Blossom-end rot in furrow-irrigated tomatoes: disease, calcium or water?
A smallholder in a semi-arid district grows 2 ha of determinate tomatoes under furrow irrigation on a sandy loam. The field was limed five years ago; the last soil test, two seasons back, gave pH 5.2, organic carbon 0.4 % and exchangeable calcium 2.1 cmol/kg. The crop was top-dressed three times with 100 kg/ha of urea and once with 150 kg/ha of muriate of potash. After 40 mm of rain, the irrigation interval stretched from 4 days to 9 days during a hot, windy spell. At the third truss, 15 % of fruit show a sunken, leathery black patch at the blossom end; the patch is firm at first and later invaded by mould. Leaves look healthy and dark green, and neighbouring plants are unaffected except on the largest-fruited trusses. The farmer plans to spray a copper fungicide and to apply more potash.
1. State your diagnosis and justify it from the evidence in the case. Say why the copper spray and the extra potash will not solve the problem.
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Diagnosis: blossom-end rot, a calcium-related physiological disorder triggered by erratic moisture - not a pathogen.
Evidence:
- The lesion is on the distal (blossom) end only, on the largest fruit and the heaviest trusses - the tissue with the greatest, fastest demand for calcium.
- The patch is firm, sunken and leathery at first, with no water-soaked spreading margin, no halo and no leaf spots. Neighbouring plants are not progressively infected; the pattern follows fruit size, not plant-to-plant spread. Mould appears only later, on dead tissue.
- No leaf yellowing, so it is not a general nutrient deficiency or a root rot. But dark green leaves plus heavy urea and muriate of potash dressings point to luxury uptake of ammonium-N and potassium, both of which compete with calcium.
- Timing fits: 40 mm of rain followed by a 9-day gap on a sandy loam with low exchangeable calcium (2.1 cmol/kg) starved the fruit of calcium exactly when cells were expanding. Calcium moves with the transpiration stream in the xylem and cannot be re-mobilised from older tissue to the fruit.
Copper fungicide is wasted money because there is no pathogen to control. Extra potash makes matters worse by competing further with calcium and magnesium uptake and by raising the salt concentration in the root zone.
2. Set out what the farmer should do in the next 14 days and what should change next season. Explain why a foliar calcium spray is only a partial measure.
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Next 14 days
- Restore even moisture: go back to 25–30 mm every 4–5 days rather than 60 mm every 9 days. Irrigate early morning or evening, and check the furrow stream actually reaches the tail end of the field.
- Mulch the beds with 5 cm of crop residue to buffer moisture and cut evaporation; keep mulch clear of the stems.
- Stop the urea and muriate of potash top-dressing for the rest of the season - the crop is already dark green and more ammonium-N and K will worsen the calcium imbalance.
- Pick off and destroy badly affected fruit so mould does not spread, and harvest earlier rather than sizing fruit that will be unsaleable.
- Foliar calcium (calcium nitrate or calcium chloride on young trusses and fruit) gives only partial, short-lived protection: very little calcium enters through the fruit skin, so it supplements water management, it does not replace it.
Next season
- Sample the soil properly (pH, Ca, Mg, K, P, lime requirement) before buying lime. At pH 5.2 on a sandy loam, aim for pH 6.0–6.5 using the laboratory's lime rate for the buffer pH, applied and incorporated 2–3 months before planting.
- If pH is corrected but exchangeable calcium stays low, use gypsum, which supplies calcium without raising pH; the soil test sets the rate.
- Split nitrogen applications, keep potassium within soil-test recommendations, and hold a constant irrigation interval. Keep a simple diary of rainfall, irrigation dates and fruit symptoms so next season's 15 % can be compared with a measured figure.
3. The farmer now wants to spread 2 t/ha of agricultural lime immediately, mid-season. Evaluate that proposal and state what you would do with the money instead.
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Verdict: do not apply lime now, mid-season.
Reasons:
- Lime is slow-acting. It needs moisture and 2–3 months to react, and it must be incorporated into the topsoil to raise pH where the roots are. Broadcast on the surface of a sandy loam under furrow irrigation, it will have almost no effect on this crop before harvest, and much of it will wash into the furrows.
- Liming does not directly fix this disorder. The immediate cause is a calcium transport failure in the fruit driven by moisture swings, not a simple soil calcium shortage. Even a well-limed soil produces blossom-end rot when irrigation is erratic.
- Risk of harm: fine lime spread on a hot, wet surface can raise pH unevenly, and without a fresh soil test the 2 t/ha figure is guesswork. If the field needs 4 t/ha, 2 t/ha buys half a correction and gives the farmer false confidence.
Better use of the money
- Spend it on the water problem: a reliable pump and furrow schedule, a simple moisture check (feel test or tensiometer), and mulch material.
- Take a proper soil sample now and pay for a lime requirement analysis. Apply and incorporate the correct lime rate after harvest, 2–3 months before the next crop.
- Monitor the next crop on the first heavy trusses and compare the disorder level with this season's 15 %. If it falls without any lime, that confirms water was the driver.