Prevent the phenomenon of black fibers on jackfruit

Black fibrosis is a physiological disorder — not a fungal or bacterial disease. Cut jackfruit has fibers and black segments, losing its commercial value. Instructions on the five causes of black fiber and how to prevent them from farming techniques for jackfruit orchards in the Mekong Delta.

Summary: Black fiber is the biggest problem with Thai jackfruit in the Mekong Delta — the cut fruit has black fibers and black segments, completely losing its selling value. Unlike pests and diseases that can be treated with crop-protection products, black sclera is a physiological disorder caused by many combined factors. Prevention must come from basic farming techniques — there are no "blackening pills." This article analyzes five main causes and preventive measures for each cause.

Applies to: Thai Changai jackfruit, red-fleshed jackfruit, To Nu jackfruit in Tien Giang, Dong Thap, Vinh Long. Especially the area has a history of high black fiber.
Duration: Room from flowering to harvest — about 4-5 months.
Difficulty level: Medium.
Estimated additional costs: 3-5 million VND per hectare per year for periodic rooms.

What is black fiber and why does it happen

Black fiber on jackfruit is a phenomenon:

The fibers between the segments (white fibers) turn black.
The jackfruit's segments are darkened locally or entirely.
In some cases, the fruit is black from the inside.
The left looks normal on the outside — only discovered when cut.

Different from:

Anthracnose (caused by fungus): clear spots on the outside of the shell.
Fruit borers: have holes.
Fruit rot: soft skin, smell.

Black rot is a physiological disorder — a combination of nutritional deficiency, water stress, plant weakness, and a number of environmental factors.

Consequence: the fruit cannot be sold at a high price. Severely damaged orchards can reduce crop revenue by 30-50%.

Five main causes

Lack of calcium — boron

This is the main reason. Calcium helps strengthen cell walls, and boron supports calcium transport in plants. Missing one of two:

Weak cells, easily bruised when exposed to stress.
The fibers and segments do not develop evenly.
Black fibers appear randomly on the fruit.

Acid soils (pH below 5.5) often lack available calcium. Soil with high nitrogen fertilization without calcium supplementation can easily become deficient.

Many stages of rain produce fruit

Too much water causes plants to absorb calcium and boron poorly — these two nutrients require a steady flow of water in the plant to move. Sudden heavy rain followed by dryness = water shock = black.

High nitrogen fertilizer during fruit growing stage

High nitrogen stimulates plant growth but is not balanced with calcium and potassium. The fruit grows quickly but the cells are weak and easily turn black.

Greed keeps the left

The tree holds over 100 fruits per tree (large Thai jackfruit) — not enough nutrition to feed them all. The tree prioritizes large fruits, small fruits are malnourished and black.

Sensitive breed

Thai Changai jackfruit is more sensitive to black fiber than To Nu jackfruit and red-fleshed jackfruit. Some non-pure seed sources can increase the rate of black fibers.

Room according to each cause

Supplement calcium — boron

Calcium-based fertilizer:

Apply 0.5-1 kilogram of lime per tree every year at the beginning of the dry season (acid soil).
Apply gypsum (gypsum) 0.3-0.5 kg per tree for alkaline soil.
Fertilize calcium nitrate (Ca(NO3)2) 200-300 grams per tree 1-2 times during the fruit growing stage.

Calcium — boron foliar spray:

Spray *Calcium-Boron* or *Calcium chloride + Boric acid* during:
Before flowering: 1 time.
15-20 days after fruit set: 1 time.
45-60 days after fruit set: 1 time.
90 days after fruit set: 1 time.
Dosage according to label — usually 1-2 milliliters of product per liter of water.

Stable water management

Water evenly — don't dry-water-dry suddenly.
Rainy season: ensure good drainage ditches. After heavy rain, check that there is no waterlogging.
Dry season: water evenly 2-3 times per week (jackfruit needs constant moisture).
Cover the base with dry grass and rice husks — reduce soil moisture fluctuations.

Balance fertilizer

Fruit growing stage:

Nitrogen reduction: fertilize *N-P-K* 12-12-17 or 15-15-15 instead of 16-16-8.
Increase potassium: add potassium sulfate (K2SO4) 200-300 grams per plant monthly.
Supplementing magnesium and trace elements: foliar spray magnesium sulfate (MgSO4) + trace element preparations (Zn, Mn, Cu).

Simple rule: during the fruit growing stage, if the tree is still pushing many young shoots — nitrogen fertilization is high, need to reduce.

Prune fruit to the correct number

Number of fruits per tree according to age:

3-5 year old trees: 15-30 fruits per tree per season.
5-7 year old tree: 30-50 fruits.
7-10 year old tree: 50-80 fruits.
Tree over 10 years old: 70-100 fruits.

Pruning according to the following principles:

When the young fruit is the size of a duck egg — prune for the first time, remove deformed fruit and small fruit in bunches.
When the young fruit is the size of a small grapefruit — prune a second time, keeping the target number of fruit.

Choose less sensitive varieties

If your orchard continuously turns black despite good care, consider less sensitive varieties:

To Nu jackfruit — small fruit but less black and fibrous.
Red-fleshed jackfruit — new variety, less black and fibrous.
New Thai jackfruit (Long Dinh) — being tested.

Intercrop varieties to reduce total orchard risk.

Scheduled room in case

Before flowering (1 month)

Lime (if the soil is acidic).
Foliar spray *Calcium-Boron* once.
Cut diseased branches, air the canopy.

Flowers bloom — young fruit sets (1 month)

Do not spray when flowers bloom.
15-20 days after fruit set: spray *Calcium-Boron* + amino acid.

Raising young fruit (1-2 months)

Prune young fruit when it's the size of a duck egg.
Fertilize *N-P-K* 12-12-17 + add potassium sulfate.
Spray *Calcium-Boron* 1-2 times.

Grow large fruits (1-2 months)

Prune the fruit a second time.
Final application of potassium sulfate.
Spray *Calcium-Boron* + KNO3 1 percent.
Consistently moisturizing — especially important.

Before harvest (15-20 days)

Stop fertilizing nitrogen.
Reduce irrigation by 30-40 percent.
Do not spray.

When the fruit is black and fibrous

Unfortunately — there is no way to "treat" blackened fruit. How to handle:

Cut fallen fibrous black fruit to reduce the tree's burden.
Increase *Calcium-Boron* spray for the remaining fruits.
Assess the black fiber rate — if it's over 30 percent, you need to review the entire care process.

Light fibrous black fruits (a few segments) can be sold at low prices for processing (dried jackfruit, fried jackfruit). The fibrous black fruit is heavy and discarded.

Monitor and evaluate

[ ] Measure soil pH twice a year — early and mid-season.
[ ] Are there any signs of calcium deficiency in the leaves (young leaves at the tips of branches are curled and wilted).
[ ] Number of fruits per tree compared to the rule.
[ ] Rain and drainage during fruit farming season.
[ ] Proportion of black fibrous fruit at harvest — graphed by year.

Common mistakes

News "medication to treat black fibrosis": none. Black fibrosis is a physiological disorder, not a disease.

Only apply calcium once a year: not enough. Leaves need to be sprayed continuously during the fruit growing stage.

High nitrogen fertilization "for big fruit": counterproductive. The fruit grows quickly but easily turns black.

Hold fruit more than 100-150 fruits per Thai jackfruit tree: definitely high black fiber rate.

Watering thoroughly then drying: water shock, increased fiber blackness. Water evenly all year round.

Take notes

Rate of black fibers in the crop (count 50-100 random fruits when cutting).
Calcium and lime fertilizer schedule.
Foliar spray schedule *Calcium-Boron*.
Number of fruits per tree.
Rainfall + unusual weather during the season.

Comparison between cases helps refine the process.

References

*Physiological disorders in jackfruit trees* — Southern Fruit Institute, 2022.
*Prevention of black fiber on jackfruit* — Department of Crop Production, 2023.
*Calcium and boron management in tropical fruit trees* — Vietnam Academy of Agricultural Sciences, 2021.