A mango block that refuses to flower a second year running
A 6-hectare Alphonso mango block near Nuzvid, Krishna district, planted 8 years ago at 10 m × 10 m (about 600 trees), yielded 14 t/ha last season but has produced only scattered flowering this year. The trees are now in dense, dark-green vegetative flush with no visible sign of dormancy. The orchard was irrigated every fortnight through October and November, received 180 mm of unseasonal rain in late October, and was given 250 kg N/ha in three splits plus 20 t/ha of FYM. The owner asks for a paclobutrazol soil drench in the first week of December for the whole block. Three years ago the same block received paclobutrazol at 10 g a.i. per tree as a single drench, flowered heavily, cropped well, and the following season carried almost no crop. Labour is scarce and the owner cannot afford a second failed year; a neighbouring Banganapalli block at 7 m × 7 m has flowered normally.
1. Explain what has gone wrong physiologically, and why the earlier paclobutrazol year was followed by a blank year. How does that change your advice now?
Read the worked answer
The block has never been allowed to reach the physiological state that permits flowering. Mango needs a mature, dark-green terminal flush held in a dormant, quiescent condition, followed by a dry/cool cue. Here continuous fortnightly irrigation and generous nitrogen kept gibberellin levels high and pushed one flush after another, so shoots stayed soft and immature. Late-October rain reinforced this. The neighbouring block's normal flowering tells you the problem is management, not climate.
The blank year after the paclobutrazol treatment is best read as biennial bearing, not as proof that the chemical 'worked too well'. A 14 t/ha crop mobilises stored carbohydrates, and the developing seed/fruit are strong sources of gibberellins and auxins that suppress the transition of the next flush. The heavy 'on' year therefore sets up an 'off' year irrespective of the drench. Residual soil paclobutrazol (it persists in soil for months) plus weak post-crop flush adds further risk of over-dwarfed, poorly filled panicles.
So the diagnosis is a carbohydrate–hormone problem compounded by over-watering and over-nutrition. The advice must shift from 'spray a PGR to force flowers' towards restoring a dormant terminal flush, correcting irrigation and nitrogen, and breaking the biennial cycle by fruit thinning and crop regulation - with any drench planned for the following cycle, not this one.
2. Will you sanction the December paclobutrazol drench for all 600 trees? Set out the decision, the dose logic and the monitoring you would insist on.
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Not as requested - a blanket December drench for every tree would spend money, create residue and probably still fail.
Timing first: paclobutrazol must reach the root zone roughly 60–75 days before the intended flowering date to block the ent-kaurene step of gibberellin biosynthesis and allow shoot maturation. A drench in the first week of December targets the next cycle, not the flowering the owner is worrying about now. If he expects this drench to rescue the present season, the expectation is physiologically impossible and should be corrected before money is spent.
Selection next: treat only trees that have a fully expanded, dark-green, mature flush and no active growth. Reject trees still flushing - they will respond poorly and be over-suppressed. Give the untreated trees water stress and a nitrogen holiday instead.
Dose: work from the approved label and current canopy volume, not from the 10 g a.i. blanket used last time to every tree of a younger orchard. Younger bearing trees need substantially less per tree, applied uniformly around the trunk in enough water to reach the feeder roots, and lighter doses on shallow or sandy soils. Keep a tree-wise record of dose and date.
Monitor: date of flush maturity, days to panicle emergence, panicle length, fruit set, and post-harvest flush vigour over two seasons. If the first response is over-dwarfing, stop dosing rather than repeating.
3. Set out a three-season plan to make the block flower and yield dependably, without excessive reliance on growth regulators.
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Season-wise, the plan should be built around canopy, water, nutrition and crop load rather than chemicals.
Immediate season: restore dormancy by stopping nitrogen from October, spacing irrigation to allow a stress period before the expected flowering window, and removing late water shoots. Do not force the crop.
Canopy: 10 m × 10 m is now too wide for 8-year-old Alphonso. Begin systematic canopy management - open the centre, keep the skirt high, and remove criss-cross limbs so light enters and future flushes mature quickly. Do not dehorn the whole block at once; do it in two halves over two seasons so one half bears while the other is restructured. Long-term, plan high-density planting with annual pruning for the new areas.
Nutrition: shift from calendar dosing to leaf and soil test-based NPK, keep organic matter high, and correct any deficiency before flowering. Heavy nitrogen is the single commonest reason for this problem.
Crop load: in an 'on' year, thin fruit in the first weeks after set so the tree carries a moderate load; this is the cheapest way to break biennial bearing. Keep a block diary of flowering date, panicle density, yield and post-harvest flush for each season. Only if a mature dormant flush and correct water management still fail to bring flowering should a PGR be used, and then on identified trees, not the whole block.