Physiological disorders

What Are Physiological Disorders?

Physiological disorders are a plant's response to abiotic stress factors—non‑living environmental influences: improper watering, temperature fluctuations, nutrient deficiencies or excesses, soil compaction, root damage, and other conditions that disturb normal growth and development (Wien, 1997). They are not infectious diseases: they do not spread from plant to plant and do not require treatment with fungicides or insecticides.

How to Distinguish Them from Diseases?

This is a key question for any gardener, because the approach to treatment is fundamentally different. Here are the main distinguishing features (Villeneuve & Geoffriau, 2021):

For physiological disorders:

• Absence of a pathogen. You will not see fungal growth, rot odour, bacterial slime, or signs of insect activity.
• Patterned occurrence. The problem often does not appear randomly, but in specific parts of the field: on elevations (where soil dries faster), in depressions (where water stagnates), on compacted wheel‑track areas, or along roadsides (Wien, 1997).
• Link to conditions. The disorder arises in response to specific events: after heavy watering following a drought, after applying large amounts of nitrogen, or during sudden cold spells.
• Mass but non‑contagious nature. The problem may affect many plants in the field, but healthy neighbouring plants do not become infected.

For (infectious) diseases:

• Presence of a pathogen. You can see fungal spores, mycelium, bacterial exudates, or signs of activity (e.g., insect damage).
• Focal spread. The disease often starts with a few plants and gradually spreads to neighbours.
• Specific symptoms. For example, wilting of individual leaves in Fusarium wilt, characteristic spots in Alternaria blight, or galls on roots caused by nematodes.

Important. Some infectious diseases at early stages can mimic physiological disorders. For instance, root infection by Pythium can cause branching of the root (as happens under soil compaction stress), while Fusarium sometimes provokes hollows resembling boron deficiency (Villeneuve & Geoffriau, 2021). That is why, when a problem appears, it is essential to examine not only the root but the whole plant, assess the pattern of spread, and recall what weather conditions and agronomic practices preceded the defects.

Why Is It Important to Know This?

Understanding the nature of physiological disorders allows the gardener to:

1. Avoid wasting money and time on unnecessary chemical treatments.

2. Adjust cultivation practices (watering, fertilisation, tillage) to prevent the problem in the next season.

3. Assess risks when planning plantings and selecting varieties, since different carrot varieties have different stress tolerances (Villeneuve & Geoffriau, 2021; Simon et al., 2008).

In the following chapters, we will examine in detail the most common physiological disorders of carrot: root forking, cracking, “hairiness,” greening, bitterness, hollows, and woodiness. For each, you will learn the causes and, most importantly, practical ways to prevent and solve the problem.

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Why Does the Root Branch?

Carrot produces a straight, smooth root only under one condition: the main root (the embryonic root) must grow downward without obstruction. As soon as the tip of this root is damaged or dies, the plant loses apical dominance, and lateral roots take over – they begin to enlarge and turn into “horns” (Villeneuve & Geoffriau, 2021). If the growing point of the root dies at a very early stage (when the root is only a few centimetres long), the main root stops elongating – resulting in a short, blunt root (Villeneuve & Geoffriau, 2021).

What kills or damages the root tip? Several causes, often acting together.

How to Prevent Branching: A Quick Checklist

1. Soil preparation:

• Deep digging (25–30 cm) with removal of stones and root fragments.
• Application of well‑rotted manure or compost (at least 1–2 buckets per 1 m²) 2–3 months before sowing.
• On heavy soils, add sand (up to 10 L/m²) and peat for looseness.

2. Sowing dates:

• Sow when the soil has warmed to +8…+10 °C in warm, dry weather.
• If prolonged rain or cold sets in, cover the bed with row cover.

3. Watering:

• Water moderately, avoiding waterlogging.
• In hot weather, water frequently but in small amounts so the soil neither dries out nor cracks (more on cracking in the next chapter).

4. Soil health:

• Follow crop rotation (carrots after potatoes, tomatoes, onions, cereals).
• Use green manures – they improve structure and suppress pathogens.
• If Pythium or nematodes are severe, use biological products based on Trichoderma or Metarhizium, or consult specialists for soil fumigant recommendations (on large areas).

5. Seedling care:

• Avoid soil compaction during weeding and thinning – do it in wet weather and gently.
• Mulch between rows with straw or compost to maintain looseness and moisture.

If the Problem Has Already Occurred

If you notice that some roots are branched – do not rush to discard the whole bed. Branched roots, though less attractive, are perfectly edible, especially for processing (juices, soups, preserves). The main thing is to correct the agronomic mistakes that caused the problem in the next season. Carefully analyse which of the factors above might have been at play in your conditions, and adjust bed preparation, timing, or watering regime.

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Why Do Carrots Crack?

The main cause is sudden changes in growing conditions, especially water regime. The internal tissues of the root (phloem parenchyma) are quite elastic, but the outer layer – the periderm (cork layer) – is much less flexible (Villeneuve & Geoffriau, 2021). When the root grows rapidly, the outer layer fails to stretch and tears. Once a crack appears, it easily propagates into the softer inner tissues (Villeneuve & Geoffriau, 2021).

Cracking During Harvest and Handling

It is important to distinguish cracks that appear in the field from those caused by mechanical impact (bumps, falls, compression during washing and packing) (Villeneuve & Geoffriau, 2021). In the field, cracks are usually longitudinal, while impacts cause transverse breaks or fractures. However, both types render the root unsuitable for storage, as pathogens enter through the cracks.

Practical take‑away: harvest carefully, especially after rains when roots are more brittle. Do not throw carrots; place them gently in containers. If you wash carrots before storage, do it quickly and gently, without strong water pressure.

A Special Case: Water Spot (Water Core)

Another disorder related to excess moisture is “water spot” – softening and breakdown of cells, leading to longitudinal cracks with uneven edges. Such cracks are quickly colonised by secondary pathogens and lead to rotting. The cause is temporary oxygen starvation (anoxia) due to water stagnation after excessive watering or heavy rain (Villeneuve & Geoffriau, 2021). Carrots are especially sensitive to this at the end of the growing season, just before harvest.

Practical take‑away: 2–3 weeks before planned harvest, reduce watering to a minimum, and if heavy rains occur, protect the beds with row cover or film (on low hoops) if possible. Ensure good drainage on the beds.

How to Prevent Cracking: Summary

1. Stable watering. Water regularly, avoiding long dry spells. In heat, water more often but in small portions. Use mulch.

2. Moderate nitrogen. Do not overfeed. Apply nitrogen in split applications during the first half of the growing season.

3. Choose the right variety. Opt for crack‑resistant varieties, especially in regions with unstable climates.

4. Correct plant spacing. Avoid dense sowing; thin in time (if not a self‑thinning type).

5. Cool the soil. Use mulch to lower soil temperature during hot periods.

6. Careful harvest. Protect roots from mechanical damage.

What to Do If Cracks Have Already Appeared?

If you notice cracks on growing carrots, it is a signal to review your watering regime. Try to smooth out moisture fluctuations immediately. Damaged roots should be used first – they are not suitable for long‑term storage. You can process them (juice, freezing, preserves) or use fresh in salads. Other roots, if damage is minor, may store for a short time – check them regularly during storage and remove any that start to rot.

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What Is “Hairiness” and Why Does It Occur?

Normally, most of the fine absorbing rootlets of carrot die off as the main root thickens – they remain only on the lower part of the root or are absent altogether in mature roots (Swiader & Ware, 1992). However, under certain conditions, the plant begins to actively grow new lateral roots over the entire surface of the root. This is a protective response to stress: the root “tries” to increase its absorbing area for water and nutrients when something impedes normal function.

There are three main causes (and they often act together):

Additional Factor: Varietal Characteristics

Some carrot varieties are more prone to forming lateral roots, especially older varieties with rough, porous skin (e.g., some Nantes types when they overgrow). Modern hybrids generally have smoother surfaces and are less “hairy” under optimal conditions (Simon et al., 2008).

Practical take‑away: if hairiness appears regularly, try changing to a more modern hybrid with smooth skin and good stress tolerance.

How to Distinguish Hairiness from Normal Roots?

Normally, young carrots (up to mid‑season) may have fine rootlets along their length – that is normal. However, by harvest time they should remain only at the very bottom of the root (the tip), while the main surface should be smooth. If rootlets densely cover the entire root and are white or slightly yellowish – that is a disorder.

What to Do If Hairiness Has Already Appeared?

If you notice roots becoming hairy, try adjusting your cultivation practices:

• Reduce watering – let the soil dry to a depth of 5–7 cm between waterings to encourage the death of excess lateral rootlets.
• Apply potassium and phosphorus fertiliser (e.g., wood ash infusion – a glass per bucket of water, or potassium monophosphate according to instructions) – this strengthens tissues and reduces the formation of new roots.
• Carefully loosen between rows – this improves aeration of the root zone.

If hairiness is severe, such roots cannot be stored long – they lose moisture and wilt quickly. Use them first or process them (juices, freezing, grated carrot). Next season, definitely review your fertilisation scheme and watering regime, and try a more resistant variety.

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Why Does the Crown Turn Green?

Carrots accumulate the green pigment – chlorophyll – when sunlight hits the root tissues. Normally, the root is entirely in the soil and protected from light. But if the upper part of the root (the shoulders) is exposed above the surface, it begins to photosynthesise like leaves and takes on a characteristic green tinge (Villeneuve & Geoffriau, 2021). Sometimes greening occurs not externally but internally – as a green stripe in the centre of the root or near the cambium. This happens when strong winds or rain expose the root and then it is covered again – green areas remain inside (Villeneuve & Geoffriau, 2021).

Why Is Greening Harmful?

Besides an unsightly appearance, green tissues contain solanine (like green potatoes). In carrots, the concentration is usually low, but it imparts a bitter taste. Consuming such roots in large quantities is not recommended, though if you remove the green layer (2–3 mm thick), the remaining part is quite edible (Villeneuve & Geoffriau, 2021). However, during storage, green areas are the first to rot, so green carrots should be processed or discarded promptly.

How to Prevent Greening?

Prevention is simple – keep the root covered with soil at all times.

What to Do with Greened Carrots?

• Cut off the green part. Scrape off a 2–3 mm layer with a knife or peeler – underneath is usually normal orange tissue.
• Check for bitterness. If the green has penetrated deeply, the root may be bitter – taste it.
• Use first. Greened carrots do not store – process them (soup, juice, freezing) or eat fresh within a few days.
• Do not keep for seed. If it is a seed root, greening does not affect seed quality, but such roots are best stored separately.

When Is Greening Not a Problem?

If the green appears only on the very tip (1–2 mm) and has not penetrated deeper, it is acceptable for personal consumption, but not for sale or long storage. Also note that some varieties naturally have light‑yellow or whitish shoulders, which is normal and not related to greening.

Summary: greening is easily avoided by timely earthing up, choosing the right sowing depth, and using mulch. It is one of the simplest disorders to prevent – just spend 5–10 minutes in mid‑season.

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Why Do Carrots Turn Bitter?

Bitterness in carrots arises from the accumulation of terpenes and other secondary metabolites (phenolic compounds). Normally their content is low, but under stress the plant starts actively synthesising these substances as a defence response (Villeneuve & Geoffriau, 2021). Additionally, bitterness may be linked to the formation of solanine during greening (see Chapter 5) or to ethylene exposure during storage alongside other vegetables (e.g., apples or pears) (Swiader & Ware, 1992). The main causes of bitterness are:

Why Does Carrot Become Woody (Tough, Fibrous)?

Woodiness is lignification of cell walls, accumulation of lignin, loss of juiciness and elasticity. The root becomes tough, dry, and feels like wood when cut. Main causes:

How to Tell If Carrots Will Become Tough and Bitter?

There are several indirect signs:

• Cracks on the root – often accompanied by bitterness.
• Pale or yellowish colour – sign of nutrient deficiency or ageing.
• Green shoulders – bitterness is guaranteed (see Chapter 5).
• Softness, limpness when pressed – the root is beginning to lose moisture and will soon become tough.

What to Do If Carrots Are Bitter or Have Become Tough?

• Check storage conditions. If carrots were stored near apples – separate them immediately. Bitterness may subside within 2–3 days after isolation from ethylene.
• Soak in cold water. Slice bitter carrots and soak for 1–2 hours – part of the bitterness will leach into the water, especially if caused by terpenes.
• Use for processing. Grated carrots can be stewed, added to soups, juices – heat treatment reduces bitterness partially.
• Do not store. Tough and bitter carrots only worsen during storage – consume them first.

Prevention: How to Grow Sweet and Juicy Carrots?

1. Choose the right variety. Select varieties suited to your region and soil type. Early varieties for quick consumption; mid‑season and late for storage.

2. Sowing and harvest timing. Sow at optimal times, harvest on time, not allowing roots to overgrow.

3. Stable watering. Regular, moderate watering is the basis of succulence. Avoid prolonged droughts.

4. Balanced nutrition. Apply nitrogen in the first half of the season, potassium and phosphorus in the second.

5. Loose soil. Deep digging, sand, compost, mulch – all improve soil structure and promote a tender, smooth root.

6. Storage. Store carrots separately from ethylene‑producing fruits (apples, pears). Maintain temperature around 0…+2 °C and humidity 90‑95 %.

Summary: bitterness and woodiness result from ageing, drought, nutritional imbalance, or improper storage. Preventing these problems is much easier than fixing them – just follow good agronomic practices and do not delay harvest.

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What Are Internal Defects?

• Hollows (cavities) – large empty spaces inside the root, often in the centre (in the woody area) or between rings.
• Loose, spongy core – the central part becomes soft, porous, loses firmness.
• Internal cracks – tissue tears not visible from the outside but revealed when cut.
• Dark spots and necrosis – areas of dead tissue (often due to boron or other micronutrient deficiencies).

Why Do Hollows and Defects Occur?

Causes can be grouped into three: nutrient deficiency (especially boron), uneven growth (stress spikes), and tissue ageing.

How to Detect Defects Before Harvest?

Unfortunately, hollows and internal defects are almost impossible to detect externally – the root may look perfectly smooth. However, there are indirect signs:

• Yellowing or premature wilting of tops – may indicate disruption of the root vascular system.
• Transverse cracks on the shoulders – sometimes accompany internal cavities.
• Overly large roots at late harvest – often have a loose core.

To check, select a few of the largest roots, cut them lengthwise, and assess the internal tissue condition. If there are widespread defects – harvest the entire crop as soon as possible, not for storage.

What to Do with Carrots That Have Hollows?

• Use first. Such carrots do not store – process them (juices, freezing, preserves, soups) or eat fresh within a few days.
• For processing. Hollows do not interfere with boiling, stewing, or juice making – just slice and remove obviously spoiled parts.
• Do not store long. Even if hollows are small, they enlarge during storage, and the root rots quickly.
• Use for animal feed. If there are many hollows and you cannot process them in time, carrots can be given to livestock (rabbits, goats, pigs), after ensuring no rot.

Prevention: How to Grow Carrots Without Hollows and Defects?

1. Boron – timely and in moderation. Apply boron (foliar or soil) 2–3 weeks before root formation. Do not overdo – excess boron is toxic.

2. Balanced nutrition. Do not overdo nitrogen, especially in the second half of the season. Favour potassium‑phosphorus fertilisers.

3. Stable growth. Water regularly, avoiding moisture fluctuations. Mulch.

4. Timely harvest. Do not keep carrots in the ground too long – follow recommended variety dates.

5. Loose soil. Deep cultivation, sand, compost – so roots can grow downward easily without deformation.

6. Choose the right variety. Some modern hybrids have denser internal structure and are resistant to hollow formation. Look for variety descriptions mentioning “good storage ability” and “crack resistance.”

Summary: hollows and internal defects result from boron deficiency, uneven growth, overgrowth, or nutritional imbalance. This is one of the most insidious problems because it cannot be seen before harvest. However, following simple rules (boron fertilisation, stable watering, balanced nutrition, timely harvest) allows you to almost completely avoid these troubles.