leaves instead of keeping flowers.
Alternating sun-rain stress causes young fruit to drop en masse, with some orchards losing 60% after just 2 unexpected periods of heavy rain.
Phytophthora and stem borers "smell" when the tree is weak. Young flowers and fruits are easy entry points.

In short: the tree is accumulating capital, one small mistake can ruin the whole crop's plan.

Prepare 15 days before the crab's eyes shine

[ ] Check sprinklers, pump valves, drippers — fix leaks first, don't wait until you need them. Pump pressure must be enough to cover ½ of the tree canopy.
[ ] Prune dry branches, diseased branches, and branches in the canopy that grow over each other. The goal is to reduce canopy humidity by 15-20% to limit fungus.
[ ] Broad spectrum spray to prevent fungus — Phosphonate or Mancozeb based active ingredient, 1 spray evenly for canopy + root collar.
[ ] Base fertilizer NPK 16-16-8 or 15-15-15 with a dose reduced by 30% compared to the leaf growing stage. Increase potassium sulfate (SOP) 0.5-1 kg/tree.
[ ] Buy a notebook or phone app to keep a orchard diary — you'll thank yourself today.

For plants that are growing buds and have not yet matured leaves, postpone flowering treatment for another 2-3 weeks. Forcing a plant to flower when the leaves are not yet old is the easiest way to end up with a failed season.

Management process in 45-60 days

Irrigation water — the most important problem

Principle: Moisturize steadily, not a lot. Moisture fluctuation is enemy number 1.

Measure soil moisture at the canopy edge (where the roots are active), not around the base. How to use a handheld moisture meter (price 500-800k, worth the investment) or use your hands: hold a handful of soil 15cm deep, the soil is clumpy but does not leak water = moderately moist.

Divide water: 2 times/day × 20-30 minutes instead of 1 time × 60 minutes. Early morning 5-6 a.m. and late afternoon 4-5 p.m. are the hours with little evaporation.

Absolutely avoid: water the roots and let them dry for 2-3 days. The tree will be shocked and the flowers will fall off the next time.

When the plant is shedding pistils (1-3 days, different hours for each plant): reduce watering by 30-50%. I know it sounds counterintuitive, but this is the core technique. If the plant is a little thirsty when the pistil is released, it will not "bud", and the flowers will pollinate much better.

After heavy rain: STOP watering for 24-48 hours, check drainage ditches. The raised tissue of durian should not be waterlogged even for just a few hours.

Nutrition — dividing doses, reducing protein

Bright crab eyes → pistil discharge: DO NOT apply nitrogen base fertilizer. Only spray Calcium-Boron (Ca+B) leaves, 2 times every 7 days, dose according to packaging.

Fruit set → young fruit the size of an egg:

Base fertilizer NPK 12-12-17 + TE: 0.5-0.7 kg/tree, divided into 2 times every 15 days.
Foliar spraying with MgSO4 + trace elements (Zn, Mn) — 1 time is enough.

Absolutely do not apply high doses of urea or DAP immediately after fruit set. High nitrogen = the tree pushes out young buds = young fruit bunches fall en masse. This is the most costly mistake I still encounter in a new durian orchard.

Pruning fruit — determines quality

When the young fruit reaches the size of a chicken egg (~30-45 days after fruiting), start pruning. Rules I apply:

Count the number of fruits on each fruit-bearing branch. Ceiling ratio: 1 left / 15 active cards.
Discard: deformed fruit, small fruit in bunches, crowded fruit, fruit growing towards the sky.
Maintain: balanced shape, even green skin, dark red stem and firm, growing downward or horizontally.

A 7-10 year old tree can hold 60-100 fruits which is enough. Greedy to keep 150+ fruits → small fruits, uneven rice, next year the tree will be exhausted. I have seen many orchard owners regret pruning and then hold the orchard "sleep crop" next year, causing even greater damage.

Main pests and diseases — prioritize prevention over treatment

Disease/Pest	Early symptoms	Treatment active ingredients	Note
Anthracnose on flowers	Flowers with brown spots, dry, stems fall off	Azoxystrobin or Mancozeb	Spray as soon as 5% of flowers show symptoms
Phytophthora left stem	The stem is dark, the sap is flowing	Phosphonate sweeps roots + improves drainage	Don't wait until the fruit falls to treat it
Mealybugs on the stem	White wax layer around the stem	Pyriproxyfen or natural enemy of ladybugs	Clean the grass around the base at the beginning of the season
Fruit borers	Small hole in the shell, young fruit falls off	Emamectin benzoate, spray every 10 days	Baking fruit with export priority trees

Spraying principles: cool afternoon (4-6pm), no wind, no rain forecast in the next 4-6 hours.

Follow up every 3-5 days

[ ] Fruit setting ratio = number of fruit set / number of blooming flowers. Target ≥ 60% with Ri6, ≥ 50% with Monthong.
[ ] Rate of young fruit dropping = number of fruit falling / number of fruit set. Target ≤ 15%. Above 25% is an alarm.
[ ] Leaf color: glossy dark green = good. Yellow = N deficiency or localized waterlogging. Purple = phosphorus deficiency. Brown spots = early fungus.
[ ] Condition of left stem: firm, dark red = good. Soft, dark = about to fall off.
[ ] Amount of irrigation water + rainfall: recorded in the book, compared every week.

Handle 3 critical situations

Young fruit falls massively (> 30% in 3 days):

Check soil moisture in 5 different spots around the canopy — does it fluctuate?
Turn over a few fallen fruits to see if the stems are infected with fungus.
Was there too much nitrogen fertilization or watering in the past 7 days?
Is there sudden rain followed by dryness?

→ Each cause has its own solution. Don't spray widely — determine first.

The tree "buds" in the middle of the fruit growing stage:

Stop watering for 5-7 days to give the plant slight stress.
Spray KNO3 (potassium nitrate) 1%, once is enough.
Prune the buds by hand when they first appear — don't let them exceed 5cm.

Left deviation to one side / distorted shape:

Usually due to irregular pollination. The next crop needs to arrange diverse seedlings or pollinate by hand in the evening from 6-9 p.m. when flowers release pistils.

The five mistakes I see the most

Hold fruit: Want more fruit → small fruit → low selling price + next year's tree will be exhausted. Pruning aggressively is a principle, not a choice.
Irrigation according to a fixed schedule: Not checking actual soil moisture → sudden dryness/waterlogging. Watering schedule is just the framework, soil moisture is the decision.
Using urea to "healthy plants": Counterproductive. The tree buds and drops its fruit. At this stage, potassium is king.
Mass spraying to prevent pests: Waste of money, drug resistance, harms natural enemies. Only spray when there are specific symptoms.
Not recorded: I don't know why the results of the next case are different from the previous case. Diary is long-term technical capital.

Minimal orchard Diary

For each week, write:

Pistil release date, fruit set date — mark each tree or lot.
Amount of irrigation water (hours × number of pumps running).
Rainfall (measured by rain gauge ~50k).
Fertilizer/spray supplies: type + dose + date.
Photos taken of leaves + fruits every weekend, saved in format `lo3-2026-05-18.jpg`.
The number of fruits counted, the number of fallen fruits.

After 2 seasons of having a diary, your orchard has its own "DNA" — no book can replace it.

References and warnings

*Durian cultivation technical handbook* — Southern Fruit Institute, 2023.
*Guidelines for Phytophthora management on fruit trees* — Plant Protection Department, 2024.

The dosage of active plant protection ingredients in the article is a reference frame. Always prioritize packaging labels + recommendations from the local Plant Protection Department. When the orchard shows strange signs that do not match the descriptions here — call the nearest agronomist, do not try it yourself.