Sunken Fruit, Healthy Leaves in a Saline Tomato Field
Mr Adeyemi grows 0.5 ha of determinate tomato on raised beds under furrow irrigation. The field is calcareous, pH 7.9, with a saturation-extract electrical conductivity of 3.2 dS/m. He irrigates 40 mm every third day, even after rain, and uses no mulch. Nitrogen and potassium are applied generously through fertigation; phosphorus was applied at planting. At 60 days, 20–30% of fruit show a dark, leathery, sunken patch at the blossom end. Leaves are dark green and healthy, with no spots or wilting; roots dug from the bed are white and clean. Mr Adeyemi has bought calcium nitrate and plans to spray it weekly. A neighbour advises a fungicide, saying the patch is a rot.
1. Diagnose the disorder and justify your diagnosis against the two proposed remedies.
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The disorder is blossom-end rot, a physiological calcium-related problem, not a pathogen.
Evidence for the diagnosis: the lesion is confined to the blossom end, sunken and leathery; foliage and roots are clean; the problem appears on fruit rather than leaves; no spreading margins or fungal growth are described. A fungal fruit rot would normally show soft decay, sporulation or a wound entry point and would spread in wet conditions.
Against the fungicide: there is no pathogen to control, so fungicide will cost money and add residue without reducing the disorder.
Against foliar calcium nitrate: calcium is phloem-immobile and foliar sprays move poorly into fruit. The soil is calcareous and pH 7.9, so soil calcium is almost certainly adequate. The real problem is calcium delivery to the fruit, driven by irregular water supply and salinity. The 40 mm every third day, including after rain, causes wide wetting and drying cycles; high salinity competes with calcium uptake and transport.
Conclusion: adding more calcium addresses a problem he probably does not have. Fungicide is wasted. The fix is water and salinity management.
2. Set out a practical management plan for the rest of the season, ranking actions by likely impact.
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1. Stabilise irrigation (highest impact). Replace the fixed 40 mm every third day with smaller, more frequent applications based on crop demand, keeping soil moisture steady rather than cycling from saturated to dry. Irrigate early morning. After rain, reduce or skip the next irrigation instead of applying the same fixed dose.
2. Reduce salinity around the root zone. Improve drainage from the beds and apply a leaching irrigation where drainage allows. Avoid fertiliser spikes, especially potassium and ammonium, which compete with calcium. Split nitrogen applications.
3. Mulch the beds. A straw or plastic mulch reduces evaporation and damps the wet–dry swings that drive the disorder.
4. Remove badly affected fruit so the plant is not wasting resources, and keep records of new clusters to see whether the plan works.
5. Monitor, do not panic-spray. Check new fruit weekly. If new clusters remain clean after two weeks of stable moisture, the diagnosis is confirmed.
Expected outcome: existing lesions will not heal, so success is measured by the proportion of affected fruit in later trusses.
3. The neighbour insists the field should be treated with a broad-spectrum fungicide as insurance. Give a reasoned response, including what evidence would change your mind.
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Insurance spraying is not free: it costs money, adds residue and may not be permitted close to harvest, and repeated broad-spectrum use can disrupt beneficial organisms and select for resistance. On the described evidence it is not justified.
What would change the diagnosis: soft, watery decay rather than dry leathery tissue; a clear margin between healthy and diseased tissue; fungal growth or spores on the lesion; lesions starting at wounds, cracks or the stem end; spread to leaves or stems; rapid increase during wet weather. Any of these would justify a sample to an extension officer before spraying.
A low-cost check: place a few affected fruit in a clean, humid container for two to three days. If no fungal growth develops and the lesion remains dry and confined, the physiological diagnosis is supported. If soft rot with sporulation appears, a pathogen may be involved as a secondary invader.
Decision rule: treat the environment first, because blossom-end rot is a management problem. Only use a fungicide if there is direct evidence of a pathogen or a documented local outbreak, and then choose a product registered for that disease on tomato and respect the pre-harvest interval.