Diseases

How to recognise downy mildew?

The first signs appear on the leaves. Look for the following symptoms:

• Pale‑yellow, slightly sunken spots appear on healthy green leaves (Conn, 2012).
• The spots gradually enlarge and take on a brownish tint.
• On the underside of the leaves, especially in the morning during dew or after rain, a grey‑violet or brown‑purple coating forms – this is the sporulation of the fungus (see Fig. 1).
• Affected leaves turn yellow from the tip downwards and die off.
• In advanced cases, the disease spreads to flower stalks (scapes) and bulbs.

Important: fungal spores are spread by wind and raindrops, so infection foci quickly spread across the plot (Brewster, 2008).

Why does downy mildew develop?

The fungus Peronospora destructor favours cool, wet weather (Conn, 2012). Spores germinate at temperatures from 3 to 25 °C, but the optimum for disease development is 10–12 °C with free moisture on leaves for 2–4 hours (Brewster, 2008). Infection occurs through stomata on the leaves, and within 9–16 days after infection the fungus begins to produce new spores.

The infection survives:

• on plant debris in the soil;
• on infected bulbs and cloves used for planting;
• on volunteer garlic seedlings (Brewster, 2008).

What to do when the disease is detected?

1. Cultural measures:

• Remove and destroy (burn) severely affected plants to prevent spore spread.
• Stop overhead watering in wet weather – the fungus develops particularly actively at high humidity.
• Ensure good ventilation of the crop: maintain optimal planting density, remove weeds that create crowding and increase humidity near the ground (Torikov and Sychev, 2018).

2. Preventive treatments:

• 20–25 days before emergence or immediately after emergence, carry out preventive spraying with copper‑containing products (1% Bordeaux mixture, copper oxychloride) (Torikov and Sychev, 2018).
• During the growing season, when the first signs of disease appear, treat plants with fungicides:
• For the home garden: products based on metalaxyl (e.g., Ridomil Gold) combined with contact fungicides (mancozeb, chlorothalonil) (Brewster, 2008; Conn, 2012).
• It is important to alternate products with different modes of action to prevent the development of fungicide resistance (Brewster, 2008).

3. Choice of varieties and planting material:

• Use healthy planting material from reliable suppliers.
• Prefer varieties resistant to downy mildew (if recommended for your region).
• Before planting, warm garlic cloves for 4 hours at 41 °C – this heat treatment can eliminate latent infection (Brewster, 2008).

What NOT to do:

• Do not leave affected leaves and plant debris on the bed – they are a source of infection for the next year.
• Do not use cloves from diseased plants for planting.
• Do not overuse nitrogen fertilisers – excess nitrogen makes leaves more succulent and susceptible to infection (Torikov and Sychev, 2018; Brewster, 2008).

Remember the key points:

Downy mildew is easier to prevent than to cure. The main preventive measures are:

• crop rotation (return garlic to the same place no earlier than after 3–4 years) (Torikov and Sychev, 2018);
• destruction of plant debris;
• planting healthy cloves;
• preventive treatments with copper‑based products at the start of the growing season.

With timely detection and a proper approach, the disease can be stopped and the crop saved.

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How to recognise Fusarium rot?

The disease develops gradually, and symptoms depend on the plant’s growth stage.

In the field:

• First signs – yellowing, curling and drying of leaf tips (Conn, 2012).
• Affected leaves turn yellow from the tip downwards, then brown and die. Wilting may start on individual leaves and then engulf the whole plant.
• Roots become dark brown, soften, rot, and sometimes take on a pinkish tint (Brewster, 2008; Engeland, 1991).
• A white or pinkish mycelial coating appears on the basal plate (the base of the bulb) – this is the fungal mycelium (Conn, 2012).
• Diseased plants are easily pulled out of the soil due to destruction of the root system.

On bulbs during storage:

• A light‑brown or pinkish rot develops on the basal plate and gradually moves upwards through the fleshy scales (Brewster, 2008; Conn, 2012).
• Affected scales become watery, soft, and the bulb dries out and turns into a "mummy".
• A white or pink coating may appear between the scales.

Important: Fusarium rot is often confused with other rots. Its distinguishing feature is that the attack starts from the base and roots, not from the neck or outer scales.

Why does Fusarium rot develop?

The fungus Fusarium is a soil inhabitant. It survives as chlamydospores (resistant resting spores) and can live in soil for several years even in the absence of host plants (Brewster, 2008; Conn, 2012).

Factors favouring disease development:

• Warm weather: the optimum for fungal growth is 28–32 °C (Brewster, 2008). In cool weather the disease develops slowly.
• Excessive soil moisture – especially when combined with high temperature.
• Damage to roots and basal plates – caused by insects (onion fly, root mite), nematodes, as well as careless weeding or mechanical injuries (Engeland, 1991; Torikov and Sychev, 2018).
• Low‑organic‑matter and heavy clay soils – roots develop weaker and are more easily infected (Engeland, 1991).
• Dense plantings – reduce ventilation and promote waterlogging in the root zone.

What to do when Fusarium rot is detected?

Unfortunately, there are no effective chemical treatments for already diseased plants in the field. Control of Fusarium rot is primarily prevention and good agronomic practice.

1. Crop rotation:

• Return garlic to the same place no earlier than after 4–5 years (6 years recommended) (Brewster, 2008; Torikov and Sychev, 2018).
• Best preceding crops – legumes, cucurbits, early cabbage, green manures (Engeland, 1991). Do not plant garlic after onions, potatoes or other crops susceptible to Fusarium.

2. Selection and preparation of planting material:

• Use only healthy, firm, spot‑free and damage‑free cloves from reliable suppliers.
• Before planting, discard all cloves showing signs of browning or softening of the base.
• As a preventive measure, treat cloves in a fungicide solution (e.g., based on tebuconazole or carboxin) according to the label – this reduces the risk of infection (Brewster, 2008).
• Heating planting material at 40–45 °C for several hours partially suppresses infection.

3. Soil improvement and care:

• Incorporate well‑rotted compost or manure – organic matter improves soil structure, promotes beneficial microflora and strengthens plant immunity (Torikov and Sychev, 2018; Engeland, 1991).
• Avoid excessive nitrogen fertilisation – it makes tissues loose and more susceptible to infection.
• Ensure good drainage; do not allow water to stagnate in the root zone.
• Remove weeds and diseased plants in a timely manner.

4. Pest control:

• Control populations of onion fly, root mite and nematodes – their damage opens the door to Fusarium (Engeland, 1991; Conn, 2012).

5. Harvest and storage:

• Harvest garlic only after full maturity and after drying in the field (in dry weather). Do not harvest wet bulbs.
• Thoroughly dry bulbs in a draught for 2–3 weeks, remove soil residues and infected specimens.
• Store garlic at 0–2 °C and 65–70% humidity – under these conditions Fusarium development is greatly slowed (Brewster, 2008; Engeland, 1991).

What NOT to do:

• Do not plant garlic after onions, potatoes or other crops susceptible to Fusarium.
• Do not use cloves with signs of rot for planting – even one diseased clove can become a source of infection for the whole bed.
• Do not overwater the soil and do not allow a crust to form on the surface.
• Do not delay harvesting – overripe bulbs with cracked scales are more susceptible to infection.

Remember the key points:

Fusarium rot is a soil‑borne disease that is difficult to cure but can be prevented. Key measures:

• long crop rotation (4–6 years);
• planting healthy, treated cloves;
• improving drainage and soil fertility;
• protecting roots from damage;
• proper harvesting and storage.

If Fusarium appears on your plot every year – it is a sign that you need to change the planting location and definitely improve the soil with green manures and organic fertilisers.

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How to recognise white rot?

Symptoms appear gradually and are often detected at later stages, when it is impossible to save the plants.

In the field:

• First signs – yellowing and wilting of leaves, starting with the lower, older ones. Leaves turn yellow from tip to base, become flaccid and lodge (Conn, 2012; Engeland, 1991).
• Roots rot, turn brown and disintegrate; the plant is easily pulled out of the soil.
• A fluffy white coating – fungal mycelium – appears on the basal plate and at the base of the bulb (see Fig. 2). Over time, the coating becomes denser.
• In the white mycelium, small (0.5–1 mm) black or dark brown round structures – sclerotia – form. These are resting structures of the fungus, resembling mustard seeds (Engeland, 1991; Conn, 2012).
• The affected bulb becomes soft, watery, then rots and mummifies.
• The disease appears in patches: in the plot you can see bare spots with yellowed and dead plants, which gradually expand (Conn, 2012).

On bulbs during storage:

• An externally healthy bulb may start to rot from the inside: tissues become soft, watery, and white coating and sclerotia appear.
• Infected bulbs rot quickly, infecting neighbouring ones.

Important: white rot can easily be confused with Fusarium or bacterial rot. The distinguishing feature is the presence of white fluffy mycelium and black sclerotia on the basal plate and at the base of the bulb (Conn, 2012; Brewster, 2008).

Why does white rot develop?

The fungus Sclerotium cepivorum is a specialised pathogen of Alliums. Its sclerotia remain viable in the soil for more than 10 years, and according to some data up to 20 years or more (Brewster, 2008; Engeland, 1991).

Factors favouring disease development:

• Moderately cool weather: the optimum for sclerotial germination and mycelial growth is 14–18 °C (Brewster, 2008). Above 25 °C the disease slows down, and at 30 °C and above it stops (Engeland, 1991).
• Moist soil: high soil moisture is necessary for sclerotial germination and root infection.
• Root exudates of garlic and other Alliums: sclerotia germinate only in the presence of specific volatile compounds (containing sulphur) released by the roots of Allium plants (Brewster, 2008; Conn, 2012). This explains why the disease affects only Allium crops.
• Dense plantings and poor ventilation – increase humidity in the root zone.
• Damage to roots and basal plates by insects, nematodes or mechanical cultivation facilitates fungal entry.

What to do when white rot is detected?

White rot is almost impossible to treat in the field. All measures are aimed at preventing introduction and reducing the number of sclerotia in the soil.

1. Quarantine and prevention of introduction:

• Never plant garlic, onions or other Alliums on plots where white rot has previously occurred (Engeland, 1991; Brewster, 2008). Return to the same place is possible no earlier than after 10–15 years.
• Purchase planting material (cloves, bulbils) only from reliable producers with phytosanitary certificates (especially if you live in a region with developed onion growing) (Engeland, 1991; Brewster, 2008).
• Do not bring soil from neighbours' gardens onto your plot, do not use untested compost that may contain Allium plant residues.
• Thoroughly clean shoes and garden tools after working elsewhere, especially where onions or garlic were grown.

2. Cultural measures when disease appears:

• When first signs are detected, immediately remove and burn all affected plants together with the surrounding clump of soil (Engeland, 1991). Do this carefully to avoid scattering sclerotia around the plot.
• Treat the affected spots (where diseased plants grew) with hydrated lime or a specific fungicide (containing tebuconazole) according to the instructions to suppress sclerotia in the soil (Brewster, 2008).
• Do not put plant residues (especially Allium) into compost – sclerotia can survive insufficient heating.

3. Methods to reduce the infectious background in soil (for enthusiasts with caution):

• Solarisation: in hot regions (with temperatures above 35 °C), cover moist soil with transparent polyethylene film for 6–8 weeks during summer. This can kill sclerotia in the topsoil (Brewster, 2008; Conn, 2012). This method is effective only under intense sunlight.
• Biological methods (experimental): applying products based on the antagonist fungus Trichoderma or bacteria (e.g., Bacillus subtilis) can reduce sclerotial numbers, but the effect is variable and depends on soil type and weather (Brewster, 2008). They have not yet been widely adopted in home gardening.
• Induced germination of sclerotia: applying products based on garlic oil or diallyl disulfide (a component of garlic essential oil) to soil can stimulate sclerotial germination in the absence of host plants, after which the fungus dies. However, this method requires precise dosage and gives variable results (Brewster, 2008; Conn, 2012).

4. Crop rotation and green manures:

• In infected regions, plan crop rotation so that Alliums (garlic, onion, leek, shallot) are returned to the plot as rarely as possible. Plant them in the same place no more than once every 5–7 years, and preferably even less often.
• Green manures from cereals (oats, rye, wheat) and legumes (lupin, vetch) give good results, as they are not hosts for Sclerotium cepivorum and can help improve soil health.

What NOT to do:

• Absolutely do not plant garlic after onions, leeks, shallots or other Alliums, even if they were healthy – sclerotia may have persisted in the soil.
• Do not use the infected plot for growing Alliums for at least 10 years.
• Do not leave diseased plants and their parts on the bed – sclerotia will remain in the soil.
• Do not compost diseased bulbs and leaves – sclerotia survive incomplete composting.
• Do not rely on quick "miracle cures" – white rot is not cured by ordinary foliar fungicide sprays.

Remember the key points:

White rot is a sentence for the plot for many years. The only reliable way to avoid it is to prevent introduction:

• buy healthy cloves from trusted suppliers;
• observe strict quarantine;
• at the first signs – immediate removal and burning of plants with soil;
• do not return garlic to the infected site for 10–15 years.

If white rot appears – switch to growing other (non‑Allium) crops on that plot, and allocate a new bed for garlic, preferably at a considerable distance.

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How to recognise neck rot?

The disease often remains unnoticed in the field and appears only 2–3 months after storage (Brewster, 2008).

In the field:

• Symptoms are almost invisible in the field; plants may look healthy until harvest.
• Occasionally, small pale spots with a greyish or brownish tinge appear on leaves and stems, especially in wet weather (Conn, 2012).
• Under severe infection, premature yellowing and lodging of leaves may occur, but this is rare.

On bulbs during storage:

• The first sign is softening of tissues at the neck (where the tops were cut). The tissue becomes watery, light brown or greyish (Conn, 2012).
• The rot gradually spreads deeper into the bulb, affecting fleshy scales. Affected scales become brown, watery and soften (Brewster, 2008).
• On the surface of affected tissues, especially between scales, a grey or grey‑brown fluffy coating – fungal sporulation – appears (Conn, 2012).
• Later, small black sclerotia (resting structures) of 1–3 mm may form on the bulb and at the neck (Brewster, 2008; Conn, 2012).
• In advanced cases, the bulb completely rots, turning into a dry mummified mass, or softens and emits an unpleasant odour (especially if secondary bacterial infection is present).

Important: neck rot can easily be confused with bacterial soft rot. The distinguishing feature of Botrytis rot is the presence of grey fluffy coating and black sclerotia, and the characteristic onset of rot at the neck, not from the base or side (Conn, 2012; Brewster, 2008).

Why does neck rot develop?

Fungi of the genus Botrytis are typical saprotrophs that live on decaying plant debris and easily enter weakened or damaged tissues (Brewster, 2008).

Factors favouring disease development:

• Infected seed and planting material: the fungus can survive on the surface of seeds and in the tissues of cloves (Brewster, 2008). This is one of the main routes of spread.
• High humidity during the growing season and maturation: rainy weather, frequent dews, dense plantings, poor ventilation favour infection of leaves and the neck (Conn, 2012; Torikov and Sychev, 2018).
• Late harvest and wet bulbs: if garlic is harvested in wet weather or after the tops have completely lodged and begun to rot, the fungus easily enters through the neck (Engeland, 1991).
• Damage during harvest: careless lifting, cutting the tops too close to the bulb, or mechanical injuries create entry points for infection (Brewster, 2008; Engeland, 1991).
• Insufficient drying of bulbs before storage: wet bulbs with an undried neck are especially vulnerable (Brewster, 2008).
• Storage at high humidity and temperature: the optimum for fungal growth is 22–23 °C, but it can develop at lower temperatures (5–25 °C), especially under high humidity (Conn, 2012; Brewster, 2008). At low humidity (65–70%) and temperature around 0 °C, the fungus develops very slowly.

What to do when neck rot is detected?

During the growing season:

• Inspect plantings carefully, especially in wet weather. If spots on leaves are found, carry out a preventive treatment with fungicides recommended for Botrytis control (e.g., based on iprodione or fludioxonil), strictly following the label (Brewster, 2008). In home gardening, biological products based on hay bacillus (Bacillus subtilis) or the antagonist fungus Trichoderma are acceptable (Conn, 2012).

During harvest and preparation for storage:

• Harvest garlic only in dry weather, when the tops have yellowed and lodged, but are not yet completely dry and rotting (Engeland, 1991).
• Do not cut the tops too short: leave a stub of 3–5 cm to reduce the risk of infection through the neck (Engeland, 1991; Torikov and Sychev, 2018).
• Thoroughly dry the bulbs after harvest for 2–3 weeks in a warm, well‑ventilated place (temperature 25–30 °C, humidity 60–70%). During this time, the neck should dry completely (Brewster, 2008; Engeland, 1991).
• Before storage, discard all bulbs showing softening of the neck, spots or damage (Conn, 2012). Even one diseased bulb can become a source of infection for the entire harvest.

During storage:

• Store garlic at 0–2 °C (or slightly higher, but not above +5 °C) and relative humidity of 65–70% (Brewster, 2008; Engeland, 1991). These conditions slow fungal development.
• Ensure good ventilation to prevent condensation on bulbs (Conn, 2012).
• Regularly sort stored garlic and remove any suspect bulbs before rot spreads to neighbours.

Prevention – the basis of control

Neck rot is a disease that is easier to prevent than to cure. Main preventive measures:

1. Healthy planting material:

• Use only healthy cloves from reliable producers. Avoid planting garlic grown from infected bulbs (Brewster, 2008).
• When buying seed, pay attention to cleanliness and absence of mould.

2. Crop rotation and agronomy:

• Follow crop rotation: do not plant garlic after onions or other Allium crops for at least 3–4 years (Torikov and Sychev, 2018).
• Remove plant debris (tops, leaves) after harvest, as sclerotia may survive on them (Conn, 2012).
• Avoid dense plantings and excessive nitrogen nutrition – this leads to succulent, loose tissues that are more susceptible to infection (Engeland, 1991).

3. Proper harvest and drying:

• Harvest garlic in dry weather, avoiding over‑ripening and lodging of tops followed by rotting.
• Thoroughly dry bulbs before storage – this is the most important step in preventing neck rot (Brewster, 2008).

4. Pre‑storage treatment (additional):

• Some gardeners warm garlic for 8–10 hours at 40–42 °C before storage to suppress surface infection, but this method requires care (Engeland, 1991).
• In commercial production, bulbs are treated with fungicides (e.g., based on tebuconazole), but this is not always available or advisable for home gardeners (Brewster, 2008).

What NOT to do:

• Do not harvest garlic in wet weather or immediately after rain.
• Do not cut the tops too short – leave a stub.
• Do not store insufficiently dried bulbs.
• Do not store garlic at high humidity (above 75%) – this promotes fungal development.
• Do not store garlic together with other vegetables (especially potatoes and cabbage) that may be sources of infection.

Remember the key points:

Neck rot is a typical storage disease that begins in the field but manifests in storage. Main protective measures:

• growing healthy planting material;
• proper drying and curing of the neck after harvest;
• careful cutting of tops leaving a stub;
• storage at low temperature (0–2 °C) and moderate humidity (65–70%);
• regular inspection and removal of diseased bulbs.

If you notice grey coating and black sclerotia on bulbs – immediately isolate and destroy them to save the rest of the crop.

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How to recognise bacterial rots?

Symptoms depend on the bacterial species and the site of infection, but there are common signs characteristic of all bacterial rots.

In the field:

• Leaves turn yellow, wilt, become flaccid and lodge. Often the leaves can be "pulled out" from the bulb – they easily separate from the neck (Conn, 2012).
• At the base of leaves and in the neck, watery, softened tissue appears, which quickly darkens (from light yellow to brown or black).
• Bulbs in the field may look healthy externally, but when cut, watery, softened scales with an unpleasant odour are visible.

On bulbs during storage:

• Soft bacterial rot (caused by Pectobacterium carotovorum, Dickeya chrysanthemi): affected scales become watery, light yellow or greyish, softening to a mushy consistency. On pressing, a foul‑smelling liquid oozes from the neck (Brewster, 2008; Conn, 2012).
• Slippery skin (caused by Burkholderia gladioli pv. alliicola): rot starts from the inner scales, which become slimy, watery and easily separate from adjacent ones. When squeezed, the core of the bulb may "slip out" through the neck – hence the name (Conn, 2012; Brewster, 2008).
• Sour skin (caused by Burkholderia cepacia): outer scales are affected, becoming yellowish‑brown and slimy. The bulb acquires a sharp vinegar‑like or sour odour due to the activity of accompanying microorganisms (yeasts) (Conn, 2012; Brewster, 2008).
• Center rot (caused by Pantoea ananatis and @bio[P. agglomerans]): rot starts in the central leaves, which become pale, watery, then brown. A strong unpleasant odour emanates from affected plants (Brewster, 2008; Conn, 2012).

Important: bacterial rots often develop secondarily on bulbs already affected by fungal diseases (Fusarium, white rot) or damaged by pests (onion fly, nematodes) (Brewster, 2008; Engeland, 1991). The distinguishing feature of bacterial infection is watery, often malodorous rot that spreads rapidly through the bulb.

Why do bacterial rots develop?

Bacteria causing rots are widespread in nature: they live in soil, on plant debris, on plant surfaces and even in the intestines of insect pests (Brewster, 2008; Conn, 2012).

Factors favouring development:

• Warm and humid weather: most bacteria multiply actively at 20–30 °C, especially under high humidity and with free moisture on leaves and bulbs (Conn, 2012; Brewster, 2008).
• Tissue damage: bacteria enter plants only through wounds – mechanical injuries during weeding, harvesting, topping, and through damage caused by pests (onion fly, thrips, nematodes) (Brewster, 2008; Conn, 2012).
• Infected planting material: many bacteria can survive on the surface of cloves and be carried with them to new beds (Brewster, 2008; Conn, 2012).
• Excess nitrogen fertiliser: succulent, loose tissues are more susceptible to infection (Conn, 2012; Torikov and Sychev, 2018).
• Improper storage: high humidity, poor ventilation, damage to bulbs during storage create ideal conditions for bacterial growth (Brewster, 2008; Engeland, 1991).

What to do when bacterial rots are detected?

Unlike many fungal diseases, bacterial infections cannot be treated in the field or in storage. All measures are aimed at prevention and preventing spread.

1. In the field:

• When plants with signs of bacterial rot (wilting, softening of the neck) are detected, immediately remove and burn them together with the surrounding clump of soil (Conn, 2012; Engeland, 1991). Do not leave diseased plants on the bed.
• Stop overhead watering, especially in hot weather, to reduce the spread of bacteria with water droplets (Conn, 2012).
• Treat plantings with copper‑based products (Bordeaux mixture, copper oxychloride) as a preventive measure, especially before bulb formation begins. Copper suppresses many bacteria (Conn, 2012; Brewster, 2008). However, remember that bacteria can develop resistance to copper, so alternate products with different active ingredients if possible.

2. During harvest and preparation for storage:

• Harvest garlic only in dry weather. Wet bulbs are an ideal medium for bacteria (Engeland, 1991).
• Avoid damage during lifting: use a fork or spade, gently lift the bulbs, do not pull on the tops (Engeland, 1991).
• Cut the tops with a sharp, clean tool, leaving a stub of 3–5 cm. Do not damage the neck and upper part of the bulb (Engeland, 1991).
• Thoroughly dry bulbs for 2–3 weeks at 25–30 °C with good ventilation. This helps heal small wounds and prevents bacterial development (Brewster, 2008; Engeland, 1991).
• Discard all bulbs with any mechanical damage, cracks, dents or signs of softening – they will be a source of infection in storage (Conn, 2012).

3. In storage:

• Store garlic at low temperature (0–2 °C) and relative humidity of 65–70% (Brewster, 2008; Engeland, 1991). Under these conditions bacteria develop very slowly.
• Ensure good ventilation to prevent condensation on bulbs.
• Regularly inspect stored garlic and immediately remove any bulbs showing signs of rot to prevent infection of neighbouring ones (Conn, 2012).

4. Pest control:

• Control onion fly, thrips, root mite and nematodes – they not only damage tissues but can also carry bacteria (Brewster, 2008; Engeland, 1991). Use approved insecticides and cultural methods (crop rotation, removal of plant debris).

What NOT to do:

• Do not harvest garlic in wet weather or immediately after watering.
• Do not cut tops too short and do not damage the neck.
• Do not store insufficiently dried, dirty or damaged bulbs.
• Do not allow storage humidity to rise above 75% – this promotes bacterial growth.
• Do not use cloves from bulbs that showed signs of rot for planting – bacteria may persist inside.

Remember the key points:

Bacterial rots of garlic are diseases of damaged and over‑moistened tissues. Main preventive measures:

• growing healthy planting material;
• protecting plants from damage (pests and mechanical);
• harvesting only in dry weather;
• thorough drying of bulbs before storage;
• storage at low temperature (0–2 °C) and moderate humidity (65–70%);
• regular inspection and removal of diseased bulbs.

Bacteria are constant companions of soil and plant debris, so it is impossible to avoid their presence completely. But by creating unfavourable conditions for them (dryness, low temperature, absence of wounds), you will preserve the harvest.

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How to recognise black mould?

Symptoms usually appear on bulbs during storage, but can also start developing in the field – on overripe, damaged or lodged plants (Conn, 2012).

In the field:

• The disease is rarely seen in the field, mostly on old, wilting or damaged leaves and stems.
• Dark, almost black spots resembling sooty coating may appear on the neck and upper part of the bulb (Conn, 2012).
• In wet and warm weather, fungal spores spread rapidly across the plot, settling on weakened plants.

On bulbs during storage:

• First signs – black, sooty or velvety coating on the outer dry scales, especially at the neck or on sites of mechanical damage (Conn, 2012).
• Often the coating forms along the veins of dry scales or between them, not immediately penetrating the bulb (Brewster, 2008).
• The tissues under the coating become watery, then brown, soften and dry out (Conn, 2012).
• At high humidity and temperature, the black coating spreads to inner fleshy scales, and the bulb completely rots and turns into a dry black mass (Brewster, 2008).
• Bacterial rots often follow black mould, because the fungus breaks down protective tissues and opens the way for bacteria (Brewster, 2008; Conn, 2012).

Important: black mould is easily distinguished from other diseases by the characteristic black velvety coating resembling soot or ash. Unlike neck rot (grey coating) or penicillium rot (greenish‑blue coating), the coating of Aspergillus niger is always intensely black (Conn, 2012; Brewster, 2008).

Why does black mould develop?

The fungus Aspergillus niger is a cosmopolitan; it occurs everywhere: in soil, on plant debris, in the air, on surfaces. Its spores are very resistant and easily spread by wind, water and insects (Brewster, 2008; Conn, 2012).

Factors favouring development:

• High temperature: the optimum for fungal growth is 28–34 °C, but it can develop at temperatures from 10 to 40 °C (Brewster, 2008; Conn, 2012). Therefore, black mould is especially dangerous in warm regions and in years with hot summers.
• Humidity: high humidity (80–90% and above) is necessary for spore germination. The fungus develops particularly actively when condensation forms on bulbs (Conn, 2012).
• Damage to bulbs: the fungus penetrates through any mechanical injuries – scratches, cracks, abrasions, cut sites, pest damage (Brewster, 2008; Engeland, 1991).
• Over‑ripeness of bulbs: overripe bulbs have cracked scales, which facilitates infection (Engeland, 1991).
• Improper drying: insufficient drying or, conversely, excessively long and hot drying (especially above 35 °C) can weaken bulbs and promote fungal growth (Brewster, 2008).
• Storage at elevated temperature: if the storage is not sufficiently cooled and the temperature stays above 20 °C, the risk of black mould increases sharply (Conn, 2012).

What to do when black mould is detected?

Affected bulbs are practically impossible to cure, so efforts should be directed at prevention and preventing spread.

1. In the field and during harvest:

• Harvest garlic in good time, avoiding over‑ripening and cracking of scales (Engeland, 1991).
• Harvest only in dry weather so that bulbs are not wet (Engeland, 1991).
• Handle bulbs gently during lifting and topping, avoid damage (Engeland, 1991; Conn, 2012).
• Cut the tops leaving a stub of 3–5 cm to avoid damaging the neck (Engeland, 1991).
• Remove all plant debris from the plot – the fungus can survive on them (Conn, 2012).

2. Drying and preparation for storage:

• Thoroughly dry bulbs after harvest. Optimal conditions: temperature 25–30 °C, good ventilation, humidity 60–70%, duration 2–3 weeks (Brewster, 2008; Engeland, 1991).
• Do not overheat during drying: temperatures above 35 °C can weaken bulbs and promote fungal growth (Brewster, 2008).
• Before storage, carefully inspect each bulb. Discard all specimens with black coating, mechanical damage, cracks and signs of softening (Conn, 2012). Even a small focus can become a source of infection.

3. Storage:

• Store garlic at low temperature (0–2 °C) and relative humidity of 65–70% (Brewster, 2008; Engeland, 1991). Under these conditions, the fungus practically does not develop.
• Ensure good ventilation in the storage to avoid condensation on bulbs (Conn, 2012).
• Regularly inspect stored garlic and immediately remove all bulbs showing signs of black mould to prevent infection of neighbours (Conn, 2012).
• When storing in warm conditions (e.g., in southern regions), additionally treat bulbs before storage. Some gardeners use dusting with chalk or ash – this creates an unfavourable environment for the fungus and reduces surface moisture on bulbs (Engeland, 1991; Conn, 2012). Treatment with sulphur products (colloidal sulphur) before storage is also acceptable – sulphur has fungicidal action (Engeland, 1991).

4. Pest control:

• Control populations of thrips, mites and other pests that damage bulbs and create entry points for infection (Engeland, 1991; Conn, 2012).

What NOT to do:

• Do not harvest garlic in wet weather or immediately after watering.
• Do not damage bulbs during harvest and topping.
• Do not cut the tops too short – leave a stub.
• Do not store insufficiently dried, dirty or damaged bulbs.
• Do not store garlic at high humidity (above 75%) – this promotes black mould.
• Do not allow the storage temperature to rise above 15–20 °C during storage, unless you are using special low‑humidity regimes.

Remember the key points:

Black mould is a disease of warm storage and damaged tissues. It is not dangerous for healthy, properly dried bulbs stored at low temperature. Main protective measures:

• timely harvest and careful handling;
• thorough drying at 25–30 °C without overheating;
• discarding all damaged and suspect bulbs;
• storage at low temperature (0–2 °C) and moderate humidity (65–70%);
• regular inspection and removal of diseased bulbs.

If black mould appears in storage, it is a signal that storage conditions are compromised: too humid, too warm, or insufficiently dried/damaged bulbs were stored.

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How to recognise penicillium rot?

Symptoms may appear both on stored bulbs and on planting material – cloves that begin to rot in the soil or during storage.

On bulbs during storage:

• On outer scales or at damage sites, pale‑yellow, watery spots appear, gradually enlarging (Conn, 2012).
• Affected tissues soften, become watery, light brown or greyish (Conn, 2012).
• On the surface of the spots and around them, a characteristic blue‑green or bluish‑green fluffy coating – fungal sporulation – forms (Conn, 2012; Engeland, 1991).
• Over time, the bulb may completely rot, turning into a soft or dry mass permeated with green coating. Often there is a musty, earthy odour (Conn, 2012).
• Unlike black mould, the coating of penicillium rot has a characteristic greenish or bluish tint, not black (Conn, 2012).

On planting cloves:

• Cloves infected with penicillium rot become soft during storage, and greenish spots and coating appear on them (Engeland, 1991).
• When planted, such cloves often do not germinate or produce weak, yellowing sprouts. Young plants may die, and the clove in the soil becomes covered with blue‑green mould (Engeland, 1991; Brewster, 2008).

Important: penicillium rot often infects cloves at damage sites – during splitting of bulbs into cloves, mechanical injuries, pest damage. The distinguishing feature is blue‑green coating and musty odour, and the onset of rot from injury sites (Engeland, 1991; Conn, 2012).

Why does penicillium rot develop?

Fungi of the genus Penicillium are widespread in nature: they live in soil, on plant debris, on plant surfaces. Spores are easily spread by wind and water (Brewster, 2008; Engeland, 1991).

Factors favouring development:

• Tissue damage: the fungus penetrates through any mechanical injuries – scratches, abrasions, cracks, points where cloves separate from the basal plate (Engeland, 1991; Conn, 2012). This is why penicillium rot is often called "splitting disease" – it actively develops on cloves damaged during "breaking down" bulbs for planting (Engeland, 1991).
• High humidity: spore germination and fungal growth require air humidity above 75–80% (Conn, 2012; Brewster, 2008).
• Temperature: the optimum for most Penicillium species is 21–25 °C, but some can grow at lower temperatures (Conn, 2012; Brewster, 2008). Importantly, Penicillium allii is almost inactive at temperatures below 15 °C, so cold storage effectively prevents the disease (Brewster, 2008).
• Long‑term storage: over time, bulbs weaken and the risk of penicillium rot increases (Engeland, 1991).
• Infected planting material: if during splitting at least one clove was infected with penicillium rot, its spores can infect all other cloves in the batch (Engeland, 1991).

What to do when penicillium rot is detected?

Affected cloves and bulbs cannot be cured. All measures should be aimed at prevention and preventing spread.

1. During preparation of planting material:

• For planting, use only firm, solid, healthy bulbs without signs of softness, spots or mould (Engeland, 1991).
• Split bulbs into cloves no earlier than 1–2 days before planting to avoid long‑term storage of damaged cloves (Engeland, 1991; Brewster, 2008).
• Work carefully: try not to damage the basal plate and the skins of cloves during splitting. Damaged cloves are the most vulnerable (Engeland, 1991).
• During splitting, carefully inspect each clove: all specimens with greenish coating, softness or spots should be immediately discarded (Engeland, 1991).
• Do not break down bulbs with obvious signs of rot among healthy bulbs – spores can spread to the entire planting material (Engeland, 1991). Such bulbs should be discarded whole.

2. Preventive treatment of cloves (for enthusiasts):

• Before planting, it is useful to dust cloves with wood ash or ground chalk – this creates an unfavourable environment for the fungus and dries the surface (Engeland, 1991).
• In commercial production, cloves are treated with fungicides (e.g., based on thiram, carboxin or tebuconazole), but this is not always available for home gardeners (Brewster, 2008; Conn, 2012). If you use fungicides, strictly follow the label.
• In arid regions, abundant watering after planting may suppress fungal development, as Penicillium is less active under high soil moisture (but this does not apply to regions with excessive moisture) (Engeland, 1991).

3. During harvest and preparation for storage:

• Harvest garlic only in dry weather and carefully, avoiding damage (Engeland, 1991).
• Thoroughly dry bulbs after harvest for 2–3 weeks at 25–30 °C with good ventilation (Engeland, 1991; Brewster, 2008).
• Before storage, discard all bulbs with mechanical damage, cracks, dents and signs of softening – they will be the first victims of penicillium rot (Conn, 2012; Engeland, 1991).

4. Storage:

• Store garlic at low temperature (0–2 °C) and relative humidity of 65–70% (Brewster, 2008; Engeland, 1991). This is the main condition preventing penicillium rot, because below 15 °C the fungus is almost inactive (Brewster, 2008).
• Ensure good ventilation in the storage to avoid condensation (Conn, 2012).
• Regularly inspect stored garlic and immediately remove any bulbs showing signs of mould (Conn, 2012).

5. Crop rotation and sanitation:

• Follow crop rotation – do not plant garlic after onions or other Alliums for at least 3–4 years (Torikov and Sychev, 2018).
• Remove all plant debris from the plot, as fungal spores can survive on them (Conn, 2012).

What NOT to do:

• Do not store planting material in a warm and humid place – this is ideal for penicillium rot.
• Do not split bulbs into cloves long before planting – damaged cloves are more quickly infected.
• Do not use cloves with signs of mould or softening for planting – even one diseased clove can infect the whole batch.
• Do not store insufficiently dried or damaged bulbs.
• Do not store garlic at humidity above 75% – this promotes mould growth.

Remember the key points:

Penicillium rot is a disease of damaged cloves and warm, moist storage. Main protective measures:

• planting only healthy, firm bulbs;
• careful splitting into cloves immediately before planting;
• discarding all damaged and suspect cloves;
• thorough drying of bulbs before storage;
• storage at low temperature (0–2 °C) and moderate humidity (65–70%);
• regular inspection and removal of diseased bulbs.

Penicillium rot is especially dangerous for planting material, because infected cloves do not produce healthy plants. If you see green mould on stored bulbs – immediately isolate and destroy them. For planting, use only cloves from absolutely healthy bulbs.

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Which diseases manifest during storage?

Almost all major garlic diseases can become active in storage. Briefly recall their signs:

Important: in storage, diseases rarely occur singly. Often one bulb shows mould, bacterial rot, and Fusarium signs together – so‑called mixed infections (Brewster, 2008).

Why do diseases develop specifically during storage?

For any infection to develop, three conditions are needed: a source of infection, susceptible tissues and favourable environmental conditions (temperature, humidity, air availability) (Brewster, 2008). In storage, all these factors may coincide:

1. The source of infection is already inside or on the surface of bulbs, having arrived from soil, plant debris, through damage or with planting material (Brewster, 2008; Conn, 2012).

2. Susceptible tissues – weakened, damaged, insufficiently dried or overripe bulbs. Pathogens easily enter them and begin to multiply actively (Engeland, 1991).

3. Favourable conditions – these are:

• Temperature above the optimum for storage (above 5–10 °C) – accelerates development of most fungi and bacteria (Brewster, 2008).
• High humidity (above 75–80%) – promotes spore germination and spread of infections (Conn, 2012; Engeland, 1991).
• Poor ventilation – leads to condensation and stagnant zones with high humidity (Conn, 2012).

How to prevent storage diseases?

Prevention of storage diseases is a single process that starts with choosing a planting site and ends with regular inspection in storage.

1. Proper agronomy in the field:

• Follow crop rotation: do not plant garlic in the same place more often than once every 3–4 years, and preferably less often. This reduces the accumulation of Fusarium, white rot and bacterial pathogens in soil (Torikov and Sychev, 2018; Brewster, 2008).
• Use healthy planting material – discard cloves with spots, damage, mould signs (Engeland, 1991; UMass Extension).
• Apply fertilisers in a balanced way: excess nitrogen makes tissues loose and susceptible to infections (Conn, 2012; Engeland, 1991).
• Control pests (onion fly, thrips, mites, nematodes) – they cause micro‑injuries through which pathogens enter (Engeland, 1991; Brewster, 2008).

2. Timely and proper harvest:

• Harvest garlic only in dry weather, when the tops have yellowed and lodged, but are not completely dry and not yet rotting (Engeland, 1991).
• Do not delay harvest: overripe bulbs with cracked scales are more vulnerable (Engeland, 1991).
• Lift carefully, using a fork or spade, to avoid damaging bulbs and tearing off the basal plate (Engeland, 1991).
• Cut the tops leaving a stub of 3–5 cm. Do not cut too short – this opens the neck to infection (Engeland, 1991; Brewster, 2008).

3. Thorough drying – the most important step:

• After harvest, spread garlic in a single layer in a warm, well‑ventilated place (under cover, in the attic, in a shed) for 2–3 weeks (Engeland, 1991).
• Optimal drying temperature: 25–30 °C, air humidity: 60–70% (Brewster, 2008). Do not allow overheating above 35 °C or direct sunlight – they can cause "steaming" and burns (Engeland, 1991).
• During drying, the neck should dry completely, become firm and closed. Bulbs should lose excess moisture and become hard, with rustling scales (Engeland, 1991; Brewster, 2008).

4. Sorting before storage:

After drying, carefully inspect each bulb. Discard all specimens with:

• mechanical damage (cracks, dents, abrasions);
• signs of rot, softening, mould;
• too thin or damaged outer scales;
• an undried or soft neck.

Even one diseased bulb can infect neighbours (Conn, 2012; Engeland, 1991).

5. Optimal storage conditions:

• Temperature: 0–2 °C (not above +5 °C) for long‑term storage (Engeland, 1991; Brewster, 2008). At this temperature, development of most pathogens is greatly slowed or stopped.
• Relative humidity: 65–70%. This is low enough to prevent spore germination and mould growth, but not so low that bulbs dry out (Brewster, 2008; Conn, 2012).
• Ventilation: essential. Ensure air circulation to avoid stagnant zones and condensation (Conn, 2012). At home, use mesh boxes, baskets, hanging bunches or stockings.
• Do not store garlic near other vegetables (especially potatoes, cabbage, carrots) that release moisture and may be sources of infection (Engeland, 1991).

6. Regular inspection during storage:

• In the first 1–2 months, check garlic every 2–3 weeks, then once a month (Engeland, 1991).
• When suspect bulbs are found, immediately remove them without waiting for infection to spread (Conn, 2012; Engeland, 1991).
• If humidity rises in storage, ventilate or use dehumidifiers (for small volumes – containers with quicklime or salt) (Engeland, 1991).

What to do if diseases have already appeared in storage?

• Sorting: immediately sort the entire harvest, remove all diseased, suspect and damaged bulbs. Even externally healthy bulbs adjacent to diseased ones should be used for food first (Conn, 2012).
• Improve conditions: lower temperature and humidity, increase ventilation. If impossible – use the infected garlic as soon as possible (Engeland, 1991).
• Do not use affected bulbs for planting or composting – burn them or throw them away (Engeland, 1991; Conn, 2012).

What NOT to do:

• Do not store wet, insufficiently dried bulbs.
• Do not allow high humidity (above 75%) in storage.
• Do not store garlic at temperatures above 5 °C for long‑term storage (exception – short‑term storage at room temperature).
• Do not cut the tops too short (less than 3 cm) – this opens access to infection.
• Do not store garlic in plastic bags or tightly closed containers – without air access, mould quickly develops (Engeland, 1991).
• Do not use cloves from bulbs that were stored with signs of disease for planting.

Remember the key points:

Storage diseases of garlic are the result of a combination of errors: improper harvest, insufficient drying, damaged bulbs, too high temperature or humidity in storage. The main "medicinal" preparation for garlic in storage is proper conditions:

• dryness (humidity 65–70%);
• coolness (temperature 0–2 °C);
• ventilation;
• regular monitoring.

And remember: healthy garlic, properly grown and prepared, stores for 6–7 months or longer without significant losses. If you lose part of the crop during storage every year – review the whole process: from site selection to cellar conditions.

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Main differences between infectious diseases and physiological disorders

Specific signs of physiological problems in garlic

Physiological disorders are often linked to agronomic mistakes, weather anomalies or genetic traits. Here are the most common "non‑infectious" problems described in the sources (Conn, 2012; Engeland, 1991; Torikov and Sychev, 2018).

Practical diagnostic algorithm for the gardener

1. Examine the plant and bulb:

• Is there any coating, spores, sclerotia, mycelium? (infection)
• Is there a putrid or sour odour? (often bacterial infection) (Conn, 2012)
• How are the damages distributed: patchy or across the whole plot?

2. Recall the weather and care over the last 2–4 weeks:

• Were there frosts, hail, strong winds? (physical damage)
• Was there drought or, conversely, waterlogging? (water stress)
• Were fertilisers or herbicides applied? (nutritional disturbance or chemical burn)
• When and how was harvesting and drying done? (for storage diseases)

3. Check spread:

• If diseased plants occur singly or in expanding foci – more likely infection (Brewster, 2008).
• If symptoms are widespread and appeared after a specific event (frost, drought, treatment) – physiological problem (Conn, 2012).

4. Try to eliminate the suspected cause:

• If after watering or feeding the condition improves – most likely a nutritional or moisture issue (Engeland, 1991).
• If no improvement and spots or rot progress – probably infection.

Why is it important to distinguish correctly?

• Treatment of infections requires removal of diseased plants, application of fungicides or bactericides, disinfection of tools and replacement of planting material (Brewster, 2008; Conn, 2012).
• Physiological problems are treated by adjusting care: watering, feeding, protection from frost, sun, wind. Applying fungicides in this case is not only useless but also harmful to the environment and your wallet (Conn, 2012).

A wrong diagnosis leads to wasted time and money, and sometimes further weakens plants (e.g., applying pesticides under drought stress).

What to do if you are in doubt?

If you cannot confidently distinguish a disease from a physiological problem – contact a regional diagnostic laboratory or use an online specialist consultation. Modern methods (PCR diagnostics, ELISA) allow accurate identification of the pathogen (Brewster, 2008). You can also send photos to thematic groups or forums, but remember: the final diagnosis should be made by a specialist.

Remember the key points:

• Infection always leaves a "trace": coating, spores, odour, specific location of rot (neck, base) and tendency to spread (Brewster, 2008).
• Physiological problem is a reaction to an external factor: it is widespread, reversible and not accompanied by signs of a pathogen (Conn, 2012).

If infection is suspected – act quickly (removal of diseased plants, treatment). For physiological disorders – adjust growing and storage conditions. Careful observation, knowledge of site history and weather are your main helpers.