Diagnostics

Step 1. Assessment of the overall condition of the planting

Before approaching an individual bush, evaluate the overall picture.

Uniformity: Has the problem affected all plants or only individual bushes? Is there a pattern (strips, patches)?

• Continuous damage often indicates weather stress (frost, hail), improper fertilisation, or a general nutrient deficiency.
• Focal damage (individual bushes or groups) is a sure sign of an infectious disease (fungal, bacterial, viral) or local pest damage (Kemble, 2022). Foci often appear at field edges or in places with poor drainage.

Step 2. Inspection of leaves (upper and lower layers)

Leaves are the most sensitive indicator of plant condition. They are the first to react to any changes.

Colour: Yellow, pale, dark green, or with a purple tint? Pay attention to which leaves the change begins with: lower (old) or upper (young).

• Yellowing of lower leaves often indicates nitrogen deficiency or natural ageing. Yellowing of upper leaves may indicate iron deficiency or a viral infection (De Jong et al., 2011; Welbaum, 2015).

Spots: What shape, colour, and border do they have? Dry spots with concentric rings are a sign of fungal diseases such as alternariosis or macrosporiosis. Wet, rapidly expanding spots on the underside of the leaf – late blight (Popkova et al., 1980).

Deformation: Are the leaves curled upward (“boat‑shaped”) or downward? Are the edges distorted?

• Curling upward with yellowing – could be potato leafroll virus (PLRV).
• Small, wrinkled leaves may indicate a viral infection (e.g., potato virus Y) or herbicide damage (De Jong et al., 2011).

Presence of coating: White, grey, or black coating on leaves – fungal sporulation (e.g., powdery mildew or grey mould).

Underside of leaf: Be sure to look under the leaf. There you can often see fungal spores, larvae, or adult insects (aphids, mites, Colorado potato beetle larvae).

Step 3. Inspection of stems

Condition: Are there cracks, spots, thickenings, or cankers?

Colour: Darkening and rotting of the stem base – sign of “blackleg” (bacterial disease) or rhizoctoniosis. Pink or orange coating on the stem indicates fusarium wilt (Popkova et al., 1980).

Cut: If the stem looks suspicious, make a cross‑section. A darkened vascular ring is a sign of bacterial wilts (ring rot) or verticillium wilt (Welbaum, 2015).

Step 4. Inspection of the root zone and roots

This step is often skipped – but it’s a mistake! Roots can tell you a lot.

Carefully dig up a bush and examine the roots and stolons.

Size and colour of roots: Roots should be white and firm. Darkened, rotting, or with nodules – a sign of fungal or bacterial diseases (Popkova et al., 1980).

Presence of growths: Swellings on roots and tubers may be caused by potato cancer or root‑knot nematodes (De Jong et al., 2011).

Step 5. Inspection of tubers

If possible, pull a few tubers from the ground, even if they are not yet mature. This will give the most complete picture.

External appearance: Cracks, growths, cankers, sunken spots (scab, late blight, rhizoctoniosis).

On a cut: The most important step! Cut the tuber.

• Rust‑brown rings or dots inside – internal rust spot or necrosis (physiological disorders, often related to heat and moisture shortage) (Dean, 1994).
• Net necrosis – symptom of potato leafroll virus or rattlevirus.
• Cavities in the centre of the tuber – hollow heart, due to uneven growth (Dean, 1994).
• Browning of the vascular ring – bacterial ring rot (Welbaum, 2015).

Step 6. Inspection of the soil

Moisture: Is the soil too dry or, conversely, waterlogged? Waterlogging leads to root suffocation and rot development, while drying causes nutrient deficiencies and tuber deformation.

Structure: Is there a compacted crust on the surface? This impedes root respiration and tuber growth.

Pests: Are there wireworm larvae, chafer grubs, or cutworm caterpillars in the soil? If tubers have small holes or gnawed tunnels – the culprits live in the soil (De Jong et al., 2011; Dean, 1994).

Step 7. Tools

For quality diagnostics you will need:

• Garden trowel or spade – for digging up roots and tubers.
• Magnifying glass (10‑20×) – for inspecting the underside of leaves for fungal spores, mites, aphids.
• Knife – for cutting tubers and stems.
• Notebook and camera – to record and photograph symptoms for comparison or consultation with a specialist.

Following this algorithm, you won’t miss important details and will be able to make the correct “diagnosis” for your potatoes. And from the next chapter, you will learn what specific symptoms mean.

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2.1. Yellowing of lower (old) leaves

This is the most favourable case, often not associated with contagious diseases.

Cause 1: Natural senescence. Towards the end of the growing season, when tubers are filling, lower leaves naturally yellow and die off. This is a normal process, as the plant moves nutrients from old foliage to young tubers. If this happens in mid‑ to late summer and does not affect the whole bush – there is nothing to worry about.

Cause 2: Nitrogen deficiency. Nitrogen is the element responsible for green mass growth. When it is lacking, the plant begins to sacrifice old leaves, moving nitrogen from them to young shoots and growing points.

• Symptoms: Leaves become uniformly light green, then yellow. The process starts with the lowest leaves, then moves upward. The leaves themselves do not curl, there are no spots, they simply become pale. The veins on lower leaves also turn yellow (Welbaum, 2015).
• What to do: Apply nitrogen fertiliser. Root feeding (e.g., manure infusion or urea) works best, or foliar spraying with a weak urea solution. This will quickly restore green colour.

Cause 3: Overwatering or soil compaction. When roots suffocate from lack of oxygen in waterlogged soil, they stop working effectively and cannot absorb nutrients, especially nitrogen. Symptoms are similar to nitrogen starvation, but on dry soil.

• What to do: Stop watering if excessive. If the soil is compacted – carefully loosen between rows to provide air access to the roots.

2.2. Yellowing of upper (young) leaves

This is a more serious alarm, as the most active, young parts of the plant are affected.

Cause 1: Iron deficiency (chlorosis). This often occurs on alkaline or over‑limed soils. Iron becomes unavailable to the plant, even though it may be present in sufficient quantity in the soil.

• Symptoms: Leaves turn yellow, but the veins remain bright green. This creates a characteristic “net‑like” or “marbled” pattern. Young leaves at the shoot tips are affected. The affected leaves do not curl or die, but the plant is severely stunted because it cannot photosynthesise properly (De Jong et al., 2011).
• What to do: Foliar feeding with iron chelate or a weak solution of ferrous sulfate. Important: applying iron to the soil is ineffective in this case; only foliar spraying works.

Cause 2: Viral infection. Many viruses (e.g., leafroll virus or some strains of potato virus Y) cause yellowing of upper leaves.

• Symptoms (key differences from iron deficiency): Yellowing is often uneven, spotted (mosaic) or ring‑shaped. Upper leaves may curl, become small, stiff, or “crinkled” (Caliskan et al., 2023). Often such bushes are stunted, and the problem appears not on one but on several bushes in groups (foci).
• What to do: Unfortunately, viral diseases cannot be cured. Infected bushes must be dug up and destroyed so they do not become a source of infection for neighbours (viruses are transmitted by aphids, Colorado potato beetles, and even through contact of leaves and tools). This is the main control measure.

2.3. Yellow or brown spots on leaves (leaf spots)

In this case, the leaf does not become yellow uniformly but is covered with spots that eventually turn brown and dry out.

Cause: Fungal diseases. Late blight, macrosporiosis (alternariosis), cercosporiosis – all start with spots on leaves.

• Late blight: Spots are brown, diffuse, often appearing on the edges or underside of the leaf. On the underside, in humid weather, a white, fluffy fungal growth is visible. The leaf quickly blackens and rots (Popkova et al., 1980; Dean, 1994).
• Macrosporiosis (dry spot): Spots are dry, brown, round or angular, with well‑visible concentric zones (like a “target”). Affected tissue becomes brittle and may crumble, leaving holes.
• Cercosporiosis: Spots are small, angular, greenish‑yellow, with a grey‑violet coating on the underside. The leaf dries up.
• What to do: Against fungal diseases, only preventive and therapeutic treatments with fungicides (Bordeaux mixture, copper‑based preparations, copper oxychloride) are effective. At the first signs of spots, immediately remove diseased leaves and treat the plants according to instructions, otherwise the disease can destroy all tops and infect the tubers.

2.4. Yellowing and wilting (loss of turgor)

This is a signal of root system problems.

Cause: Lack of moisture. In hot weather, plants may wilt during the day and recover in the evening. If leaves do not recover, turn yellow at the edges and dry out – it is lack of water in the soil.

Cause: Root diseases. Verticillium and fusarium wilts, as well as bacterial rots (blackleg), block the plant’s vascular system. Roots or stems cannot supply water to the leaves.

• Symptoms: Yellowing and wilting often start on one side of the bush. On a cross‑section of the stem or tuber, a darkened vascular ring is visible. With “blackleg”, the base of the stem blackens and rots (Welbaum, 2015).
• What to do: For bacterial wilts, remove diseased plants, disinfect the soil. Prevention – healthy seed material and proper crop rotation.

Summary of Chapter 2: “Yellow light” for your potatoes

When you detect yellow tops, the main thing is not to panic and answer a few questions:

1. Which leaves turned yellow? If lower – most likely nutrition or ageing. If upper – possible virus or micronutrient deficiency.

2. Is it uniform yellowing or spots? Spots indicate a fungal disease. Uniform yellowing – nutritional disorder or virus.

3. Is there curling or wilting? Curling often points to a virus, wilting – to root problems.

4. Is the problem widespread across the field or localised? Localised lesions (foci) – sign of infection.

By answering these questions, you can determine whether you need to urgently rescue the crop with fungicides, remove diseased bushes, or simply feed the plants with urea.

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3.1. Curling due to viruses (the most dangerous option)

Viral diseases of potato often cause characteristic leaf deformation, and curling is one of the main symptoms (Caliskan et al., 2023). A distinctive feature of viral infection is that it is usually not limited to curling but is accompanied by colour changes (mosaic, chlorosis, necrosis) and growth suppression.

1. Potato leafroll virus (PLRV)

This is one of the most harmful viruses. It is transmitted by aphids and infected tubers.

Symptoms: The main sign – leaves roll upward, “boat‑shaped”, along the midrib. Rolled leaves become stiff, “leathery”, brittle. They turn pale green or yellow, and sometimes have a purple tinge on the edges. Affected bushes are low‑growing, dwarfed. On cut tubers, “net necrosis” (brown spots and arcs in the flesh) may be visible (Chikh‑Ali & Karasev, 2023; De Jong et al., 2011).

What to do: This is incurable. Infected bushes must be removed immediately with roots and destroyed to prevent spread by aphids. The main protection is the use of healthy seed material and control of vectors (aphids).

2. Other viruses (potato virus Y, potato virus M, gothic)

Different viruses cause different types of curling. They are often grouped under “virus mosaic” or “gothic”.

• Potato virus Y (PVY): Can cause downward or upward leaf curling, as well as crinkling, rugosity, and necrotic spots (Popkova et al., 1980; Chikh‑Ali & Karasev, 2023).
• Potato virus M (PVM): Causes “mosaic curling” – leaves curl and become mosaic‑coloured.
• Gothic (spindle tuber): Leaves become small, sticking upward at a sharp angle, sometimes curling into a tube. Tubers become elongated, spindle‑shaped (Popkova et al., 1980).
• What to do: Same measures as for PLRV. Remove diseased bushes, protect healthy ones from insect vectors. Varieties with genetic resistance to viruses are the best prevention.

How to distinguish viral curling from other causes:

• Curling is almost always accompanied by colour changes (mosaic, yellowing, reddening of veins).
• Affected bushes are stunted, have shortened internodes, small leaves.
• Often occurs in foci (several bushes nearby), as it spreads from diseased to healthy via aphids or contact.
• Tubers may show internal necrosis or deformities.

3.2. Curling due to physiological causes (non‑infectious)

This is the most common cause, and it rarely leads to plant death if care mistakes are quickly corrected.

1. Heat and drought (heat stress)

In hot weather, especially when temperatures stay above 30 °C, leaves curl upward to reduce evaporation. This is a protective mechanism.

• Symptoms: Curling often starts with young upper leaves. Leaves do not change colour (remain green), do not become stiff, but simply look “clamped”. When temperature and humidity normalise, they unroll (Demirel, 2023; Welbaum, 2015).
• What to do: Provide regular watering, preferably drip or furrow irrigation in the evening. Mulch the soil to retain moisture. In extreme heat, overhead irrigation can cool the tops, but it increases the risk of late blight.

2. Potassium deficiency

Potassium is one of the key elements for regulating water balance. When deficient, leaves lose turgor and curl upward.

• Symptoms: Leaf edges turn yellow, then brown and die (marginal scorch), while the leaf blade curls upward. Older leaves develop bronze spots and become wrinkled. Leaves often become “boat‑shaped” (Popkova et al., 1980; Welbaum, 2015).
• What to do: Apply potassium fertiliser (potassium sulfate, potassium magnesium). Liquid root feeding works best. Usually helps quickly.

3. Excess chlorine in the soil (chlorosis)

This is not a deficiency but a toxic effect. Occurs when irrigating with water high in chlorine or applying potassium chloride in large doses.

• Symptoms: Leaves become pale green, curl “boat‑shaped” along the midrib, edges die and turn light brown. Affected leaves droop but do not fall. Stems become short and thick (Popkova et al., 1980).
• What to do: Stop using chlorine‑containing fertilisers (if you have applied them recently). Flush the soil with plenty of water to leach chlorine into deeper layers (if drainage allows). In future, use chlorine‑free fertilisers (e.g., potassium sulfate).

4. “Deforming jaundice” (functional disorder)

This is not an infectious disease, but a condition caused by poor soil aeration on heavy, waterlogged soils.

• Symptoms: Leaves become chlorotic (yellowish), severely deformed, curled, often with necrosis, sometimes reduced (almost without lobes). Bushes are depressed (Popkova et al., 1980).
• What to do: Improve soil structure – add organic matter, loosen, install drainage. The problem is reversible: if such a plant is transplanted into loose aerated soil, it recovers.

3.3. Herbicide stress (herbicide damage)

Curling, especially in the form of “downward twisting” (epinasty) or “thread‑like” leaves, can be caused by herbicides.

• Symptoms: Leaves and stems become distorted, thinned, leaf blades narrow, “feather‑like”. This resembles the effects of 2,4‑D or other hormonal herbicides, even as vapours from neighbouring fields (De Jong et al., 2011).
• What to do: If this happens, the only help is improved care (watering, feeding) to help the plant outgrow the stress. In severe cases – plantings may die.

Summary of Chapter 3: How to tell a virus from physiology?

To avoid mistakes, ask yourself the same questions:

1. Is there a colour change? If leaves are just curled but green and turgid – it’s heat. If they turn yellow, show mosaic or necrosis – it’s a virus or deficiency.

2. Does the whole bush suffer? With heat or potassium deficiency, usually all leaves are affected uniformly or by tiers. With a virus, often one side of the bush or several shoots are affected, while others look healthy.

3. Is there growth retardation? With a virus – yes, the bush is often dwarfed. With heat and deficiency – growth slows, but the bush does not become “ugly”.

4. Is there a systemic pattern? Viruses often appear in foci. Heat – over the entire area.

5. Examine tubers and roots. If there are deformities, necrosis – it’s a virus. If roots are healthy and leaves curled – it’s physiology.

Main rule: if you suspect a virus – remove diseased bushes without mercy! This is the only way to stop an epidemic.

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4.1. Problems with seed material (tuber “ageing”)

Potato is propagated vegetatively, and with each generation tubers can physiologically “age”. This is one of the main causes of degeneration if seed stock is not renewed.

Cause 1: Physiologically old (over‑ripe) seed tuber.

If tubers were stored at too high temperatures or for too long, they “age”. Inside, growth inhibitors accumulate.

• Symptoms: Such a tuber produces many weak, thin shoots (thread‑like sprouts) or, conversely, one or two puny stems. The bush turns out “ruffled”, with small, pale leaves. It is clearly weaker than neighbours. Yields drop sharply (De Jong et al., 2011; Welbaum, 2015).
• What to do: Such tubers are unsuitable for planting. They should be discarded during sorting. The best seed material is medium‑sized tubers (50–80 g) with short, thick, green sprouts (after vernalisation).

Cause 2: Latent infection in the seed tuber.

Viruses (especially leafroll virus, potato virus Y) often sit in tubers asymptomatically, but when planted they give weakened plants.

• Symptoms: Bush is stunted, leaves small, chlorotic, may be curled or covered with mosaic spots (as described in Chapter 3). Unlike “old” tubers, here there are characteristic visual signs on leaves (Chikh‑Ali & Karasev, 2023).
• What to do: This is viral infection. The bush must be removed entirely so it does not become a source of infection. Planting material for the next year should be taken only from verified healthy lots (preferably certified).

4.2. Problems with roots and soil (physiology and diseases)

The bush does not grow because its root system cannot perform its function – supplying water and nutrients.

Cause 1: Cold and wet soil.

Potato is a temperate‑climate crop, but its roots work poorly at soil temperatures below 8‑10 °C.

• Symptoms: Sprouts emerge but develop very slowly. The bush seems to “freeze”. Leaves may turn purple because roots cannot absorb phosphorus in cold conditions (Welbaum, 2015; Demirel, 2023).
• What to do: This is temporary. As soon as the soil warms to 12‑15 °C, growth usually accelerates. For the future: plant potatoes only in well‑warmed soil. Use black mulch or ridge planting to warm the soil faster.

Cause 2: Root rots and fungal diseases.

Rhizoctoniosis, blackleg, verticillium wilt, and fusarium attack roots and the lower stem, blocking nutrition.

Symptoms:

• Rhizoctoniosis (“black scurf”): Dry cankers appear at the base of the stem, or the stem becomes “constricted”. Emergence may fail, or weak, twisted sprouts appear. In dry weather, the bush may stop growing (Popkova et al., 1980).
• Blackleg (bacterial rot): The lower stem blackens, becomes watery, rots, and the bush pulls out easily from the soil. The plant wilts quickly.
• Verticillium and fusarium wilt: Wilting starts from lower leaves, the bush yellows and dries, but the stem remains standing. On a stem cut, darkened vessels are visible.

What to do: Remove diseased bushes immediately. Water the soil with fungicide solutions (Fitosporin, Trichoderma) to suppress infection. The main prevention – crop rotation (do not plant potatoes after potatoes or other solanaceous crops for 3‑4 years) and pre‑planting treatment of seed material.

Cause 3: Soil compaction and oxygen deficiency.

Roots must breathe. If a crust forms after rains or the soil is too compacted – the root system develops poorly.

• Symptoms: Bush is low‑growing, leaves pale green, often with yellowing. Stunting is combined with a generally depressed appearance.
• What to do: Regular loosening between rows (especially after watering and rains) is the “second watering” for potatoes. It breaks the crust and allows air to reach the roots.

4.3. Nutrient deficiency (especially nitrogen)

One of the most common causes – lack of major nutrients, especially nitrogen, which is responsible for green mass growth.

Signs of nitrogen starvation:

The bush is pale green or yellowish, weakly branched, internodes short. Stems thin, leaves small. Yellowing starts from lower leaves (as described in Chapter 2). With phosphorus deficiency – leaves may be dark green with a purple tint, and growth also slows (Welbaum, 2015).

What to do: Apply nitrogen fertiliser. For quick effect – foliar feeding with urea (carbamide) on the leaves. For long‑term – root feeding (manure infusion, chicken manure, ammonium nitrate).

4.4. Weeds (competition)

This is a common cause that is often underestimated. Strong weeds (couch grass, bindweed, sow thistle) take water, light, and nutrients from potatoes.

• Symptoms: The bush is clearly depressed, grows slower than neighbours, surrounded by a “wall” of weeds. Potato roots cannot compete with weeds.
• What to do: Regular weeding, especially at early stages, before the bush closes the canopy. Mulching between rows helps suppress weeds and retain moisture.

Summary of Chapter 4: Why potato “stands still”?

Stunted growth is always a systemic problem. Check three levels:

1. Seed tuber: Was it healthy? Was it properly prepared (vernalised)?

2. Roots and stem: Are there rots, cankers, mechanical damage? Is the soil loose enough?

3. Nutrition: Is there enough nitrogen? Is there competition from weeds?

The key differential sign:

• If the bush is small, pale, but without obvious spots or rot – look for a problem in nutrition or soil.
• If the bush is ugly, with necrosis, mosaic, or rot at the base – it is infection (viruses or bacteria). Such plants must be ruthlessly removed.

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5.1. Lack of moisture (especially during tuber filling)

This is the most common cause of small tubers. Potato is a drought‑sensitive crop. The critical period is budding‑flowering, when tuber set and filling occur.

• Why it works: With water shortage, leaf stomata close, photosynthesis slows or stops (Demirel, 2023). The plant cannot produce carbohydrates. Also, without water, tuber cells cannot expand. If during this period there is heat and no rain, tubers stop growing and remain small, even if the variety is large‑fruited. In arid regions without irrigation, a good harvest is hardly possible (Caliskan et al., 2023).
• Symptoms: During flowering and after, tops look droopy, leaves lose turgor (especially during the day), leaf edges may curl (as described in Chapter 3). When dug, tubers are small, sometimes deformed.
• What to do: Provide regular watering. The greatest need for moisture is during budding, flowering, and the first 3‑4 weeks after flowering (filling phase). The soil during this period should be moist but not waterlogged (optimally 75‑80% of full field capacity) (Popkova et al., 1980). Drip irrigation or furrow irrigation gives better results than overhead irrigation (less risk of late blight).

5.2. Nutrient deficiency (especially potassium and phosphorus)

Even with sufficient water, tubers have nothing to form from if the plant lacks nutrients.

Potassium: The main element for tuber filling. It regulates cell water regime, participates in starch synthesis and transport of sugars from leaves to tubers.

• Symptoms: Small tubers with relatively normal tops. Leaves may be dark green, wrinkled, with a bronze tint, edges curl upward (marginal scorch). On a tuber cut – darkening of flesh around the vascular ring (Welbaum, 2015; Dean, 1994).
• What to do: Apply potassium fertilisers (potassium sulfate, potassium magnesium) throughout the season. Best – fractionally, with watering. Avoid chlorine‑containing potassium fertilisers (potassium chloride) – they are toxic to potatoes.

Phosphorus: Affects overall plant growth and root formation. With its deficiency, the bush is weak, roots poorly developed, and tubers lack strength to grow.

• Symptoms: Tops are stunted, leaves dark green with a purple tint. Tubers small, yield low.
• What to do: Apply phosphorus fertilisers (superphosphate, bone meal) in autumn or spring under digging. Phosphorus is immobile in soil and must be applied to the root zone.

5.3. Dense planting (competition)

Too close spacing of bushes leads to competition for water, light, and nutrition.

• Why it works: Each bush needs a feeding area. If bushes are crowded, roots intertwine, and resources are insufficient for forming large tubers. There may be many tubers per bush, but all are small (Dean, 1994).
• What to do: Follow optimal planting scheme. For most table varieties, it is 70‑80 cm between rows and 25‑35 cm between tubers in the row. For early‑maturing varieties with compact bushes, the distance can be slightly reduced; for late varieties – increased.

5.4. Weak tops (insufficient photosynthetic surface)

“Weaklings can’t produce big tubers.” If the bush is weak, with small or sparse foliage, it simply cannot “feed” the tubers.

Causes of weak tops:

• Nitrogen deficiency at the start. Poor green mass does not provide enough carbohydrates (see Chapter 4).
• Diseases and pests. Late blight, macrosporiosis, Colorado potato beetle destroy leaves. If tops die prematurely, tubers stop growing and remain small.
• Shading. Potato loves light. Planting in shade of trees or in dense rows leads to elongation and weakening of tops.

What to do:

• Provide nitrogen nutrition at early stages.
• Actively control diseases and pests.
• Plant potatoes in well‑lit areas.
• Do not delay planting – early spring potatoes have time to build tops before summer heat.

5.5. Overheating and high night temperatures

Potato is a temperate‑climate crop. For tuber formation and filling, optimal temperature is about 18‑21 °C.

• Why it works: At temperatures above 25‑27 °C, transport of carbohydrates from leaves to tubers is blocked. Everything synthesised in leaves goes to respiration, not to tubers. High night temperatures are especially dangerous (Demirel, 2023; Welbaum, 2015).
• What to do: Plant potatoes at the optimal times for your region so that the filling phase coincides with the coolest periods. Use early‑maturing varieties in regions with hot summers. Mulching soil helps lower its temperature.

5.6. Varietal characteristics

Remember: do not expect fist‑sized tubers from early‑maturing varieties. Early varieties yield faster, but their tubers are often medium or small. Yield and size are genetically determined (De Jong et al., 2011).

What to do: When buying seed material, focus on varieties recommended for your zone and consider their characteristics: maturity, purpose, tuber weight potential.

Summary of Chapter 5: How to grow large potatoes?

To get large tubers, you need to create three conditions:

1. Abundant and healthy tops. Leaves must work at full capacity until the very end of the season.

2. Sufficient moisture. Especially during budding‑flowering‑filling.

3. Optimal number of set tubers. You don’t need to leave all flowers (that’s debatable), but better adjust planting density and apply potassium so that part of the energy goes to size rather than number.

Key differential sign for small tubers:

• If tops are vigorous and green, but tubers small – look for the cause in water, potassium, or heat.
• If tops are weak, yellow, or diseased – first solve nutrition and plant protection problems.

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6.1. Physiological deformations (non‑infectious) – the most common cause

These deformations arise from external factors that disrupt normal tuber growth. Often we ourselves are to blame – improper watering, nutrition, or poor soil.

1. Tuber outgrowths (“secondary tubers” or second growth)

This is when additional small tubers or protrusions form on the main tuber.

Cause: The plant starts forming tubers, then tuber growth stops (drought, heat), and then resumes (watering or rain). During the second growth wave, new stolons develop from the eyes of the main tuber, giving rise to small “daughter” tubers. This is called second growth (Popkova et al., 1980; Dean, 1994).

Symptoms: On the main tuber, protrusions of various sizes appear, resembling small potatoes. The tuber becomes ugly.

What to do: Provide uniform watering, avoid sharp fluctuations in moisture. Mulching helps smooth out fluctuations.

2. Cracks on tubers (growth cracks)

Deep or superficial cracks on the tuber surface.

Cause: A sharp growth spurt of the tuber after a dry period. The skin (periderm) cannot stretch fast enough, and the tuber cracks. Often happens after heavy watering or rain following drought (Dean, 1994; Welbaum, 2015).

Symptoms: The tuber looks “torn”, with deep furrows that may heal, but marketable appearance is lost.

What to do: Uniform watering is the main prevention.

3. Dumbbell‑shaped, pear‑shaped, or “beak‑shaped” tubers

Distorted, with thickening on one end and narrowing on the other.

Cause: Uneven development due to soil compaction, too shallow planting, or mechanical obstacles (stones, clods). The root system cannot expand, and the tuber seeks paths for growth, becoming distorted (Dean, 1994; Popkova et al., 1980).

What to do: Thorough soil preparation, removal of stones, deep loosening. Plant potatoes in loose, cultivated soils.

4. “Spindle tuber” and “gothic” (physiological)

This is a special case that can be either viral (see below) or physiological. But in some cases, such deformation is caused not by viruses but by high temperatures during tuber growth or poor soil aeration (Popkova et al., 1980).

Symptoms: Tubers become elongated, with a pointed end, like a spindle. Eyes become deeper or more prominent.

What to do: If it is a physiological cause, it is not contagious. Next year under normal conditions, tubers will return to their varietal shape. Viral gothic, on the contrary, will progress, and bushes will look depressed.

6.2. Deformations due to viruses (infectious, systemic)

Viral diseases often cause typical tuber deformations, and these deformations are inherited with seed material.

1. Spindle tuber (gothic) – viral

Caused by potato spindle tuber viroid (PSTVd). This is a very harmful disease.

Symptoms: Tubers become elongated, spindle‑shaped or pear‑shaped, with an increased number of eyes. Eyes are often prominent, with a sharply defined “eyebrow”. Skin may be cracked. Bushes – dwarfed, with small upward‑pointing leaves (Popkova et al., 1980; De Jong et al., 2011).

What to do: Remove and destroy diseased bushes and tubers. The viroid is highly contagious – transmitted through contact, tools, seeds. Use of certified seed material is the only reliable protection.

2. Deformation due to virus R (rattlevirus) and potato mop‑top virus (PMTV)

These viruses do not so much deform the shape as cause internal necrosis and arcs. But sometimes affected tubers can be ugly with cracking.

Symptoms: On the cut, rust‑brown arcs, rings, or bands are visible. Sometimes external cracks and deformations (Chikh‑Ali & Karasev, 2023; Dean, 1994).

What to do: Remove diseased plants. These viruses are transmitted by soil pests (nematodes, fungi), so the main prevention – healthy seed material and pest control in the soil.

6.3. Deformations due to bacterial and fungal diseases (infectious, local)

Many diseases cause cankers, growths, and lesions that make the tuber unusable.

1. Potato cancer (quarantine disease)

A very dangerous fungal disease. Not found everywhere, but you need to know about it.

Symptoms: On tubers and stolons, white, then brown or black growths appear, resembling cauliflower or coral. Size – from pinhead to fist (Popkova et al., 1980; De Jong et al., 2011).

What to do: If detected, immediately report to the quarantine service. Plants and tubers are destroyed on site. Use only zoned, cancer‑resistant varieties.

2. Common scab and powdery scab

These do not so much deform as form cankers, growths, and crusts on the surface.

Symptoms: On tubers, dry, corky cankers, bumps, “star‑like” pustules appear (Popkova et al., 1980).

What to do: Do not plant potatoes on acidic or over‑limed soils. Regulate soil moisture during tuber set. Use resistant varieties.

3. “False cancer” (physiological proliferation of eyes)

This is not a disease, but a reaction to adverse conditions (cold and wet soil, lack of air).

Symptoms: From the eyes of the seed tuber or new tuber, many thread‑like short sprouts grow, forming a dense mass, sometimes with small tubercles (Popkova et al., 1980).

What to do: Plant potatoes in warm, loose soil, ensure good ventilation of tubers during storage.

Summary of Chapter 6: Main fork – “physiology or infection?”

To avoid mistakes, use a simple test:

1. Evaluate tuber shape:

• If the tuber is simply crooked, with constrictions or cracks, but skin is smooth, colour normal – it is most likely physiology. The culprits are uneven watering or poor soil.
• If the tuber has growths, “warts”, cankers, or deep necrosis – it is infection (fungi or bacteria).
• If the tuber is elongated, “spindle‑shaped”, with prominent eyes – it is viroid (gothic).

2. Evaluate the bush (see Chapters 2‑4):

• If the bush is vigorous, green, and tubers are crooked – the problem is physiological.
• If the bush is dwarfed, diseased, with mosaic or necrosis – it is a viral infection, and the whole plant must be removed.

Main advice: with strong viral deformations (gothic, internal necrosis), tubers are unsuitable even for planting. They must be ruthlessly culled to avoid infecting the next season.

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Step 1. Look at the nature of damage: uniform or focal?

This is the first and most important question.

Continuous, uniform damage of the whole field or most of it.

• What this means: Symptoms appear on all plants roughly equally. This almost always indicates physiological stress (weather, nutrition, soil). For example, yellowing of all tops after frost, chlorosis from iron deficiency on alkaline soils, or mass wilting from drought.
• What to do: Look for the cause in weather, soil, and cultivation practices.

Focal, local damage (spots, groups, individual bushes).

• What this means: Disease or pest spreads from a source of infection. This is a classic sign of infectious disease (fungal, bacterial, viral) or pest infestation. For example, late blight always appears in foci, Colorado potato beetle first colonises field edges, and viruses spread from diseased to healthy bushes via aphids.
• What to do: Look for a specific pathogen or pest in that focus.

Step 2. Look for traces: pest, fungus, bacterium, or virus?

If damage is focal, determine the nature of that focus. Each group has its own “signatures”.

A. Signs of pests (insects, mites, nematodes):

Pests leave mechanical damage and themselves.

• Leaves: Chewing, holes, skeletonisation (tissue eaten between veins). On the underside, you can see eggs, larvae, or adults (Colorado potato beetle, aphids, mites).
• Stems and tubers: Tunnels, gnawed holes, traces of activity (excrement, webbing). For example, wireworms bore narrow tunnels in tubers, and mole crickets gnaw roots.
• Key sign: The pest (or its traces) can be seen with the naked eye or with a magnifying glass. With severe infestation, webbing (mites) or sticky secretions (aphids) are often visible on plants.

B. Signs of fungal diseases (late blight, macrosporiosis, scab, etc.):

Fungi reproduce by spores and often leave a coating on the surface of leaves and tubers.

• Leaves: Characteristic spots (dry with concentric circles, or conversely, wet, diffuse). In humid weather, a coating appears on the spots (white, grey, pink, black) – fungal sporulation.
• Tubers: Sunken spots, cankers, growths, and on the cut – brown or black areas (rot). Often affected tubers have an unpleasant odour.
• Key sign: Presence of spores (coating) on affected tissues and characteristic spotted damage that enlarges over time.

C. Signs of bacterial diseases (blackleg, ring rot):

Bacteria cause rotting and often affect the vascular system.

• Leaves and stems: Wilting (loss of turgor), yellowing, then blackening and softening of tissues. Often there is slime with a foul odour. With blackleg, the base of the stem blackens and softens.
• Tubers: On the cut, a darkened vascular ring or rot that starts from the stolon end is visible. Upon pressing, yellow or white slime may ooze from affected vessels.
• Key sign: Rotting and slime, odour, vascular involvement. Bacteria often enter through mechanical damage.

D. Signs of viral diseases (potato virus Y, leafroll, gothic):

Viruses do not kill cells immediately but disrupt their function, causing systemic changes throughout the plant.

• Leaves: Mosaic colouration (alternating dark and light areas), curling, crinkling, rugosity, filiformity (leaves become narrow like strings). The bush is often dwarfed with shortened internodes.
• Tubers: Deformation (spindle‑shaped, pear‑shaped), internal necrosis (rusty arcs, rings), poor sprouting (thread‑like sprouts).
• Key sign: Systemic deformation of the whole bush and tubers, absence of rot and coating, dwarfism. Viruses cannot be cured, and diseased plants must be removed.

Step 3. Recall weather and care (physiological factor)

If you have not found any pests or signs of infection, and damage is uniform – it is most likely a physiological disorder.

• Heat and drought: Leaves curl, lose turgor, tubers small and cracked.
• Overwatering: Leaves yellow (nitrogen starvation due to poor root function), tubers may “suffocate” and rot without signs of infection.
• Nutrient deficiency: Characteristic colour changes (iron chlorosis, potassium starvation, etc.), stunting.
• Key sign: Symptoms are reversible when the cause is eliminated (watering, fertilisation, loosening). No focal spread.

Quick diagnostic checklist (for the field)

Final conclusion

Golden rule of diagnostics: never diagnose based on a single symptom.

Always examine leaves, stems, roots, and tubers (see Chapter 1). Cross‑check signs. If you see coating, rot, or slime – it is infection (fungi or bacteria). If you see chewing, tunnels, or insects – it is a pest. If there is deformation, mosaic, or dwarfism – it is a virus. If it is simply yellowing, curling without spots and without pests – it is physiology; look for care mistakes.

And remember: viruses cannot be cured! If you suspect a virus, the only correct decision is to remove the diseased plant without mercy. This will save the rest of the crop.