Blossom-end rot in a glasshouse tomato crop
A 0.3 ha glasshouse tomato crop on rockwool is at truss 3. Fruit on truss 2 show brown, sunken, leathery patches at the blossom end. Feed EC is 4.2 mS/cm (target 2.8–3.5), root-zone EC is 5.1, and root-zone pH is 5.2. Relative humidity is 55% by day and 60% at night. Irrigation is programmed at eight applications of 150 mL per plant per day regardless of solar radiation. The grower proposes adding a calcium foliar spray and raising feed calcium from 180 to 260 ppm. Night temperature is 15 °C and day temperature is 24 °C. Drain percentage is 35%, and fruit load on trusses 1–2 is heavy. The crop is nine weeks from sowing.
1. Diagnose the disorder and explain why the proposed calcium spray is unlikely to fix it.
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The symptoms are classic blossom-end rot, a calcium-related disorder of the fruit rather than a shortage of calcium in the feed. Calcium moves in the xylem with water and is delivered mainly to leaves, which transpire most. Fruit transpire little, so when the plant is under water stress, calcium is partitioned away from the fruit and the blossom end collapses.
Several factors here point to water stress rather than low feed calcium. Feed EC of 4.2 and root-zone EC of 5.1 are above target, which lowers the water potential of the root zone and reduces water uptake. Low daytime humidity of 55% increases transpiration from leaves, competing with the fruit for calcium. Irrigation is fixed at eight shots per day regardless of radiation, so the plant may be overwatered at low demand and underwatered at high demand, producing exactly the transient calcium shortage that causes blossom-end rot.
A foliar calcium spray is unlikely to help. Calcium is phloem-immobile, and foliar uptake is directed mainly to leaves, not to fruit. Raising feed calcium from 180 to 260 ppm also will not fix the problem if water flow to the fruit is the limiting factor.
2. What should the grower change first, and in what order?
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First, reduce EC. Bring feed EC toward 2.8–3.0 and root-zone EC toward 3.5 by increasing the drain fraction and reducing feed concentration. High EC is the most direct cause of reduced water uptake here.
Second, replace fixed irrigation with radiation-based scheduling. Increase the number of smaller shots, especially around midday, so water uptake is steady and the plant does not swing between wet and dry. Check the drain percentage after each change; 35% is high, but it should not be reduced until EC is under control.
Third, raise humidity to about 65–75% by day using vents or fogging, to reduce leaf transpiration and leave more water for the fruit. Keep night temperature stable and avoid large day–night swings.
Do not increase feed calcium as the first move. A foliar spray may green up leaves but will not correct fruit symptoms. Also check for root disease or poor drainage, because damaged roots cannot take up water even if the feed is correct. Remove badly affected fruit to reduce sink demand, but treat that as a consequence, not the cure.
3. How would you judge whether the changes have worked, given that the crop is nine weeks from sowing?
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The test is whether new fruit on trusses 3 and 4 develop symptoms, not whether truss 2 improves. Truss 2 fruit will not recover. Set a simple monitoring routine: record root-zone EC and pH daily, drain percentage, and the number of new fruit with blossom-end rot on each truss. Take measurements at the same time each day so the trend is meaningful.
Expect a lag of about one to two weeks before new fruit show the effect. If new symptoms continue after EC is corrected and irrigation is radiation-based, look for another cause, such as root damage, poor rockwool drainage, or uneven irrigation. If symptoms stop, the diagnosis was correct and the changes should be maintained for the rest of the crop.
Record the cost of the changes, including any yield loss from removing affected fruit, so the grower can compare it with the cost of the proposed calcium spray. The judgement should be based on new fruit and on the trend in EC and drain, not on a single observation.