Blossom-end rot in a saline greenhouse tomato crop
A 0.25-ha plastic greenhouse near Tripoli grows tomato cv. Ikram for the spring market. The crop is at early fruit set. Drip irrigation applies 6 mm/day of water with EC 3.9 dS/m; the root-zone saturated extract is ECe 5.1 dS/m. Daytime relative humidity is 88% and night 95%. The first truss shows brown, leathery patches on the blossom end, while leaves show no necrosis and no pathogen is isolated from the lesions. Leaf Ca is 0.9% dry weight, just below the usual sufficiency range, and distal fruit tissue Ca is 0.06% dry weight versus 0.12% in healthy fruit. The grower applied two foliar sprays of calcium nitrate, but new affected fruit continue to appear. The soil is a clay loam with a compacted plough pan at 20 cm. The grower asks whether the problem is a disease and whether more calcium spray will help.
1. Is this an infectious disease? Explain the physiological mechanism that best accounts for the fruit symptoms and for the failure of foliar calcium.
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It is not an infectious disease. The symptoms fit blossom-end rot, a localized calcium-related disorder of the fruit. The key evidence is the distribution: leaf Ca is only slightly below sufficiency, but distal fruit tissue Ca is half the healthy value, and no pathogen is isolated. Calcium moves mainly in the xylem and is not redistributed in the phloem. Fruit transpire little, so the distal blossom end depends on xylem inflow at a time when rapid fruit expansion dilutes calcium. High humidity reduces transpiration, and the high root-zone ECe (5.1 dS/m) plus the compacted plough pan reduce water uptake, further limiting xylem flow to the fruit. Foliar calcium nitrate is retained largely on the leaves and is not phloem-mobile, so it cannot supply the fruit. The near-adequate leaf Ca confirms that the problem is calcium distribution, not a simple whole-plant calcium shortage.
2. Recommend two management changes that target calcium delivery to the fruit, and one measurement the grower should use to judge whether the changes work. Justify each recommendation.
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- Reduce the root-zone salinity and improve water uptake. If lower-EC water is available, leach the root zone; otherwise reduce fertilizer concentration and apply irrigation more frequently so the root zone does not dry between cycles. This lowers osmotic stress and supports steadier xylem flow to rapidly growing fruit. Breaking the 20-cm plough pan or improving drainage also helps root exploration and water uptake.
- Avoid competition and peaks of demand. Do not over-apply potassium, magnesium or ammonium, which can compete with calcium uptake, and keep soil moisture uniform during fruit set. Foliar calcium should not be the main strategy because it does not reach the fruit.
- Monitor with root-zone ECe (saturated extract) and by tagging trusses to count new blossom-end rot cases. A fall in ECe toward the crop's target range, together with fewer new lesions on tagged fruit, shows that the changes are working. Fruit Ca analysis can confirm the trend but is slower and more expensive.