Diagnostics

Yellowing of Leaves (Chlorosis)

Yellowing is the most common symptom, and its causes can be many and varied.

• Natural senescence: If yellowing begins at the tips of the oldest, lower leaves and they gradually dry up, and this happens towards the end of the growing season (during bulb maturation), it is a completely normal process. The bulb is preparing for harvest, and the leaves are transferring their nutrients to it (Brewster, 2008).
• Nitrogen deficiency: Leaves become pale green and then yellow, starting from the tips; plant growth slows down. This is a sign of starvation. Nitrogen is the primary element for green mass growth, and its deficiency immediately affects coloration (Rubatzky and Yamaguchi, 1997; Akhatov et al., 2013).
• Potassium deficiency: Yellowing begins at the leaf edges, which may then turn brown and dry out. Leaves become wrinkled and lose turgor (Rubatzky and Yamaguchi, 1997; Wien and Stützel, 2020).
• Excess moisture or lack of oxygen in the soil: When waterlogged or when a soil crust forms, roots suffocate, and this reflects on the leaves. They turn yellow and plants look depressed. This is especially dangerous for young seedlings (Akhatov et al., 2013).
• Root rots and pest damage: Any damage to the roots (e.g., by onion fly larvae or bulb mites) or stem nematode (Ditylenchus dipsaci) prevents the plant from taking up water and nutrients. As a result, leaves turn yellow and wilt en masse (Conn, 2012; Akhatov et al., 2013).
• Bacterial and fungal diseases: For example, with Fusarium basal rot (Fusarium oxysporum) and white rot (Sclerotium cepivorum), yellowing begins at the leaf tips and rapidly spreads downward, leading to plant death (Conn, 2012; Akhatov et al., 2013).

Pale and Whitening of Leaves

• Micronutrient deficiency (iron, zinc, manganese): Manifests as uniform or interveinal chlorosis—the leaf becomes lighter, while veins may remain green. This primarily affects young leaves (Rubatzky and Yamaguchi, 1997; Conn, 2012).
• Thrips damage: Leaves become covered with many small silvery‑white streaks and spots. This is the result of the pest (Thrips tabaci) sucking sap from the cells. In severe infestations, leaves completely pale and dry up (Brewster, 2008; Conn, 2012).
• Herbicide burn: When herbicides come into contact with leaves, pale, white or yellow spots, deformation, and curling appear. Symptoms are often one‑sided or noticeable along field edges (Conn, 2012; Akhatov et al., 2013).
• Viral infections: In mosaic or yellow dwarf, characteristic light, yellow‑white stripes and speckles alternate with green areas on leaves. This is a very specific symptom (Brewster, 2008; Akhatov et al., 2013).

Drying and Wilting of Leaves

• Lack of moisture: In hot, dry weather with insufficient watering, leaves lose turgor, become limp, turn yellow, and dry starting from the tips. During prolonged drought, plants may die completely (Rubatzky and Yamaguchi, 1997; Akhatov et al., 2013).
• Root damage (nematode, onion fly): The plant cannot take up moisture, and the above‑ground part wilts and dries en masse, even though the soil may be wet. This is easy to verify by pulling out such a plant: its roots will be rotten or damaged (Conn, 2012; Akhatov et al., 2013).
• Fusarium wilt: When infected with the fungus Fusarium oxysporum, the plant’s vessels become blocked, and leaves wilt and dry, often starting with one or two central leaves (Conn, 2012; Akhatov et al., 2013).
• Downy mildew (Peronosporosis): The disease is caused by the fungus Peronospora destructor. Pale, yellowish or brownish spots appear on leaves, which over time become covered with a grayish‑purple mildew. Affected areas dry out. Sometimes leaves may curl and become deformed (Brewster, 2008; Conn, 2012; Akhatov et al., 2013).

Curling and Deformation of Leaves

• Herbicide damage: Curling, crinkling, and thickening of leaves are typical signs of exposure to hormonal herbicides (e.g., 2,4‑D). Leaf shape becomes severely distorted (Akhatov et al., 2013).
• Stem nematode attack (Ditylenchus dipsaci): This pest, penetrating plant tissues, causes swelling, twisting, and deformation (”bloating”) of stems and leaves. Leaves become thick, short, with unusual bends (Brewster, 2008; Conn, 2012; Akhatov et al., 2013).
• Viral diseases (Onion Yellow Dwarf Virus - OYDV): In this disease, leaves not only turn yellow but also become flattened, shrivelled, crinkled, and easily lodge. Plants are severely stunted and look dwarfed (Brewster, 2008; Conn, 2012; Wien and Stützel, 2020).
• Insect damage: Larvae of some pests (e.g., the onion weevil) may damage tissues inside the leaf, causing deformation, twisting, and the formation of pale stripes and spots (Akhatov et al., 2013).

Summary for the Gardener:

When inspecting leaves, always pay attention to the complex of symptoms. Assess which leaves are affected (old, young, all of them), how the damage starts (from the edge, tip, or base), and whether there are any additional signs such as mildew, spots, or insect traces. This will help you pinpoint the cause of the problem and take timely measures.

Bulb Too Small (Poor Weight Gain)

If the bulb does not reach the expected size for the variety, it is almost always the result of stress that prevented the plant from accumulating enough dry matter.

• Excessively high plant density: In thick sowings, plants compete for light, water, and nutrients. As a result, bulbs turn out small, even if the variety is large‑fruited (Rubatzky and Yamaguchi, 1997; Wien and Stützel, 2020).
• Lack of moisture or nutrition: In dry periods or on poor soils, bulb growth slows. Particularly critical is the lack of water and nitrogen during the period of active leaf growth and the start of bulb formation (Rubatzky and Yamaguchi, 1997).
• Early or premature bolting: If the plant goes to flower, all its resources are directed to seed formation rather than bulb filling. Such a bulb remains small and hard (Akhatov et al., 2013; Wien and Stützel, 2020).
• Premature aging or death of leaves due to diseases (downy mildew, alternaria) or pests (thrips): Loss of leaf area sharply reduces photosynthesis, and the bulb simply does not have time to gain mass (Brewster, 2008; Conn, 2012).
• Genetic characteristics: Short‑day varieties grown under long‑day conditions may bulb prematurely without gaining sufficient vegetative mass, resulting in small bulbs (Rubatzky and Yamaguchi, 1997).

Bulb Soft, Flabby, or Rotting

This is an alarming signal that most often indicates bacterial or fungal rots.

• Bacterial soft rot (wet rot): Caused by bacteria (e.g., Pectobacterium carotovorum, Burkholderia spp.). The bulb softens, tissues become watery, translucent, and emit an unpleasant putrid odour. The infection often starts at the neck or through wounds (Conn, 2012; Akhatov et al., 2013). As noted in sources, when such a bulb is squeezed, a foul‑smelling liquid may be expelled (Brewster, 2008).
• Slippery Skin: Caused by the bacterium Burkholderia gladioli. Inner scales soften, become slimy, and when pressed, the ”core” of the bulb may slip out through the neck. This is a characteristic symptom that distinguishes this disease from other rots (Conn, 2012; Brewster, 2008).
• Sour Skin: Caused by the bacterium Burkholderia cepacia. Outer fleshy scales turn yellow‑brown, soften, a slimy mass accumulates between them, and the bulb emits a vinegar‑like or sour odour (Conn, 2012).
• Fusarium basal rot: The fungus Fusarium oxysporum infects the basal plate, which turns brown and crumbly. The rot moves upward through the bulb. Affected bulbs are soft, and the base may easily separate (Conn, 2012; Akhatov et al., 2013).
• White rot of the base (sclerotial rot): Caused by the fungus Sclerotium cepivorum. A white, cottony mycelial growth appears on the base and scales, in which small black sclerotia (resembling poppy seeds) later form. The bulb softens and rots away (Brewster, 2008; Conn, 2012; Akhatov et al., 2013).

Cracks on the Bulb

Cracks may appear both in the field and during storage.

• Irregular watering: Sharp fluctuations in moisture—from drought to heavy watering—cause uneven growth of internal tissues, leading to cracking of bulbs. This is especially common on light soils (Conn, 2012; Rubatzky and Yamaguchi, 1997).
• Frost damage or sunburn: As Conn (2012) points out, freezing or sudden overheating of tissues can lead to their death and cracking. With frost damage, tissues become watery and translucent.
• Physiological disorders: Sometimes cracks appear due to too rapid bulb swelling under favourable conditions, especially in thin‑skinned varieties.

Splitting of the Bulb into Segments (Twinning, Multi‑centredness)

This defect greatly reduces marketable quality, especially for processing (e.g., for rings).

• Genetic predisposition: Some varieties are more prone to forming multiple centres. However, this trait is strongly modified by growing conditions (Rubatzky and Yamaguchi, 1997; Wien and Stützel, 2020).
• Unfavourable conditions at the start of growth: Stress (drought, temperature fluctuations, pest damage) at early stages can stimulate the awakening of lateral buds instead of suppressing them by dominant apical growth. As a result, several independent bulbs or one deformed bulb form (Wien and Stützel, 2020).
• Excess nitrogen: Excessive nitrogen nutrition in the first half of the growing season may provoke enhanced branching, leading to bulb splitting (Rubatzky and Yamaguchi, 1997).
• Planting depth of sets: Experiments have shown that too deep planting can increase the number of multi‑centred bulbs, especially in some varieties (Wien and Stützel, 2020).

Summary for the Gardener:

A healthy bulb should be firm, with dry, glossy or matte outer scales typical of the variety, and of correct shape. Any signs of softening, mould, cracks, or deformation are serious reasons for concern. Many rots begin in the field but manifest during storage, so thorough inspection of bulbs at storage time is the key to preserving the harvest.

Neck Too Thick and Juicy (”Thick‑neck” or ”Bull‑neck”)

This is one of the most common problems for gardeners. Instead of drying out and closing by harvest time, the neck remains thick, elastic, and juicy. This means that the process of maturation and the translocation of nutrients from leaves to bulb has been disrupted.

• Excess nitrogen fertiliser during bulb formation: Nitrogen stimulates vigorous growth of green mass. If you applied too much nitrogen at the stage when onions should start forming bulbs, the plant continues to ”fatten”, growing leaves and neck at the expense of bulb maturation (Brewster, 2008; Rubatzky and Yamaguchi, 1997).
• Watering during the maturation period: Continuing heavy watering when onion leaves have already started to yellow and lodge delays their natural drying and consequently the drying of the neck. The onion does not enter dormancy (Akhatov et al., 2013).
• Late‑maturing variety in a short‑summer region: Long‑day varieties planted in areas with insufficient day length may simply not have time to mature by harvest. The bulb‑formation process is prolonged, and the neck remains wet (Rubatzky and Yamaguchi, 1997; Wien and Stützel, 2020).
• Damage or loss of leaves in mid‑season (e.g., by hail, diseases): This may stimulate the growth of new leaves from dormant buds, which also disrupts the normal maturation cycle and delays neck drying (Conn, 2012).

Why does the gardener need to know this?

A thick, juicy neck is an open gateway for infections, especially for the agents of neck rot (Botrytis allii) and bacterial rots. Through an undried neck, fungi and bacteria easily penetrate the bulb, causing it to rot within the first few months of storage (Brewster, 2008; Conn, 2012). Such onions do not store well.

Neck Does Not Dry Out for a Long Time (or Does Not Dry at All)

This problem is closely related to the previous one, but here the emphasis is on the drying process.

• Unfavourable weather before harvest: Rainy, cloudy weather prevents natural drying of plants in the field. Moisture is retained in tissues, and the neck‑drying process stops (Brewster, 2008; Akhatov et al., 2013).
• Poor harvesting practices: Onions pulled from wet soil require more thorough and prolonged drying. If leaves are cut immediately after lifting, especially if they were still succulent, infection easily enters through the fresh cut into the neck (Conn, 2012).
• Root system damage: If roots are damaged by pests (onion fly, nematode) or suffer from rot, the plant cannot effectively complete its life cycle. Nutrient translocation is disrupted, and the neck may remain moist (Wien and Stützel, 2020; Akhatov et al., 2013).

Why does the gardener need to know this?

A well‑dried neck is the main condition for successful storage. Rapid and thorough drying in the first days after harvest allows the neck to ”seal”, preventing the entry of pathogens and reducing moisture loss. Many sources, including Brewster (2008) and Conn (2012), emphasize this.

Neck Softening, Mould Appearance

This is a direct sign of incipient rot. The most common causes:

• Neck rot (Botrytis allii): This is a fungal disease. A grey, fluffy mould appears on the neck, followed by small black masses (sclerotia). Surrounding tissues soften, become watery and brown. The disease often starts precisely at the neck, especially if it was damaged during harvest or did not dry properly (Brewster, 2008; Conn, 2012; Akhatov et al., 2013).
• Bacterial soft rot: The neck becomes wet, slimy, and softens. An unpleasant odour appears. Bacteria (Pectobacterium, Burkholderia) enter through stomata or micro‑damages, especially under high humidity (Conn, 2012).
• Fusarium: Although Fusarium rot more often affects the base, it can move up through the vessels and affect the neck, causing browning and softening (Conn, 2012).

Summary for the Gardener:

The neck is the weakest point of the bulb after harvest. The main secret of long storage is a dry and thin neck. To achieve this, you must:

1. Stop watering 2–3 weeks before the expected harvest.

2. Harvest only in dry weather, when the leaves of most plants have yellowed and lodged.

3. Dry onions thoroughly after lifting for 7–10 days (in a dry, airy place or in the sun if weather permits). Do not cut wet leaves too short.

Roots Weak, Thin, Sparse, Poorly Developed

If upon digging you see that the root system is scanty, short, without lateral branches, it is a sure sign that the plant is suffering severe stress.

• Lack of moisture or, conversely, waterlogging and lack of oxygen: Onions are very sensitive to water regime. Under drought, roots stop growing and die; under waterlogging, especially on heavy, slumping soils, roots suffocate, begin to rot, and their number decreases sharply (Rubatzky and Yamaguchi, 1997; Akhatov et al., 2013).
• High soil density or soil crust: Onion roots cannot penetrate a dense, compacted soil layer, especially in the top horizon. This often happens with untimely loosening or on clay soils (Wien and Stützel, 2020).
• Pest damage: Larvae of onion fly (Delia antiqua), wireworms, mole crickets may eat roots, leaving only ”stumps”. The bulb mite (Rhizoglyphus echinopus) feeds on the base and roots, making them crumbly. This is one of the most frequent causes of a weak root system (Akhatov et al., 2013; Brewster, 2008).

Why does the gardener need to know this?

Weak roots are the reason plants look depressed, leaves yellow, and bulbs do not fill. Without a powerful root system, onions simply cannot extract necessary nutrients from the soil, even if they are present.

Roots Brown, Dark, with Signs of Rot

This is a classic symptom of root rots caused by fungi or bacteria.

• Fusarium basal and root rot (Fusarium oxysporum f. sp. cepae): Roots become brown, then blacken and rot. The base of the bulb also turns brown and crumbly. On a cross‑section of the bulb, darkened vessels ascending from the base are visible. This is one of the most dangerous and widespread diseases (Conn, 2012; Akhatov et al., 2013).
• Pythium and Rhizoctonia rots (Pythium spp., Rhizoctonia solani): Cause ”damping‑off” in seedlings and root rot in adult plants. Roots become watery, brown, and disintegrate. These diseases are especially dangerous in cold and wet weather (Conn, 2012; Akhatov et al., 2013).
• White rot of the base (Sclerotium cepivorum): The fungus attacks roots and the base, covering them with a white cottony mycelium in which small black sclerotia later form. Roots die off rapidly, and the plant perishes. This disease is very persistent; sclerotia can survive in soil for up to 20 years (Brewster, 2008; Conn, 2012).
• Bacterial rot: Bacteria (Pectobacterium, Burkholderia) often secondarily colonise roots already damaged by other pathogens or pests. Roots become slimy, dark, and emit a foul odour (Conn, 2012).

Pink or Reddish‑Purple Hues of Roots (Pink Rot — Pyrenochaeta terrestris)

This is a very characteristic symptom that is easy to recognise. Roots turn pink, then reddish‑purple, become translucent, brittle, and eventually disintegrate. This disease is particularly widespread in warm regions but occurs everywhere. It prefers soil temperatures above 16 °C and is especially active at 24‑28 °C (Conn, 2012; Brewster, 2008). Resistant varieties are few but exist (Rubatzky and Yamaguchi, 1997).

Why does the gardener need to know this?

Pink rot does not kill the plant instantly, but it severely weakens it. Affected plants tolerate drought poorly, produce small bulbs, and yellow early. The pink colour of roots is one of those rare cases where a single symptom can give a precise diagnosis.

Roots Die Off but Do Not Rot (Dry Out)

If roots appear brown, dry, and brittle rather than wet and slimy, it is most often the result of abiotic stress.

• Drought: During prolonged lack of moisture, roots lose turgor and die. The finest absorbing rootlets suffer first.
• Sunburn: In hot weather when the soil is strongly heated, especially on light sandy soils, surface roots may literally ”cook” and die (Conn, 2012; Akhatov et al., 2013).
• Soil salinity: Onions are very sensitive to salinity. With excess salts, roots dehydrate, turn brown, and die (Wien and Stützel, 2020).

Summary for the Gardener:

When digging up plants, always inspect the roots. Pay attention to their colour (pink is a sure sign of pink rot), the presence of mould (white cottony — white rot), whether they are wet or dry, and how easily they detach from the base. Remember that many root problems start from improper watering or soil compaction, so prevention (loosening, mulching, proper crop rotation) is often more important than treatment.

Bolting (Formation of Flower Stems)

Bolting is the premature transition of the plant to flowering in the first year. Instead of forming a large bulb, the plant spends energy on producing a flower stalk and seeds. This is one of the most common and frustrating problems for gardeners.

• Incorrectly chosen variety for the region: As Rubatzky and Yamaguchi (1997) and Wien and Stützel (2020) describe in detail, different onion varieties have different requirements for photoperiod (day length) and temperature for bulb initiation. If you plant a long‑day variety under short‑day conditions or vice versa, the plant may either fail to bulb or bolt.
• Improper storage of planting material (sets): This is one of the most frequent causes. If sets are stored at +5... +15 °C (the temperature range that stimulates vernalisation), they undergo a stage preparing for flowering. When planted, such plants are very likely to bolt. Recommended storage temperatures for sets are below +5 °C or above +20 °C (Brewster, 2008; Wien and Stützel, 2020).
• Early planting in cold soil: Prolonged exposure to low positive temperatures (around +5... +10 °C) on young plants that have already passed the juvenile phase can trigger vernalisation. This is especially dangerous for bolting‑prone varieties (Rubatzky and Yamaguchi, 1997).
• Large size of sets: The larger the planting bulb, the higher the probability it will bolt. For bulb production, sets of 1.5–2 cm diameter are usually used; larger bulbs (picker‑size) are grown only for green leaves (Akhatov et al., 2013).

Why does the gardener need to know this?

Bolting almost always results in yield loss. Bulbs from such plants will be small, hard, and unsuitable for long storage. Choosing the right variety and maintaining proper storage conditions for sets are the foundations of prevention.

Does Not Form a Bulb (Fails to Bulb)

The plant continues to produce leaves and does not develop the characteristic thickening at the base. This is the complete opposite of bolting, but the result is equally disappointing.

• Insufficient day length: Each variety has a critical day length that triggers bulb formation. If the day length is shorter than this threshold, the plant will keep growing leaves indefinitely (a property used for growing green onions) (Rubatzky and Yamaguchi, 1997; Wien and Stützel, 2020).
• Excess nitrogen during bulb formation: Nitrogen stimulates green growth. If you apply too much nitrogen when onions should start bulbing, the plant continues to ”fatten” and the bulb does not set (Brewster, 2008).
• Too low or too high temperature: The bulbing process requires a specific temperature range. Under extreme heat or cold, it may be delayed (Rubatzky and Yamaguchi, 1997).
• Juvenile stage: Young plants that have not reached a certain age simply do not ”understand” the signals to bulb. They must first produce a certain number of leaves (Wien and Stützel, 2020).

Why does the gardener need to know this?

If onions do not form bulbs, it means you either chose the wrong variety for your climate zone or made mistakes in cultivation (especially with nitrogen fertilisation). This directly leads to yield reduction.

Does Not Mature (Leaves Stay Green and Do Not Lodge)

A bulb has formed, but leaves do not yellow, do not lodge, and the neck remains thick and juicy. This is a disruption of the final developmental phase—maturation and entry into dormancy.

• Excess nitrogen: As already mentioned, nitrogen delays leaf senescence. If applied too late, the plant cannot finish the growing season in time (Brewster, 2008).
• Excessive watering late in the season: High soil and air moisture stimulates new leaf growth and delays their dieback. Watering must stop 2–3 weeks before harvest (Akhatov et al., 2013).
• Cool, rainy weather: As described in research, cool summer temperatures and high humidity slow senescence. In such cases, onions simply do not have time to mature before cold weather sets in (Wien and Stützel, 2020; Brewster, 2008).
• Leaf damage in the second half of the season: As Conn (2012) and Brewster (2008) point out, severe leaf damage (e.g., by hail or downy mildew) may stimulate new leaf growth from lateral buds, disrupting the natural maturation cycle and leading to a thick neck.

Why does the gardener need to know this?

Unripe onions with green foliage and a juicy neck will not store well. They are prone to rots, and their outer scales are not properly formed. In such a situation, you can try to force‑dry the onions by laying them in windrows directly on the bed to accelerate nutrient translocation from leaves to bulb, but this does not always help (Brewster, 2008).

Summary for the Gardener:

Developmental disorders in onions result from a complex interaction of genetics (variety) and growing conditions (weather, care). The golden rule for a good harvest: choose region‑adapted varieties, strictly follow planting dates and fertiliser rates, especially nitrogen. And remember that proper storage of sets is the key to having onions that will last all winter.

STEP 1. Assess the Big Picture

Before examining individual symptoms, answer a few general questions. They will help narrow down the search.

1. What was the weather like in the last 2–3 weeks?

• Hot and dry? The problem is most likely related to lack of moisture or root overheating.
• Wet and cold? Fungal and bacterial infections, especially root rots, are highly probable.
• Sharp temperature fluctuations? These can cause stress, cracking of bulbs, and trigger diseases.

2. When did you plant the onions and what variety?

• Is the variety suitable for your region? (Long‑day or short‑day? (Rubatzky and Yamaguchi, 1997)).
• Did you plant too early or too late? (This is especially important for preventing bolting).

3. How did you care for the crop?

• How often did you water? (Especially during bulb formation).
• What fertilisers and when did you apply? (Did you over‑feed with nitrogen?).

Why is this necessary?

The answers to these questions will give you 70% of the information about the likely cause of the problem. Often, problems arise not from a single mistake but from a chain of unfavourable factors.

STEP 2. Conduct a ”Head‑to‑Toe” Examination of the Plant

Now, with a general picture in mind, move on to a detailed inspection. Use the knowledge from the previous chapters. Proceed sequentially:

1. Examine the roots (dig up 2–3 plants).

Normal: Roots light, firm, well‑branched.

Warning signs:

• Brown, rotting roots. (Likely: Fusarium, white rot, waterlogging).
• Pink or red hue. (Precise diagnosis: pink rot (Pyrenochaeta terrestris) (Conn, 2012)).
• Very few roots, thin and weak. (Possible: drought, soil compaction, pest damage).

2. Examine the bulb.

Normal: Firm, elastic, with dry outer scales.

Warning signs:

• Soft, with an unpleasant odour. (Likely: bacterial or fungal rot (Brewster, 2008; Conn, 2012)).
• White cottony mould with black dots on the base. (Precise diagnosis: white rot (Sclerotium cepivorum) (Akhatov et al., 2013)).
• Cracks. (Likely: irregular watering).
• Small despite good leaves. (Likely: root problems, unsuitable variety, high plant density).

3. Examine the neck.

Normal: Dry, thin, easily twisted.

Warning signs:

• Thick, juicy, not drying. (Likely: excess nitrogen, late watering).
• Softened, with grey or black mould. (Precise diagnosis: neck rot (Botrytis allii) or bacterial soft rot (Conn, 2012)).

4. Examine the leaves.

Normal: Green, firm, uniformly coloured.

Warning signs:

• Yellowing from tips. (Possible: lack of nitrogen or moisture, natural senescence).
• Paleness, silvery streaks. (Precise diagnosis: thrips damage (Brewster, 2008)).
• Curling, deformation. (Likely: herbicide damage, viruses, or stem nematode (Akhatov et al., 2013)).
• Spots with mould. (Likely: downy mildew or alternaria (Conn, 2012)).

STEP 3. Correlate Symptoms and Make a Preliminary Diagnosis

Now you have a set of symptoms. Correlate them with each other and with the conditions in which the onions grew (STEP 1).

Examples:

Scenario A: Leaves wilt and yellow from the tips, bulb soft, base brown. Weather was wet and cool.

• Preliminary diagnosis: Fusarium basal rot or a complex of root rots. Check the roots—they will be brown and rotted (Conn, 2012).

Scenario B: White elongated spots on leaves, bulb covered with black mould under scales. There was recent heat.

• Preliminary diagnosis: Aspergillosis (black mould). Often affects weakened or damaged bulbs in hot weather. Check storage conditions (Akhatov et al., 2013).

Scenario C: Onions are growing vigorously but not forming bulbs; leaves are succulent, dark green, neck thick. You heavily applied manure or nitrogen fertilisers.

• Preliminary diagnosis: ”Fattening” due to nitrogen over‑feeding. Stop watering and nitrogen application (Brewster, 2008).

Scenario D: Leaves covered with silvery streaks, curled, bulb small. Weather was hot and dry.

• Preliminary diagnosis: Thrips. Turn a leaf over—you will see tiny insects (Brewster, 2008; Conn, 2012).

STEP 4. Refine the Diagnosis and Take Action

If you have doubts, try to find more information on your suspected diagnosis in reliable sources (e.g., plant disease handbooks). However, for decision‑making in a home garden, a preliminary diagnosis is usually sufficient.

Important! Many diseases and pests have similar symptoms. For instance, leaf yellowing can have a dozen causes. Your task is not to make a 100% accurate diagnosis, but to identify the most likely cause and start acting.

Action plan:

1. If the problem is agronomic (watering, nutrition): Correct the mistake immediately.

2. If the problem is a pest or disease: Apply appropriate control methods (biological, folk, or chemical). In most cases, for amateur gardeners, preventive measures and timely removal of diseased plants are sufficient.

3. If the problem persists: You may be dealing with a viral disease or soil infestation (e.g., white rot). In that case, the best strategy is to remove diseased plants and avoid planting onions in that spot for several years (crop rotation) (Brewster, 2008).

Summary: Quick Diagnostic Table

For convenience, I have summarised the main symptoms and their possible causes in a table. Use it as a cheat sheet.

Final advice:

Keep a garden diary. Record which varieties you planted, when and how you cared for them, and what problems occurred in different years. Over time, this will help you better understand your garden and anticipate issues. Remember that the best defence is prevention: healthy sets, proper crop rotation, and timely care. Then your onions will reward you with abundant harvests and excellent keeping quality!