Mango cv. Amrapali orchard with poor fruit set after wet, foggy flowering
Ramesh Sahu owns 0.8 ha of 12-year-old mango cv. Amrapali at Kamakhya Nagar in Dhenkanal district, planted at 5 m x 5 m and never pruned since planting. The orchard carried a very heavy crop last year (about 9 t). This year it flowered well in the first week of January. During February there were three spells of unseasonal rain totalling 48 mm, night temperatures of 12-14 degrees C and dense morning fog that cleared only around 9 a.m. At the pea stage he finds only 8-10 fruits retained per panicle against the 20-25 he expected, black sunken lesions on several panicle stalks and young fruits, sticky honeydew with black coating on the lower leaves, and a heavy population of small green hoppers on the panicles. In January he applied 500 g urea per tree and has since given one round of a broad-spectrum insecticide. He wants to know why the crop failed, what to do for the fruit still on the tree, and how to stop this repeating.
1. Rank the likely causes of the poor fruit set in this orchard from most to least important, and state the field evidence that would confirm or rule out each one.
Read the worked answer
Judge the problem in three separate stages, because fruit number is the product of pollination, retention and protection.
Weather at flowering (primary). Mango pollen germinates best around 25-30 degrees C. Nights of 12-14 degrees C depress pollen germination and tube growth, 48 mm of rain washes pollen off the stigma and dilutes the stigmatic exudate, and fog keeps the flower wet for hours. Evidence: compare set on early panicles that opened before the rain with late panicles that opened after it. If late panicles hold 18-20 fruits, weather is the driver and no spray would have saved the early crop.
Carbohydrate exhaustion after a 9 t crop (strong secondary). Heavy bearing draws down reserves, so the tree flowers profusely but cannot fill the fruit. Evidence: short, thin, pale panicles, negligible new shoot growth, and a yield record showing the previous heavy year. The 500 g urea in January makes this worse, not better, because nitrogen at panicle emergence pushes vegetative growth.
Anthracnose (Colletotrichum gloeosporioides). Fog and rain are ideal for it; the black sunken lesions on the panicle stalk and young fruit are characteristic. Evidence: count incidence on 10 tagged panicles. If lesions are absent on the panicles that escaped the fog, weather plus fungus together explain the loss.
Hopper. The green nymphs, honeydew and sooty mould confirm a heavy population, but hopper alone rarely takes a crop from 25 to 8 fruits; it is a co-factor, not the root cause.
Ranking: weather > carbohydrate status > anthracnose > hopper.
2. Give a corrective spray and nutrition plan for the next six weeks, specifying product, dose and timing, and justify each choice.
Read the worked answer
The crop still on the tree is worth saving, but nothing should be aimed at 'more flowers'. Everything should protect the retained fruit and stop the tree from spending energy on leaves.
Immediate (within 3 days)
- Carbendazim 50 WP at 1 g/L plus a sticker, to stop anthracnose spreading from the lesions into the fruit stalk. Mancozeb 75 WP at 2.5 g/L is the alternative if carbendazim has been used repeatedly.
- Imidacloprid 17.8 SL at 0.3 mL/L for hopper, directed at the panicles and the underside of leaves. Justification: the honeydew and sooty mould show the population is well past the damage threshold, and hopper feeding scars the fruit stalk.
At pea-to-marble stage (about 10 days later)
- NAA at 20 ppm, roughly 0.45 mL of a 4.5% NAA formulation per litre, as a directed spray on the fruit clusters, to reduce drop.
- Boron only if a soil test shows deficiency; a 0.2% borax spray at panicle emergence next year is safer than guessing now.
Nutrition
- Stop nitrogen completely for this season. Apply 40-50 kg well-rotted FYM per tree in a ring at the drip line, plus muriate of potash at 500 g and single super phosphate at 750 g per tree, based on soil test. Potassium and phosphorus support fruit filling and reserve build-up; more nitrogen only produces a flush that competes with the fruit.
Cultural
- Remove and burn the fallen infected fruits and pruned panicle debris, since these carry the anthracnose inoculum.
- Withhold irrigation for 10 days before harvest to raise TSS, then irrigate lightly. Do not repeat the broad-spectrum insecticide; it destroys the natural enemies that keep hopper in check.
3. What long-term changes would convert this orchard into a regular bearer, and in what order would you implement them?
Read the worked answer
Regularity in mango is a canopy, carbohydrate and pruning problem, not a spray problem.
Year 1 - open the canopy first. The 5 m x 5 m spacing with no pruning has produced a closed, humid canopy, which is why sooty mould and anthracnose are severe. Remove the central leader and 2-3 inward-growing limbs per tree in the first fortnight after harvest, keeping the skirt at least 60 cm above ground and letting light into the centre. Paint the exposed limbs with Bordeaux paste. Do not prune and de-blossom in the same year on the same tree.
Year 1 - manage the heavy year. If the tree carries a heavy crop, remove 60-70% of panicles at the 1-2 cm stage on marked branches. This sacrifices some yield this year but builds the carbohydrate reserve for next year, which is the standard de-blossoming treatment for biennial bearing. Amrapali is more regular than Dashehari but still alternates under a heavy load.
Year 2 - calendar-based nutrition. Split NPK into a post-harvest basal dose plus a flowering-time dose based on soil and leaf analysis; keep nitrogen out of the panicle-emergence window. Apply FYM every year and mulch the basin.
Year 2 - pest and disease calendar. Two protectant sprays (carbendazim or mancozeb) from panicle emergence to pea stage, one hopper spray only when the population crosses the threshold, and sticky traps for monitoring instead of routine spraying.
Year 3 - evaluate. Record yield per tree for three seasons. A tree that still alternates after de-blossoming and canopy opening needs a heavier pruning or a replant decision. Also consider intercropping the alleys only with a low, non-competitive crop during the first two years after canopy opening.