Tomato Physiological Disorders

How Is This Different from a Disease?

Visually, symptoms can be very similar. For example, leaf chlorosis (yellowing) can be caused by both nitrogen deficiency and viral infection. The key difference is that when the stress factor is removed, the physiological disorder gradually disappears, whereas the infection continues to develop (Blancard, 2012).

Main Groups of Causes

1. Water stress — lack or excess of moisture

2. Temperature stress — heat, cold, sharp fluctuations

3. Light stress — lack or excess of light

4. Nutritional disorders — deficiency or excess of elements

5. Mechanical damage — injury to roots, stems, fruits

How to Tell It's a Physiological Disorder?

Here are several key indicators:

• Check the roots. If the root system is healthy (white or light brown roots, no rot or galls), the problem is most likely non-infectious (Blancard, 2012).

• Assess the spread. If the disorder appears simultaneously on many plants and has a clear connection to a specific area of the field, greenhouse, or weather event—this is a characteristic sign of a non-infectious nature (Akhatov, 2010).

• Observe the dynamics. If symptoms begin to disappear after adjusting care (watering, feeding, shading)—you're on the right track.

• Rule out pests and pathogens. Careful inspection of leaves on both sides, stems, and fruits will help distinguish, for example, spider mites from physiological leaf curling.

Remember the main point: physiological disorders are the plant's signals that its living conditions have moved beyond its comfort zone. By learning to "read" these signals, you can quickly adjust care and preserve your harvest.

In the following chapters, we'll examine each type of stress in detail and learn how to properly respond to the signals the plant sends.

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How to Tell a Plant Is Suffering from Water Deficiency?

When moisture is scarce, the tomato activates survival mechanisms that are immediately noticeable:

• Leaves lose turgor and wilt, especially during hot daytime hours. The plant may recover at night—this is an early sign.
• Lower leaves turn yellow and dry up—the plant sacrifices old leaves to conserve water for young ones and fruits.
• Leaf edges curl upward—this reduces the evaporating surface.
• Flowers and ovaries drop—with moisture deficiency, the plant cannot support them nutritionally and "sheds ballast."
• Fruits become smaller and ripen prematurely, becoming less juicy.
• With prolonged drought, stems may crack—this results from uneven water supply after a dry period (Akhatov, 2010).

Why this happens: Tomato roots absorb water and minerals from the soil solution. When moisture is lacking, osmotic pressure drops, and roots cannot effectively absorb water. Additionally, calcium transport to fruits suffers, which provokes blossom-end rot (more on this in Chapter 6).

What Happens with Excess Moisture?

Overwatering is no less dangerous. Tomatoes cannot tolerate water stagnation at the roots.

• Lower leaves yellow and wilt, even though the soil is moist—this is a paradoxical symptom of root damage.
• Leaves may take on a dark green hue and become brittle.
• Watery blisters appear on stems and leaves (edema) —this results from the plant absorbing more water than it can transpire, and cells "burst" (Blancard, 2012).
• Roots darken, rot, and lose root hairs—their absorptive capacity decreases.
• Plants become looser and more susceptible to fungal infections—especially gray mold and late blight.
• Fruits may crack—with sharp humidity fluctuations (after drought—heavy watering).

The tomato root system needs oxygen (Akhatov, 2010). Excess water displaces air from the soil, roots suffocate, and normal nutrient absorption ceases. The plant "starves" even in wet soil.

How to Find the "Golden Mean"? Practical Recommendations

1. Consider the plant's development stage

(Akhatov, 2010)

2. Water correctly

• In open ground—water deeply but infrequently so water penetrates deep (40–50 cm). Shallow frequent watering encourages root development in the upper layer, where they dry out faster.
• In greenhouses—drip irrigation gives the best results: water is supplied in measured doses directly to the root zone. This prevents both overwatering and drying out.
• Watering time—morning or early evening. On hot sunny days, watering with cold water causes stress (shock). In greenhouses, finish watering 1–2 hours before sunset to avoid nighttime moisture stagnation and condensation (Akhatov, 2010).
• Water temperature—not below 18–20°C. Cold water (from wells, boreholes) inhibits roots.

3. Consider weather and soil type

• In hot sunny weather, water consumption rises sharply—increase watering frequency.
• On cloudy and cool days, reduce watering.
• On light sandy soils, water drains faster—water more frequently but in smaller volumes.
• On heavy clay soils—less often but more abundantly, so water saturates the entire root zone without stagnating.

4. Mulching is your ally

A layer of mulch (straw, rotted sawdust, black film) significantly reduces evaporation, minimizes moisture fluctuations, and protects roots from overheating. This is especially relevant for hot climate regions.

5. Learn to "read" the plant

• If leaves are wilted in the morning but recover by evening—water slightly earlier or increase the volume.
• If leaves are constantly wilted even though the soil is moist—check roots for rot or damage (e.g., nematodes or root rots) (Blancard, 2012).

What to Do If Stress Has Already Appeared?

For drying out:

• Water plants gradually, preferably in several passes, so water has time to soak in.
• Thoroughly moisten the soil, then mulch.
• In greenhouses, increase air humidity (spraying pathways, turning on fogging)—this reduces the load on leaves.

For overwatering:

• Stop watering until the topsoil layer dries.
• Loosen the soil around plants to improve aeration.
• In greenhouses, increase ventilation.
• If roots are severely damaged, you can lightly hill the stem with moist soil to stimulate the formation of adventitious roots (Akhatov, 2010).

Water Stress and Other Problems

It's important to remember: water regime disruption often provokes or worsens other physiological disorders:

• Blossom-end rot — results from poor calcium supply due to moisture fluctuations.
• Fruit cracking — a sharp transition from drought to heavy watering.
• Leaf chlorosis — often associated with the roots' inability to absorb elements during overwatering.

Main takeaway: regular but moderate watering, considering weather conditions and the plant's development stage, is the foundation of tomato health. Watch your plants closely, and they will tell you when and how much water they need.

In the next chapter, we'll examine how temperature—both heat and cold—affects tomato condition and how to protect plants from temperature stress.

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Why Is Tomato So Sensitive to Temperature?

The optimal range for growth and fruiting is 18 to 27°C during the day and 15–18°C at night (Akhatov, 2010). When temperatures exceed these limits, key processes are disrupted:

Tomato is a self-pollinating crop, but for successful pollination, temperature matters: pollen matures and germinates optimally in the range of 15–32°C. At higher or lower values, fruit set drops sharply (Wien & Stützel, 2020).

Signs of Cold Stress

Tomato suffers from cold even at positive temperatures. Below +12°C, growth stops; at +10°C, flowering ceases; and during frost, the plant dies (Akhatov, 2010).

Main symptoms:

• Leaves become dark green with a purple tint (anthocyanosis) —a defensive reaction: the plant accumulates anthocyanins to better absorb red light and warm itself (Akhatov, 2010).
• Curling, thickening, and deformation of young leaves —during prolonged cooling during meristem growth.
• Slowed growth and development —the plant seems to "freeze."
• Flower and ovary drop —when pollination occurs below 13–15°C, pollen doesn't mature or becomes sterile.
• "Silvering" (white tip) —silvery or light areas on leaves, especially in varieties lacking genetic resistance (Atherton & Rudich, 1986). Appears during sudden cooling during active growth.
• Watery necrotic spots on leaves after frost —when thawing, damaged tissues become transparent, then darken.

Why this happens: Cooling reduces membrane fluidity, slows water and nutrient transport, and disrupts enzyme function. Young growing tissues are especially affected—they contain more water and are less resistant.

Signs of Heat Stress

Daytime temperatures above 30°C are already an alarm signal. At 35°C, pollen dies, growth nearly stops, and at 40°C, the plant can suffer irreversible damage.

Main symptoms:

• Leaves become smaller, stems thinner, internodes lengthen —the plant tries to cool itself by increasing the evaporative surface.
• Leaf curling upward —reduces the evaporation area, protects from overheating (Akhatov, 2010).
• Flower and ovary drop —the main cause is pollen sterility. Lycopene (red pigment) is not synthesized above 30°C, fruits remain orange-yellow (Atherton & Rudich, 1986).
• Sunburn on fruits —discolored, depressed areas on the sun-exposed side.
• Fruit cracking —especially when heat is combined with sudden watering.
• Wilting even with moist soil —roots cannot supply water fast enough; evaporation exceeds absorption.

Why this happens: At high temperatures, respiration increases—the plant spends energy on cooling rather than growth. Photosynthesis is inhibited. Additionally, proteins denature, and cell membrane structure is disrupted.

Temperature Regime Features in Greenhouses and Open Ground

Open ground:

• Daily fluctuations can reach 20°C or more.
• Plants harden, so they are more resistant to short-term fluctuations.
• Strong heat combined with dry air is most dangerous.
• Spring and autumn frosts are the main threat.

Greenhouses:

• Temperatures can rise to 40°C and above with insufficient ventilation.
• At night, sharp cooling is possible.
• Greenhouse plants are more delicate and tolerate cold less well.
• It's important to maintain a ratio: daytime temperature 2–3°C higher than nighttime (Akhatov, 2010).

Connection to light levels: On sunny days, higher temperatures are acceptable (up to 25°C) than on cloudy days (no higher than 22°C). This allows the plant to use light effectively for photosynthesis without overheating.

How to Protect Tomatoes from Temperature Stress

From cold:

• Choose cold-resistant varieties and hybrids —many regionalized varieties are resistant to short-term cold spells.
• Harden seedlings before transplanting —gradually acclimate them to lower temperatures over 5–7 days.
• In open ground, use temporary covers —non-woven material, film, plastic bottle caps.
• In greenhouses, monitor nighttime temperature: don't let it drop below 16–17°C during fruiting.
• When frost threatens—water —moist soil releases heat more slowly, and sprinkling (fine spraying) at night protects from freezing (Akhatov, 2010). However, watering is only done at positive temperatures.
• Use anti-stress preparations (Megafol, Epin-Extra) —they help plants tolerate cold more easily by stimulating protective mechanisms.

From heat:

• Ensure ventilation —in greenhouses, open vents and doors, create cross-ventilation (tomatoes are not afraid of drafts).
• Provide shade —in open ground, use nets, shields; in greenhouses—whitewashing roofs, shade nets, internal screens (Akhatov, 2010).
• Increase watering, but only in the morning —so moisture has time to soak in and doesn't provoke root overheating.
• Spray pathways in the greenhouse—this increases air humidity and lowers temperature through evaporation.
• Don't remove excess leaves in hot weather —they protect fruits and soil from overheating.

Universal tips:

• Maintain optimal soil temperature (18–20°C)—cold soil slows root growth even when air temperature is normal.
• In hot regions, prefer heat-resistant varieties with a powerful root system and dense leaf cover.
• Timely potassium and calcium feeding increases resistance to temperature stress.

How to Distinguish Temperature Stress from Disease?

The main rule: maintain temperature within the comfortable range for tomatoes. If weather goes beyond these limits—respond quickly: shade, ventilate, cover, water. The plant will surely respond to care with a healthy appearance and abundant harvest.

In the next chapter, we'll examine how light—its quantity, quality, and duration—affects tomato growth and development, and how to avoid problems related to lighting.

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Why Does Tomato Need Light?

Photosynthesis occurs in tomato leaves—the conversion of carbon dioxide and water into organic substances under the influence of light. The more light (within reasonable limits), the more active photosynthesis, the more nutrition the plant receives for growth, flowering, and fruit filling.

However, three characteristics of light matter:

• Intensity (illuminance) —the amount of light falling on the leaf.
• Photoperiod —day length.
• Spectral composition —the ratio of rays of different wavelengths (red, blue, far-red, etc.).

Tomato can grow with different day lengths, but 12–16 hours of light per day is considered optimal. With less than 10–11 hours of daylight and low intensity, flowering and fruiting are significantly delayed (Wien & Stützel, 2020).

Signs of Light Deficiency

If a tomato receives too little light, it tries with all its might to "reach" the source—this is called etiolation. Main symptoms:

• Stems stretch, become thin and fragile, internodes lengthen.
• Leaves become smaller and paler, turning light green or yellowish—chlorophyll breaks down as the plant uses its reserves for survival (Akhatov, 2010).
• Leaves may be directed upward —toward the light source.
• Flowering is delayed or doesn't occur —the plant lacks energy to form generative organs.
• Flowers and ovaries drop —especially on lower inflorescences.
• Fruits ripen slowly and unevenly, often remaining pale-colored.

Why this happens: With insufficient light, photosynthesis doesn't meet the plant's carbohydrate needs. The tomato is forced to use reserve substances (starch, proteins, even chlorophyll) to sustain life. Flowering and fruiting are the most energy-intensive processes, so the plant postpones them (Akhatov, 2010; Heuvelink, 2018).

Light deficiency is especially critical during:

• Seedling growing (especially in northern regions in February–March).
• Dense plantings—when plants shade each other.
• Cloudy and rainy weather lasting several weeks.
• In greenhouses with dirty or fogged coverings.

Signs of Light Excess

You might think more light is good. But in southern regions and on hot sunny days, excess light can be harmful:

• Leaves become smaller and lighter —the plant protects itself from overheating and photooxidation.
• Internodes shorten, stems become thicker and stiffer.
• Leaf edges curl upward —reduces the area for evaporation and light absorption.
• Discolored areas appear on fruits (sunburn) —tissues are destroyed by intense ultraviolet radiation.
• Pollination is disrupted —in combination with high temperature, pollen becomes sterile (Wien & Stützel, 2020).

Unlike light deficiency, excess is more common in open ground in southern regions and in poorly shaded greenhouses.

Spectral Composition and Photoperiod

Tomato is day-neutral, meaning it flowers at any day length. However, there are nuances:

• With short days (less than 10–12 hours) and low light, flowering is delayed.
• With long days (more than 16 hours), vegetative growth may dominate over generative growth—the plant "luxuriates" but produces few fruits.
• Optimal photoperiod for tomato is 12–14 hours (Akhatov, 2010).

Spectral composition also matters. Red light stimulates flowering and fruit set; blue light promotes compact growth and leaf development. In greenhouses, LED lamps with predominant desired spectra can be used for supplemental lighting (Heuvelink, 2018).

Practical Recommendations: How to Manage Light Regime

Regional Features

How to Distinguish Light Stress from Diseases and Other Disorders?

Practical Takeaway

• Light for tomatoes is fuel. Without it, there's no photosynthesis, no harvest.
• The main task is to find balance: don't let plants stretch in shade, and don't allow burns in the sun.
• Watch the leaves: they are the main indicator of light comfort. Pale, stretched, with long internodes—they need more light. Stiff, small, with curled edges—time to shade.
• In greenhouses, light regime can be regulated: clean coverings, use reflectors, install lamps. In open ground, rely more on proper variety selection and planting location.

Golden rule: the more light—the higher the temperature should be (but not above 27°C), and vice versa: on cloudy days, lower the temperature so the plant doesn't waste energy on useless respiration.

In the next chapter, we'll examine nutritional disorders—how to tell which elements tomatoes lack and what to do about fertilizer excess.

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How to Tell the Problem Is Nutritional?

The plant receives 14 essential elements from soil or substrate, each playing its role. Deficiency or excess of any manifests with characteristic symptoms:

• Old leaves suffer first —when mobile elements (nitrogen, phosphorus, potassium, magnesium, zinc) are lacking, the plant moves them to young growing parts.
• Young leaves and growing points suffer first —when immobile elements (calcium, boron, iron, copper, manganese, sulfur) are lacking, as they cannot be redistributed (Akhatov, 2010).

Important: deficiency symptoms often arise not from the actual absence of an element in the soil but from:

• Inaccessibility (wrong pH, cold soil, overwatering).
• Antagonism (excess of one element blocks uptake of another).
• Root damage (rot, nematodes, soil compaction).

Always assess the root system and growing conditions before "treating" with fertilizers.

How to Quickly Identify What's Missing—Symptom Table

Visual diagnosis is the first step. Use the following table (Akhatov, 2010; Hochmuth & Sideman, 2023):

Detailed Look at Major Elements

Excess of Elements—Equally Dangerous

Salt excess (soil or substrate salinization) manifests as:

• Marginal scorch on old leaves.
• Leaves curling upward, tissue darkening.
• Slowed growth, ovary drop.
• With high chloride or sodium content—chlorosis and necrosis (Akhatov, 2010).

Causes of salinization: frequent fertilizing with high fertilizer content, watering with hard water, insufficient drainage.

What to do: flushing soil with large amounts of water (especially on light soils) washes out excess salts. In greenhouses with drip irrigation, increase drainage volume (30–40% of supplied solution).

How to Properly Diagnose and Fertilize

1. Start with visual inspection:

• Which leaves are affected (old or young)?
• What type of damage (interveinal, marginal, general chlorosis, necrosis)?
• Are there symptoms on fruits?

2. Check soil or substrate pH:

• Optimum for tomato is 6.0–6.8. At pH below 5.5 and above 7.5, many elements become unavailable (Hochmuth & Sideman, 2023).

3. Assess the root system:

• If roots are damaged (rot, nematodes), even with adequate nutrition, elements won't be absorbed. In this case, treat roots first.

4. Analyze fertilization history:

• What fertilizers and in what doses were applied?
• Was there overfeeding?

5. Apply foliar feeding for quick correction of micronutrient deficiencies—they work within 1–2 days.

6. In greenhouses with substrates (mineral wool, peat, coconut) —regularly analyze drainage solution for macro- and micronutrients, adjust nutrient solution composition.

7. Use leaf diagnostics (element content analysis in leaves)—the most accurate method. Recommended ranges for tomato:

• Nitrogen: 4.5–5.0%
• Phosphorus: 0.5–1.0%
• Potassium: 4.0–5.5%
• Calcium: 2.2–4.8%
• Magnesium: 0.5–1.05%
• ochmuth & Sideman, 2023)

Table: How to Distinguish Deficiency of One Element from Another

Practical Takeaway

• The plant is the best indicator of its nutrition. Regularly inspect leaves, especially lower and young ones.
• Don't rush with fertilizers —first rule out other causes (pH, watering, roots).
• Maintain balance: overfeeding is as dangerous as underfeeding.
• During fruiting —increase potassium share, reduce nitrogen share.
• Foliar feeding with micronutrients is a quick and effective way to correct deficiencies.

In the next chapter, we'll examine in detail one of the most common and frustrating disorders—blossom-end rot of fruits, its causes, prevention, and treatment.

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What Is Blossom-End Rot?

Blossom-end rot (scientifically—apical necrosis of fruits) is a physiological disorder caused by local calcium deficiency in fruit tissues, especially in the distal (blossom) end. Calcium is an element that doesn't move from old tissues to young ones, so its delivery to fruits depends on continuous water flow through vessels (Akhatov, 2010).

The disorder appears as tissue necrosis (death) at the fruit tip, usually during active fruit growth—from the 14th to 21st day after flowering (Akhatov, 2010).

Important: blossom-end rot is NOT an infectious disease. It doesn't spread from plant to plant, but in wet weather, secondary pathogens (e.g., gray mold or head mold) can colonize affected tissues, which then become infectious.

What Does Blossom-End Rot Look Like? (Symptoms)

• On green fruits: a small watery spot appears at the tip (on the blossom scar side), initially light green or whitish. Gradually it enlarges, becoming dark brown or black, sunken, dry or leathery (Blancard, 2012).
• In advanced cases: necrosis can cover up to half the fruit. Tissue becomes hard and shriveled.
• In moist conditions: mold growth (Penicillium, Botrytis, Erwinia) may appear on the necrotic surface—this is already secondary infection (Akhatov, 2010).
• Internal symptoms: when cut, the fruit shows browning and necrosis of tissues in the blossom end, sometimes blackening of seeds and placenta.

Important to distinguish from herbicide damage, hail, or sunscald—with blossom-end rot, necrosis is always localized to the blossom scar area, not on the sides or near the peduncle (Blancard, 2012).

Why Does Blossom-End Rot Occur? (Causes)

The main cause is imbalance between calcium supply and fruit growth rate. Calcium enters the fruit only with water (through xylem) and only while the fruit is growing. As growth slows, water flow decreases, and calcium stops reaching the tip (Wien & Stützel, 2020).

What factors trigger this disorder?

Critical level: if fruit calcium content (on dry matter) falls below 0.08%—blossom-end rot is almost guaranteed. Above 0.12%, it practically doesn't occur (Atherton & Rudich, 1986).

Preventing Blossom-End Rot—The Main Strategy

Treating already affected fruits is useless—they will remain defective. But you can and should prevent new ones.

What to Do If Blossom-End Rot Has Already Appeared?

1. Remove affected fruits. They won't recover and will only drain the plant's energy. Burn them or bury them away from beds to avoid attracting secondary infection.

2. Immediately normalize watering. If there was drought—water abundantly but gradually. If overwatering—let soil dry out.

3. Apply foliar calcium feeding (0.5% calcium nitrate solution). Repeat after 3–5 days.

4. Adjust fertilizing: temporarily exclude ammonium fertilizers, reduce potassium dose.

5. Reduce plant load —remove some ovaries so remaining ones get more calcium.

6. Mulch the soil —this stabilizes root zone moisture and temperature.

7. In the greenhouse —increase ventilation or, conversely, apply shading depending on the situation.

How to Distinguish Blossom-End Rot from Similar Problems?

(Blancard, 2012; Akhatov, 2010)

Why Is Calcium So Important?

Calcium is a structural element of cell walls and membranes. It provides tissue strength and elasticity. When deficient, cells lose stability, begin to break down, and necrosis occurs (Jones, 2008).

Interestingly: even if there's enough calcium in the soil, it may not reach the fruit due to water regime disruption. Calcium moves through the plant only with water, through xylem, and only in the direction of transpiration flow. Fruits transpire little water, so calcium reaches them on a "residual basis"—after leaves and growing points have been supplied (Wien & Stützel, 2020).

Practical Takeaway

• Blossom-end rot is a signal of imbalance between water and calcium supply and fruit growth rate.
• Prevention is 90% of success. Regular watering, balanced feeding, and root care are the best protection.
• Large-fruited varieties and beefsteak tomatoes are at risk—give them special attention.
• At first signs —quickly adjust watering, apply foliar calcium feeding, and remove affected fruits.

In the next chapter, we'll examine another common problem—fruit cracking, learn why it occurs, how to prevent it, and what to do if cracks have already appeared.

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What Is Fruit Cracking?

Cracking is a physiological disorder caused by imbalance between fruit growth and skin elasticity (skin + cuticle). When internal fruit tissues grow faster than the skin can stretch, it splits. Cracks can be of different types and appear on both green and ripening fruits (Akhatov, 2010; Blancard, 2012).

Types of Cracks

(Akhatov, 2010; Blancard, 2012)

Why Do Fruits Crack? (Main Causes)

The main cause is a sharp change in water regime, causing abrupt fruit growth that the skin can't keep up with.

Mechanism: During drought, fruit growth pauses, and skin becomes less elastic. After heavy watering or rain, water suddenly enters the fruit, cells swell, internal pressure rises, and the skin splits (Blancard, 2012).

Preventing Cracking—The Best Protection

Cracks are easier to prevent than to treat. Main strategies focus on stabilizing water regime and strengthening skin.

What to Do If Cracks Have Already Appeared?

1. Immediately stabilize watering. If there was drought—water gradually, don't flood. If overwatering—let soil dry out.

2. Remove cracked fruits. They won't recover and will spoil, potentially becoming a source of secondary infections (gray mold, alternaria, bacterial rots).

3. Apply foliar potassium feeding (potassium nitrate 0.5–1%)—this will strengthen the skin of remaining fruits.

4. In greenhouses —increase ventilation, reduce air humidity.

5. Mulch the soil if you haven't already.

6. For widespread cracking in open ground, consider installing film cover during rainy periods.

Which Varieties Are More Prone to Cracking?

(Akhatov, 2010; Blancard, 2012)

How to Distinguish Cracking from Other Damage?

(Blancard, 2012; Akhatov, 2010)

Practical Takeaway

• Cracking is a consequence of sharp water regime fluctuations. The main enemy is alternating drought and heavy watering.
• Prevention —regular watering, mulching, balanced feeding (especially potassium and calcium).
• For greenhouses —monitor nutrient solution EC and don't allow sharp drops.
• Choose resistant varieties, especially if you're in an area with unpredictable rainfall.
• Remove cracked fruits immediately —they not only lose marketable quality but are also entry points for infection.

In the next chapter, we'll examine sunscald —how to protect fruits and leaves from overheating and UV radiation to preserve harvest and quality.

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What Is Sunscald in Tomatoes?

Sunscald is thermal tissue damage caused by direct sunlight exposure when the surface temperature of the fruit or leaf exceeds tolerable limits. Unlike many other disorders, sunscald is irreversible cell damage that does not heal (Blancard, 2012).

Tomato is especially vulnerable during fruit ripening. Green fruits are more sensitive to burns than ripe ones, as they contain more water and have more delicate skin. At temperatures above 30–35°C on the fruit surface, protein denaturation and cell membrane destruction begin (Akhatov, 2010).

How Does Sunscald Manifest? (Symptoms)

On fruits:

• Discolored, yellowish or white patches appear on the side facing the sun.
• Patches become sunken and dry, the skin acquires a papery texture (Blancard, 2012).
• In mild cases—only yellowing (sun-induced yellowing) without necrosis, caused by chlorophyll destruction and disruption of lycopene (red pigment) synthesis at temperatures above 30°C (Atherton & Rudich, 1986).
• In severe burns, the patch turns white, becomes depressed, tissues die, and saprophytic fungi (e.g., Alternaria alternata) may colonize, causing black mold.

On leaves:

• Pale, discolored areas with clear boundaries appear, often between veins.
• Leaves become dry and brittle, edges may curl upward (protective response).
• With severe burns, leaves turn brown and dry up completely, especially young and most exposed ones.

On stems:

• Rarely, longitudinal pale areas with subsequent corking may occur, especially where the stem is not protected by leaves.

Why Does Sunscald Occur? (Causes)

The main cause is high solar radiation intensity combined with high temperature, when the fruit surface temperature reaches above 40°C (at air temperature 35–38°C and no shading). Under these conditions:

• Lycopene (red pigment) is not synthesized above 30°C—fruits remain yellow-orange even when ripe.
• Chlorophyll breaks down faster than it can be restored.
• Epidermis and mesophyll cells lose water, shrink, and die.

Additional risk factors:

• Poor foliage cover —natural (few leaves in the variety) or caused by diseases, aging, leaf removal.
• Sharp transition from cloudy to sunny weather —leaves and fruits don't have time to adapt.
• High air humidity —impairs cooling through evaporation.
• Leaf damage from diseases (late blight, alternaria, septoria) or pests—fruits lose natural protection.
• High temperature in greenhouses without effective ventilation and shading.

Preventing Sunscald

Protection from burns is a set of measures aimed at reducing light intensity and improving plant cooling.

What to Do When Burns Appear?

1. Immediately shade the plants (in greenhouse—activate shading; in field—install nets or shields).

2. Remove severely damaged fruits —they won't recover, will spoil, and may become a source of infection for neighboring fruits.

3. Lightly spray plants (not with cold water!)—this will reduce leaf and fruit temperature, but don't allow water stagnation on fruits to avoid fungal diseases.

4. Apply foliar potassium feeding (potassium nitrate 0.5–1%)—potassium strengthens cell walls and increases stress tolerance.

5. Increase ventilation in the greenhouse—reducing humidity and temperature will speed cooling.

6. With severe damage —harvest fruits early so they can ripen indoors, not exposed to further sunlight.

How to Distinguish Sunscald from Other Damage?

(Blancard, 2012; Akhatov, 2010)

Regional Characteristics

(Akhatov, 2010; Heuvelink, 2018)

Practical Takeaway

• Sunscald is thermal damage, not an infection. It can't be treated—only prevented.
• Main enemies: lack of shading, leaf removal, sudden weather changes, moisture deficiency.
• Prevention is key: shade plants during hot hours, maintain leaf cover, water on time.
• When burns appear —remove damaged fruits and adjust conditions (shading, watering, ventilation).
• Choose resistant varieties, especially if you're in a high-insolation zone.

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In the next chapter, we'll examine tissue edema (oedema) —a lesser-known but frustrating disorder that often occurs in greenhouses and with overwatering, and learn how to deal with it.

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What Is Edema (Oedema)?

Oedema (intumescence, oedema) is a physiological disorder related to water balance disruption in the plant. When roots absorb excess water and evaporation (transpiration) is limited (e.g., due to high humidity, low light, or low temperature), excess water accumulates in cells. Cells become overfilled, their walls can't hold, and swellings, blisters, and then necrotic spots appear on the surface (Blancard, 2012; Akhatov, 2010).

Edema is a non-infectious disorder, but damaged tissues become entry points for secondary infections (gray mold, bacterial diseases).

How Does Edema Manifest? (Symptoms)

On leaves (usually lower, older ones):

• Small, watery blisters (vesicles) appear on the underside along veins or across the blade (Blancard, 2012).
• Blisters may be transparent, yellowish, or greenish, resembling water droplets.
• With increased humidity or without changes, blisters burst, releasing cell sap, and in their place brown, corky (suberized) spots appear, resembling warts.
• The leaf blade deforms, curls, becomes bumpy (Akhatov, 2010).
• With severe damage, leaves yellow and drop.

On stems and petioles:

• Watery longitudinal streaks or blisters may appear, which then turn into corky growths or cracks.

On fruits:

• Edema on fruits is rare but possible—as watery spots that then darken and become corky.

Key sign: oedema always starts with watery, transparent swellings that then turn into brown, corky tissues—unlike necrosis from sunscald or infections.

Why Does Edema Occur? (Causes)

The main cause is imbalance between water uptake and its evaporation. The plant absorbs more water than it can transpire. This occurs with a combination of several factors:

Mechanism: Water entering leaves cannot evaporate through stomata (closed due to high humidity or darkness). It accumulates in intercellular spaces and within cells, creating excess turgor pressure. Epidermal and parenchyma cells rupture, forming blisters. After blisters burst, tissues suberize (cork) to protect damaged areas (Akhatov, 2010).

Preventing Edema—Managing Microclimate

Edema is a "disease of poor ventilation and excess moisture." Prevention aims to create conditions where transpiration exceeds water absorption.

What to Do When Edema Appears?

1. Immediately increase ventilation —open all vents, turn on fans, create drafts.

2. Stop watering until the topsoil layer is completely dry (3–5 cm deep). Even if the plant looks wilted, don't water—edema indicates overwatering.

3. Reduce air humidity in the greenhouse—turn on heating, open doors, use dehumidifiers.

4. Remove severely damaged leaves (with extensive necrosis)—they won't recover and will be sources of infection.

5. Apply foliar potassium feeding (0.5% potassium nitrate or potassium sulfate)—potassium improves water balance regulation and strengthens cell walls.

6. Check the root system —if there are signs of rotting (brown, soft roots), fungicide treatment may be needed (but this goes beyond physiology).

How to Distinguish Edema from Other Disorders?

(Blancard, 2012; Akhatov, 2010)

Special Considerations for Greenhouses and Open Ground

(Akhatov, 2010; Heuvelink, 2018)

Practical Takeaway

• Edema is a signal that the plant is "suffocating" from moisture. The main cause is not water in the soil but the inability to evaporate it (high humidity, lack of light, poor ventilation).
• There's no treatment —only prevention and condition adjustment.
• Main measures: ventilation, moderate watering, humidity control, adequate lighting.
• If edema appears —immediately improve ventilation, reduce watering, remove damaged leaves.
• In greenhouses this is especially relevant: that's where conditions for edema are most often created.

In the next, concluding chapter, we'll summarize everything we've learned and address the main question: how to distinguish a physiological disorder from a real disease, so you don't waste time and effort on incorrect "treatment."

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Why Is It So Important to Distinguish?

(Blancard, 2012; Akhatov, 2010)

Key Distinguishing Criteria—Quick Reference Table

Step-by-Step Diagnosis Algorithm in the Garden or Greenhouse

1. Start with a general inspection:

• Are all plants affected or only some?
• Is there a clear boundary between healthy and affected areas? (This is characteristic of infection.)
• Has there been any extreme weather in recent days (frost, heat, heavy rain, drought)?

2. Examine the roots:

• Dig up 2–3 plants with obvious symptoms and 2–3 healthy ones (for comparison).
• Carefully wash roots free of soil.
• Assess color, presence of galls, rot, spots (Blancard, 2012).
• If roots are healthy—the problem is most likely not a soil-borne infection.

3. Carefully study the leaf lesion pattern:

• Where are symptoms located—on old or young leaves? (See Chapter 5.)
• Is there fungal coating, spores, pycnidia, bacterial "tears"? (Use a 10–20× magnifier.)
• Are there clear spot boundaries, rings, "bird's eye," mosaic? (This indicates viruses or bacteria.)
• Are symptoms symmetrical on the leaf and across the plant? (Asymmetry—suspect vascular infection, virus, or bacterial wilt.)

4. Assess dynamics:

• How quickly are symptoms spreading?
• If over 3–5 days without care changes, symptoms progress sharply—likely infection.
• If with improved watering or feeding, symptoms on new leaves disappear—likely stress.

5. Use simple tests:

• Bacterial wilt test (Ralstonia solanacearum): place a cut stem in a glass of water. If a white cloudy stream emerges from the cut within 1–2 minutes—it's bacterial wilt (Blancard, 2012).
• Vascular browning test: cut the stem lengthwise. If vessels are dark brown, not just yellowed—suspect Fusarium or Verticillium wilt (Jones, 2008; Atherton & Rudich, 1986).

6. Recall recent agronomic activities:

• When were fertilizers applied? Was there an overdose?
• When and how were plants watered? Was there drying out or, conversely, water stagnation?
• Were pesticide treatments done? Was there phytotoxicity?

When to Consult a Specialist?

Sometimes even an experienced gardener can't pinpoint the cause. In such cases, it's better to contact a nearby agrochemical or phytopathological diagnostic center (at agricultural universities, plant protection stations, private laboratories). Do the following:

• Collect samples of affected organs (leaves, stems, fruits, roots) separately for each "type" of symptom (if different).
• Place them in clean paper bags (not plastic, to prevent rotting). If sending soil—take samples from the root zone (depth 15–25 cm) at several points.
• Record all observations (weather, care, timing, varieties) so the specialist can diagnose faster.

Laboratory methods (microscopy, PCR, ELISA, culture plating) allow precise pathogen identification (Blancard, 2012; Heuvelink, 2018). This saves time and money in the next season.

Examples of Complex Cases

Case 1: Yellow spots on lower leaves.

• Possible causes: magnesium, potassium, nitrogen deficiency, as well as late blight, alternaria, septoria.
• Algorithm: Look at spot shape (interveinal or marginal?), is there sporulation? Check roots. Recall feeding. If spots are interveinal with green veins—likely magnesium deficiency. If spots have brown centers with yellow halos—possibly fungal infection.

Case 2: Leaf curling upward.

• Possible causes: heat, drought, light excess, as well as viral diseases (e.g., TYLCV or other Begomovirus, see Blancard, 2012) or herbicide damage.
• Algorithm: If curling is widespread, started after heat, and leaves are pale—likely heat stress. If curling is accompanied by mosaic, yellowing veins, and individual plants are more severely affected—suspect virus. Check for herbicide drift from neighboring fields.

Case 3: Dark sunken spots on fruit tips.

• Cause: almost always blossom-end rot (see Chapter 6), unless there's obvious damage (hail) or diseases (anthracnose, late blight). However, if spots are not only on the tip but on the sides, accompanied by concentric rings—this could be alternaria or anthracnose (Blancard, 2012).

What to Do If the Diagnosis Is Unclear?

If you're not 100% sure, follow the principle of minimal intervention:

1. First eliminate stress: Check watering, pH, humidity, temperature, feeding—bring them to normal for 3–5 days.

2. If there's no improvement and symptoms continue spreading—conduct a trial treatment with a narrowly targeted product against the most likely pathogen (e.g., copper-based—against bacteria and some fungi, or a biological fungicide based on Trichoderma).

3. If that doesn't help—immediately send samples to the laboratory.

Never mix several strong pesticides "just in case"—this can worsen stress and kill beneficial insects.

Table: Key Signal Signs for Quick Orientation

(Blancard, 2012; Akhatov, 2010; Hochmuth & Sideman, 2023)

Final Words

Tomato is a living organism that constantly responds to environmental conditions. Most problems that gardeners mistake for diseases are actually the result of improper care. By learning to "read" the plant, you can timely adjust watering, nutrition, lighting, or temperature and avoid using chemicals where they're not needed.

But if you see that the problem doesn't disappear after normalizing conditions, don't hesitate to consult specialists. An accurate diagnosis is the key to effective treatment and harvest preservation.

The main rule: always start diagnosis by evaluating growing conditions, not by searching for a "miracle" product.

Article Summary

Now you're equipped with knowledge to confidently recognize and address tomato physiological disorders. Remember: a healthy plant is the result of harmony between variety genetics and growing conditions. Observe, analyze, adjust—and tomatoes will reward you with abundant, high-quality harvest.