Blossom-end rot and fruit cracking in drip-irrigated tomato
A 0.4 ha block of indeterminate tomato is drip-irrigated on a sandy loam with pH 6.8, exchangeable Ca 6.0, Mg 2.1 and K 0.9 cmol/kg. Fertigation delivers 220 kg N/ha, 35% of it as ammonium sulphate, and 260 kg K2O/ha. The irrigation water is 1.9 dS/m and the root-zone extract has risen to 3.8 dS/m; the grower applies three heavy irrigations a week 'to push the salt down'. From the first truss, 18% of fruit show a water-soaked spot at the stylar end that turns black and leathery; the worst rows are the windward outside rows. After 40 mm of rain following two dry weeks, 12% of ripening fruit split radially. Leaf Ca is 1.9% (adequate) and four weekly foliar sprays of 0.5% calcium chloride have changed nothing. Yield is 48 t/ha, but 26% of fruit is unmarketable and the buyer has warned about the next delivery.
1. Explain why blossom-end rot has appeared in this crop and why the foliar calcium programme has not reduced it. What does the leaf analysis tell you?
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The soil is not short of calcium; the fruit is.
Calcium moves almost entirely in the xylem, carried by the transpiration stream, and is phloem-immobile. The fruit transpires very little, so its calcium supply depends on a continuous, steady flow arriving through the pedicel while the tissue is expanding. Any interruption in that flow during the two to three weeks after fruit set produces a localised deficiency at the stylar end, where cells are dividing fastest and are furthest from the vascular supply. The tissue collapses, and the water-soaked spot blackens and becomes leathery.
Three factors in this field are interrupting supply:
- Wet–dry cycling. Three heavy irrigations a week followed by drying gives exactly the fluctuation thatstarves expanding fruit, and it matches the 40 mm rain event that later cracked ripening fruit.
- Root-zone salinity of 3.8 dS/m. Osmotic stress plus Na⁺ competing with Ca²⁺ at the root surface reduces uptake, and the 'leaching' pulses are not lowering it because the water itself is 1.9 dS/m.
- Ammonium and potassium load. 35% of 220 kg N as ammonium sulphate (about 77 kg N) acidifies the rhizosphere and suppresses Ca uptake; 260 kg K2O/ha is far above crop removal (roughly 180–200 kg K2O/ha for a 48 t crop) and K competes directly with Ca.
The windward outside rows show the worst symptoms because high vapour-pressure deficit there drives leaf transpiration, and the leaves outcompete the fruit for the same calcium stream.
Why the sprays failed: calcium chloride is not phloem-mobile and mango, tomato fruit has a waxy, almost stomata-free surface, so very little of the spray enters the fruit. Leaf Ca of 1.9% confirms adequate supply; the problem is distribution within the plant, not total uptake, so adding more calcium to the leaves cannot correct it.
2. List, in priority order, the changes you would make for the next planting, with the numbers you would change. How would you monitor whether the strategy is working?
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Priority 1 – stabilise soil moisture. Convert from three heavy pulses to small, frequent applications (e.g. 4–6 short pulses a day by drip) so the root zone stays near field capacity without saturation, and mulch the beds. This is the single change most likely to cut blossom-end rot, because it removes the supply interruptions.
Priority 2 – bring root-zone EC down towards 2.5 dS/m. Blend with a lower-EC source if one exists, reduce the concentration of each fertigation dose rather than applying large leaching pulses, and check emitter uniformity. Leaching with 1.9 dS/m water cannot achieve much.
Priority 3 – rebalance nutrition.
- Replace ammonium sulphate with calcium nitrate or a nitrate-dominant N source; keep ammonium N below about 10% of total N.
- Reduce K2O from 260 to about 180–200 kg/ha, split through the season.
- Supply calcium through the drip (e.g. calcium nitrate plus a Ca chelate) rather than by foliage; keep Ca and phosphate in separate injection cycles to avoid precipitation.
- Do not increase Mg; at 2.1 cmol/kg it is already competing with Ca.
Priority 4 – reduce transpiration demand on exposed rows (windbreak, shade net, or a border row) so leaves do not out-compete fruit.
Monitoring: tag trusses 4–6 on 20 plants in the windward and central rows and count new BER lesions weekly; record root-zone EC and soil moisture daily; track the percentage of unmarketable fruit per harvest. Recovery is realistic - later trusses formed under stable moisture should show BER below 5% - but fruit already affected cannot recover, so assess new set only.
For cracking, harvest fruit at the breaker-to-pink stage before forecast rain and keep irrigation steady rather than compensating after a dry spell.
3. The grower proposes switching to a determinate variety and raising K2O to 400 kg/ha 'to harden the fruit'. Evaluate both proposals.
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Raising K2O to 400 kg/ha would make the problem worse, not better. The disorder is a calcium delivery problem, and potassium is the strongest competitor with calcium at the root surface and in the xylem. Doubling K to 400 kg/ha also raises the osmotic concentration of the fertigation solution, pushing root-zone EC above the already excessive 3.8 dS/m. There is no evidence that high potassium 'hardens' tomato fruit; the usual result is more blossom-end rot and more competition for magnesium. The correct K rate is close to crop removal (about 180–200 kg K2O/ha for 48 t/ha), and any extra is money spent to reduce quality.
A determinate variety is not a cure either, but it has a modest role. Determinate types ripen their crop in a shorter window, which makes irrigation scheduling, harvest labour and spraying easier to manage, and some determinate types differ in crack resistance and in the number of fruit per truss. That said, a determinate variety is subject to exactly the same calcium physiology; if the water and salinity problems continue, it will show blossom-end rot identically. Changing variety without changing irrigation would simply produce a smaller plant with the same disorder.
A better package: choose a variety with a documented low susceptibility to blossom-end rot and radial cracking, keep the Ca and K balance correct, stabilise moisture, and lower root-zone EC. If a variety change is made for other reasons (market preference, labour), it should be tested on a small block for one season with the new irrigation schedule before the whole 0.4 ha is committed. The key judgement is that cultural and water management, not variety or extra potassium, is the lever that controls this disorder.