Not Everything Wrong Is a Disease
Brown leaf margins that look like burn, a plant that throws a flower stalk three weeks early, a tomato fruit with a black base that tests negative for pathogens — a large share of the problems that reach a grower’s desk are not infectious at all. They are physiological: the plant’s transport or development failed under a particular set of conditions.
That distinction matters because the responses are completely different. You cannot spray your way out of calcium transport failure, and you cannot cool your way out of a genetics problem. This guide covers the disorders we see most often in commercial hydroponic houses, starting with lettuce tipburn in hydroponics, and gives you the decision order that avoids chasing the wrong cause.
Tipburn
What it looks like
Young inner leaves develop brown, papery edges that expand inward as the head grows. It appears first on the newest leaves, often hidden inside a head that looks fine from above — which is why it is frequently discovered by the grader rather than the grower.
Why it happens
Tipburn is a calcium transport problem, not usually a calcium supply problem. Calcium moves in the transpiration stream and is poorly redistributed once deposited, so the fastest-growing tissue can be locally short even when the solution contains plenty. Anything that slows local transpiration, or pushes growth too fast, raises the risk.
| Driver | How it contributes | What to change |
|---|---|---|
| High humidity / low airflow in the canopy | Reduces transpiration at the leaf margin | Increase air movement within and beneath the canopy; review night humidity |
| Fast growth from high light or warm nights | Leaf expansion outpaces calcium delivery | Adjust temperature setpoints, particularly night temperature |
| High EC in the root zone | Tightens water and calcium uptake competition | Review EC against crop stage and season; check for root-zone accumulation |
| Ammonium-based nitrogen proportion | Shifts uptake balance against calcium | Review nitrogen form in the recipe |
| Genetics | Varieties differ substantially in susceptibility | Choose tolerant varieties for high-risk seasons |
| Restricted root volume or poor root health | Limits uptake capacity | Check root condition; confirm solution oxygenation |
What actually helps
- Air movement, especially inside the head and at night. This is the single most effective lever in many houses
- Night humidity control so the plant keeps moving water after lights and fans reduce
- Moderating night temperature where the schedule allows it
- Variety selection for high-risk seasons — cheap insurance compared with grade-out
- Foliar calcium sprays help only where they reach the affected tissue, which is often inside a closed head. Treat them as support, not as a cure
Bolting
Heat-driven premature flowering in lettuce and other leafy crops. Plants elongate, develop a bitter stem and lose market value quickly.
- Triggers: sustained high temperature, particularly night temperature; long day length; water stress; sometimes transplant shock
- Variation: varieties differ widely in bolt tolerance, and summer-adapted genetics exist for most markets
- Control: seasonal variety selection, temperature management, avoiding stress at transplant, and shortening the cycle into the risky period
- Harvest decision: once a crop starts to bolt, earlier harvesting salvages more than waiting does
Blossom End Rot
Dark, leathery patches on the blossom end of tomato and pepper fruit — the same story as tipburn, applied to fruit.
- Cause: localised calcium deficiency at the fruit tip, driven by transport rather than supply
- Common triggers: fluctuating root-zone moisture, high EC, damaged roots, rapid fruit set in hot conditions
- Fixes: stabilise irrigation frequency and volume, review EC and nitrogen form, check root health, avoid extreme swings in root-zone conditions
- Reality check: affected fruit will not recover; the goal is protecting subsequent trusses
Edema and Related Symptoms
Small water-soaked blisters, usually on the underside of leaves, that may later turn corky. It happens when leaves take up water faster than they can transpire, typically under high humidity and low airflow combined with warm roots — the mirror image of tipburn drivers.
- Improve airflow and reduce excess humidity
- Balance root-zone and air temperature — warm roots with cool, still air is the classic setup
- Review irrigation timing so uptake is not pushed late in the day into falling transpiration
Fruit Cracking, Puffiness and Poor Set in Fruiting Crops
| Symptom | Common cause | Check first |
|---|---|---|
| Tomato fruit cracking | Rapid water or humidity swings during fruit expansion | Irrigation consistency and day/night humidity difference |
| Puffy or hollow fruit | Poor pollination or seed set | Pollination programme and flower viability |
| Uneven ripening | Temperature extremes, particularly heat | Cooling capacity and ventilation during peak periods |
| Pepper misshapes | Flowering-time temperature stress or incomplete pollination | Temperature at flowering and set rates |
| Cucumber bitterness or shape defects | Environmental stress and pollination gaps | Climate consistency and pollinator activity |
Diagnostic Order That Saves Time
- Establish the pattern. Random across the house usually means environment; a block or row boundary usually means layout, airflow or plumbing
- Identify which tissue. Newest leaves point to transport issues; older leaves point to mobility of nutrients or root problems
- Check environment records before chemistry. Most of these disorders have a climate signature in the log
- Inspect roots. Poor root condition converts a mild nutritional imbalance into a visible disorder
- Check variety notes. Some disorders are effectively expected behaviour for a given genetics in a given season
- Change one thing and verify. Simultaneous changes leave you unable to read the result
Records That Make Diagnosis Possible
- Climate logs including night humidity and temperature, not just daytime averages
- Irrigation and EC data per zone per day
- Variety and batch records so susceptible lines can be identified after the fact
- Grade-out records by symptom rather than only total reject figures
- Photographs with dates — progression over time is diagnostic in itself
FAQ
Is tipburn caused by too little calcium in the solution?
Usually not. Most affected crops have adequate calcium available; the failure is in transport to fast-growing tissue, driven by humidity, airflow and growth rate.
Will adding more calcium fix it?
Occasionally, where supply genuinely is short. Far more often the answer is airflow, night humidity control and variety choice. Adding calcium without addressing transport gives disappointing results.
Do fans help?
Yes, particularly horizontal airflow directed into and around the canopy. Still air inside a developing head is one of the strongest tipburn predictors.
Is it infectious?
No. Tipburn does not spread plant to plant, and that is part of why it looks so unlike a disease pattern once you know to look for the layout signature instead.
Can affected leaves recover?
No. Damaged tissue stays damaged. Management protects the next flush of leaves, which means the value of early detection is in future harvests, not the current one.
Why is it worse in winter in some houses?
Because dehumidification and airflow are often reduced when heating is running, pushing humidity up exactly when growth continues. Reviewing night strategy usually helps more than adding nutrition.
Get the Environment Right First
Physiological disorders are a layout and climate problem before they are a nutrition problem. Send us your crop plan, house dimensions and climate constraints and we will review airflow, zoning and layout for the risk factors above. Reach us through the quote form.
Related reading: most disorders trace back to climate or diagnosis — see humidity control and VPD, ventilation and HAF fans and diagnosing nutrient deficiencies; also relevant are root health and commercial lettuce production in NFT.