Diseases

1.1. Downy Mildew (Peronospora)

Pathogen: Peronospora destructor (Berk.) Casp. — an obligate parasite that infects only Allium crops (Akhatov et al., 2013; Conn, 2012).

How to recognize. Downy mildew can manifest in two forms:

• Systemic form — the plant becomes infected "from the inside," via an infected bulb or seeds. Leaves become pale green, turn yellow, become distorted, and are covered with a grayish-purple coating of sporulation. Such plants are stunted and often die (Akhatov et al., 2013; Autko et al., 2012).

• Local form — infection occurs through stomata via airborne spores (conidia). Pale yellow, elongated spots appear along the leaf veins. In humid weather, a grayish-purple "velvety" coating—the fungus's sporulation—forms on the underside of the spots. The spots enlarge, leaves turn brown and die (Conn, 2012; Brewster, 2008).

A key feature of downy mildew: the coating appears on leaves that look healthy—this is an important diagnostic sign (Akhatov et al., 2013).

Conditions for development:

• Optimal temperature — 15–18 °C (Conn, 2012).
• Requires high air humidity (above 95%) and leaf wetness — dew, rain, heavy watering (Brewster, 2008).
• The disease spreads rapidly during cool, rainy weather and in dense plantings (Autko et al., 2012).

What to do:

1. Prevention. Grow varieties resistant to downy mildew (e.g., hybrids involving Allium roylei). Practice crop rotation (do not return onions to the same spot for at least 3–4 years). Remove plant debris and discard diseased plants (Brewster, 2008; Autko et al., 2012).

2. Cultural practices. Plant onions in well-ventilated areas, orient rows in the direction of prevailing winds. Avoid crowding and excessive nitrogen fertilization, which stimulates the growth of tender, susceptible foliage (Akhatov et al., 2013; Conn, 2012).

3. Fungicide treatments. When the first symptoms appear, use approved products: copper-based (Bordeaux mixture, Abiga-Peak), Ridomil Gold MC, Revus, Quadris. Treatments are applied preventatively or at the first signs of disease at 7–14 day intervals (Akhatov et al., 2013). Important: downy mildew is a systemic disease, so preventative treatments are more effective than curative ones (Brewster, 2008).

Why this works. Downy mildew spores only germinate in a water droplet and in cool weather. Ventilating the planting reduces the time leaves remain wet, thus breaking the infection cycle. Copper-based products act on spores before they penetrate the tissue, so they are applied before rain or at the first symptoms.

1.2. Alternaria (Purple Blotch)

Pathogen: Alternaria porri (Ellis) Cif. — a facultative saprophyte that infects weakened and senescent leaves (Akhatov et al., 2013; Conn, 2012).

How to recognize. Small, watery spots with a whitish center appear on leaves and flower stalks. Over time, they enlarge up to 2 cm in length, becoming brownish-purple with concentric rings (like a "target") and a yellow border (Akhatov et al., 2013; Brewster, 2008). In high humidity, the spots are covered with a dark olive or black coating of sporulation (Conn, 2012).

Key difference from downy mildew: Alternaria spots have distinct concentric zones and a purple hue, whereas downy mildew spots are more diffuse, and the coating is grayish-purple and appears mainly on the underside of the leaf (Akhatov et al., 2013).

Conditions for development:

• Favored by warm weather (optimum around 25 °C) and humidity (dew, rain, leaf wetness lasting more than 12 hours) (Conn, 2012; Akhatov et al., 2013).
• The disease more often affects old, physiologically weakened leaves, as well as plants with injuries (e.g., from thrips) (Brewster, 2008).
• Spores are spread by wind and rain splash (Conn, 2012).

What to do:

1. Cultural practices. Practice crop rotation, remove plant debris. Avoid excess nitrogen—it promotes disease development. Provide plants with potassium and phosphorus to increase resistance (Conn, 2012; Akhatov et al., 2013).

2. Protection. When symptoms appear, apply fungicides — copper-based, Skor, Bravo, chlorothalonil. Repeat treatments every 7–10 days depending on weather (Akhatov et al., 2013; Brewster, 2008).

3. Thrips control. Thrips damage leaves, creating "entry points" for the fungus, so pest control is part of Alternaria protection (Conn, 2012).

Why this works. The fungus Alternaria lives on plant debris in the soil. Removing leaves after harvest reduces the number of spores that can infect a new crop. Thrips control is important because even minor leaf damage becomes an entry point for infection.

1.3. Stemphylium Leaf Blight

Pathogen: Stemphylium vesicarium (Wallr.) E.G. Simmons (Akhatov et al., 2013; Conn, 2012).

How to recognize. Symptoms are very similar to Alternaria, often leading to misdiagnosis. Small, light yellow or brownish, watery spots appear on leaves. Over time, they become elongated, with darker edges and a lighter center. Spots coalesce, causing leaves to dry up (Akhatov et al., 2013). Difference from Alternaria: on Stemphylium spots, a weak grayish or black coating may appear, but concentric "targets" are usually absent, and sporulation looks more uniform (Conn, 2012; Akhatov et al., 2013).

Conditions for development:

• The disease develops in warm, humid weather, often as a secondary infection on tissues already damaged by other pathogens or insects (Brewster, 2008; Akhatov et al., 2013).
• The pathogen survives on plant debris and seeds (Conn, 2012).

What to do:

1. Protection measures are the same as for Alternaria: crop rotation, removal of plant debris, fungicide treatments (copper products, chlorothalonil, Skor) (Akhatov et al., 2013; Conn, 2012).

2. Pay special attention to preventing leaf damage and controlling other diseases, as Stemphylium often attacks already weakened plants (Brewster, 2008).

Why this is important. Stemphylium often "masks" itself as Alternaria, but control measures are common. The main difference in strategy: with Stemphylium, the priority is to reduce plant stress—ensure adequate nutrition, moderate watering, and pest protection.

Comparative Table of Leaf Diseases

General Preventative Measures for Leaf Diseases

Regardless of the specific pathogen, the following practices help reduce the risk of infection:

1. Crop rotation. Do not plant onions in the same spot more than once every 3–4 years. This reduces the accumulation of fungal spores in the soil (Autko et al., 2012; Brewster, 2008).

2. Remove plant debris. After harvest, burn or deeply incorporate all leaves and husks into the soil—spores and mycelium survive on them (Conn, 2012; Akhatov et al., 2013).

3. Proper watering. Use drip irrigation or water at the base to keep leaves dry. Avoid overhead watering in cool weather (Conn, 2012; Brewster, 2008).

4. Ventilation. Plant onions in elevated areas, maintain spacing between plants, orient rows with the wind to allow leaves to dry faster after dew or rain (Akhatov et al., 2013; Brewster, 2008).

5. Balanced nutrition. Excess nitrogen makes tissues loose and more susceptible. Moderate nitrogen doses combined with phosphorus and potassium increase resistance (Conn, 2012; Akhatov et al., 2013).

6. Select healthy planting material. Heat-treat sets before planting (40–42 °C for 8 hours)—this reduces systemic downy mildew infection (Akhatov et al., 2013; Autko et al., 2012).

2.1. Neck Rot (Gray Neck Rot)

Pathogens: Fungi of the genus Botrytis — predominantly Botrytis allii Munn (teleomorph — Botryotinia allii), less commonly B. byssoidea and B. squamosa (Akhatov et al., 2013; Conn, 2012; Brewster, 2008).

How to recognize.

In the field, the disease remains unnoticed for a long time. The first signs appear after harvest—1–2 months into storage. Here's what to look for:

• Initial stage. The tissue at the neck becomes soft, slightly watery, and takes on a yellowish-brown tint ("cooked" appearance). When cut, the affected scales have a dirty-yellow color (Akhatov et al., 2013; Autko et al., 2012).

• Development of rot. Infection spreads from the neck downwards, towards the base. Affected tissues soften, and the bulb becomes flabby. A gray, fluffy coating—the fungus's sporulation—appears on the neck surface and between scales (Conn, 2012; Brewster, 2008).

• Late stage. Small (1–4 mm) black sclerotia—dense, rounded structures resembling "crumbs" or "scabs"—form on the coating and outer scales. The affected bulb completely rots, emitting an unpleasant odor (Akhatov et al., 2013; Brewster, 2008).

Important diagnostic sign: Neck rot starts specifically from the neck, not from the sides or base. If rot starts from the side, it is likely mechanical damage or another infection (Conn, 2012).

Conditions for development:

• The fungus prefers moderate warmth: optimal temperature for development is 15–20 °C, but it can grow from 3 °C up to 33 °C (Akhatov et al., 2013).
• High air humidity (especially in storage) accelerates rotting. In a damp storage, the disease spreads from diseased bulbs to healthy ones through contact (Autko et al., 2012).
• In the field, infection is favored by: rainy weather before harvest, late harvest (when the neck is still juicy), damage to leaves and neck during cutting or mechanical harvesting, and excess nitrogen fertilizer delaying maturation (Conn, 2012; Brewster, 2008).

Sources of infection:

• Fungal sclerotia survive in soil and on plant debris for up to 2 years or more.
• Spores (conidia) are spread by wind and rain splash from diseased plants and rotten bulbs in waste piles.
• Infected planting material (sets, shallots) is a major source (Akhatov et al., 2013; Conn, 2012).

What to do:

Protection from neck rot involves a set of measures starting long before harvest.

Before planting and during the growing season:

1. Crop rotation. Do not return onions to the same spot for at least 3–4 years. The fungus accumulates in the soil, and frequent onion planting increases infestation (Autko et al., 2012; Brewster, 2008).

2. Variety selection. Varieties with red and yellow dry scales are generally more resistant to neck rot than white-scaled ones (Conn, 2012; Akhatov et al., 2013). If the disease is a frequent problem in your region, prefer colored varieties.

3. Balanced nutrition. Avoid excessive nitrogen doses in the second half of the growing season—they delay maturation, the neck remains thick and juicy, facilitating fungal entry. Apply potassium and phosphorus—they accelerate maturation and strengthen tissues (Conn, 2012).

4. Control other diseases and pests. Damage from downy mildew, Alternaria, or thrips weakens the plant and creates "entry points" for Botrytis (Akhatov et al., 2013). Timely protection against these problems is indirect prevention of neck rot.

During harvest and after—most crucial:

5. Harvest timing. Harvest onions only after complete leaf lodging and drying of the neck. Unripe onions with a thick, moist neck are prime candidates for rot (Autko et al., 2012; Brewster, 2008).

6. Drying. After lifting, onions must be dried: first in the field (in good weather) for 5–7 days, then in a well-ventilated shed or dryer with forced air at 30–35 °C for 5–8 days. It is crucial that the neck dries completely—only then can the fungus not penetrate inside (Akhatov et al., 2013; Conn, 2012).

7. Trimming. Cut the leaves when they are completely dry, leaving a neck 4–6 cm long. Early or too-low cutting of a green neck injures tissues and opens the way for infection (Autko et al., 2012).

8. Culling. Before storage, remove all bulbs with signs of damage, thick or moist necks, or mechanical injuries. Even one diseased bulb can become a source of infection for neighbors (Brewster, 2008).

9. Storage conditions. Store onions at 0–2 °C and 75–80% relative humidity. Under these conditions, Botrytis development is greatly slowed. The storage should be well-ventilated to prevent condensation on bulbs (Autko et al., 2012; Conn, 2012).

Additional measures:

• Heating sets. Before planting, heat sets at 40–42 °C for 8 hours—this reduces internal infection (Akhatov et al., 2013).
• Fungicide treatments. During the growing season, when the risk of neck rot is high (rainy summer), products based on chlorothalonil, mancozeb, and systemic fungicides (e.g., tebuconazole) are used. Treatments start 2–3 weeks before harvest (Akhatov et al., 2013; Brewster, 2008). In private gardens, preventative measures without chemicals are often sufficient.

Why this works. Drying and proper trimming are key. A dry neck acts as a dense plug through which the fungus cannot penetrate. If the neck is wet or damaged, Botrytis spores, which are constantly present in the air, easily germinate and colonize the tissues. Cold storage slows fungal growth to such an extent that it does not cause significant rot over the entire storage period.

2.2. What else can affect the neck?

Besides classic neck rot, other diseases can manifest in the neck area:

• Bacterial rots (soft rot) — start at the neck but are accompanied by a sharp unpleasant odor, slime production, tissues become gelatinous rather than just soft (Conn, 2012; Brewster, 2008). Bacterial rots are discussed in more detail in Chapter 4.

• Black mold (Aspergillus rot) — caused by the fungus Aspergillus niger. More often appears on bulbs stored in warmth (above 28 °C). A black "dusty" mass of spores forms on the neck and under dry scales. The bulb may look healthy externally, but inside scales are covered with black coating (Conn, 2012; Akhatov et al., 2013). This disease is characteristic of hot regions or storages with elevated temperatures.

• Fusarium basal rot — although it affects the base, sometimes in later stages it spreads upward, affecting the neck too. But its primary focus is always at the bulb's base (Conn, 2012).

During diagnosis, it's important to remember: if rot started from the neck, it is almost always neck rot (Botrytis) or bacteriosis. Fungal neck rot is distinguished by gray coating and black sclerotia, bacterial rot by unpleasant odor and slimy consistency.

Brief Summary of Neck Diseases

3.1. Fusarium Basal Rot (Fusarium)

Pathogen: Fungus Fusarium oxysporum f. sp. cepae (W.C. Snyder et H.N. Hansen) — a specialized form infecting specifically Allium crops. Other Fusarium species are sometimes found, but this is the main one (Akhatov et al., 2013; Conn, 2012; Brewster, 2008).

How to recognize.

Symptoms depend on the plant's growth stage:

• Seedlings and young plants. Leaves turn yellow, starting from the tips, then wilt and die. Roots become dark brown, rot, often with a pinkish coating. Diseased plants are easily pulled from the soil as the root system is destroyed (Conn, 2012; Autko et al., 2012).

• Adult plants. First signs in the field are yellowing and drying of leaves, often on one side of the bulb. If a bulb is cut lengthwise, the base appears watery, turns brown, and rot spreads upward through the fleshy scales. A pinkish or white mycelial coating may be visible on the base and between scales on the cut (Akhatov et al., 2013; Conn, 2012).

• During storage. Bulbs infected in the field may show no symptoms at harvest, but 1–2 months into storage, the base softens, becomes watery, and white or pinkish mycelium appears on it. Rot rises upwards, and the bulb rots completely (Brewster, 2008; Conn, 2012).

Important diagnostic sign: In Fusarium, rot begins from the base and spreads upwards, while roots darken and die. If you gently press on the bulb's base, it may "give way"—the base tissue becomes soft and crumbly (Conn, 2012).

Conditions for development:

• The fungus prefers warmth: optimal temperature for infection and development is 25–28 °C, though it can grow in the range 15–32 °C (Conn, 2012; Akhatov et al., 2013).
• High soil moisture promotes spore spread. Infection is facilitated by damage to roots or the base (e.g., by onion fly, wireworms, cultivation) (Brewster, 2008).
• The fungus survives in soil as chlamydospores (thick-walled resting spores) for several years, and on plant debris (Akhatov et al., 2013).

Sources of infection:

• Infected soil and plant debris are the primary reservoir.
• Infected bulbs (sets, shallots, mother bulbs) can carry infection from field to field.
• The fungus can also live on weed roots, though the main host is onion and other Alliums (Brewster, 2008).

What to do:

Fusarium wilt is a difficult-to-control soil-borne infection. The main strategy is prevention and reducing plant stress.

Before planting:

1. Resistant varieties and hybrids. There are varieties with partial resistance to Fusarium, e.g., some F1 hybrids (Sedona, Performer, Mundial). If Fusarium is a problem in your region, choose locally adapted resistant varieties (Akhatov et al., 2013; Brewster, 2008).

2. Crop rotation. Do not grow onions in the same spot more than once every 4–5 years. Rotate with cereals, legumes, which are not affected by Fusarium (Autko et al., 2012; Conn, 2012).

3. Select planting material. Carefully inspect sets before planting—discard bulbs with dark spots on the base or signs of rot. Heating sets at 40–42 °C for 8 hours can reduce internal infection, though it does not eliminate it completely (Akhatov et al., 2013).

During the growing season:

4. Regulate soil moisture. Avoid overwatering and waterlogging in the root zone. On heavy soils, use raised beds or drainage. The fungus develops more actively in waterlogged soil (Brewster, 2008; Conn, 2012).

5. Pest control. Onion fly, bulb mite, and other soil-dwelling pests damage the base and roots, creating "entry points" for Fusarium. Pest control reduces infection risk (Akhatov et al., 2013; Brewster, 2008).

6. Nutrition. Excess nitrogen weakens plants, while adequate potassium and phosphorus increase resistance to root rots (Conn, 2012).

During storage:

7. Drying and culling. Thoroughly dry onions before storage. During sorting, remove all bulbs with even the slightest signs of base damage (Brewster, 2008).

8. Storage conditions. Store onions at 0–2 °C and 75–80% humidity. At low temperatures, Fusarium development is slowed but not completely stopped, so culling is especially important (Autko et al., 2012).

Why this works. Fusarium is a soil fungus that can live in the ground for years. Resistant varieties have the ability to restrict the spread of the fungus in root and base tissues (so-called "horizontal" resistance). Crop rotation reduces the pathogen population in the soil, and pest control reduces the number of wounds—the main "gates" for infection.

3.2. White Rot (Sclerotial Rot)

Pathogen: Fungus Sclerotium cepivorum Berk. (teleomorph — Stromatinia cepivora) (Akhatov et al., 2013; Conn, 2012; Brewster, 2008). This is one of the most dangerous onion pathogens, infecting all Allium crops.

How to recognize.

• Field symptoms. In young plants, leaves turn yellow and die, starting from the tips. A dense white, cottony coating (mycelium) appears on the base and at the base of the scales. Over time, small black sclerotia (poppy seed-sized, 0.5–1 mm)—the fungus's resting structures—form on the coating and around diseased bulbs (Akhatov et al., 2013; Brewster, 2008).

• Later infection. If infection occurs just before harvest, external signs may be weak or absent. However, during storage, bulbs begin to rot from the base, covered in white coating and sclerotia (Conn, 2012).

• On garlic and leek. Symptoms are similar: leaves become chlorotic, plants wilt, the base is destroyed, garlic cloves mummify and become light, hollow inside (Akhatov et al., 2013).

Important difference from Fusarium: In white rot, the mycelium is always white, dense, felt-like, not pinkish or weak. Sclerotia are black, round, clearly visible to the naked eye. Rot progresses even at relatively low temperatures (optimum 14–18 °C for sclerotia germination) (Conn, 2012; Brewster, 2008).

Conditions for development:

• The fungus prefers cool, wet weather. Sclerotia germinate at 9–24 °C, most actively at 14–18 °C (Brewster, 2008). In hot weather (above 25 °C), development slows.
• Soil moisture is critical: sclerotia germinate best near field capacity.
• Acidic soils (pH 5.5–6.0) favor disease development (Akhatov et al., 2013).

Danger: Sclerotia survive in soil for more than 15–20 years! They are resistant to drying, frost, and many chemicals. Moreover, their germination is stimulated by onion root exudates—an evolutionary adaptation allowing the fungus to "wait" for the host's appearance (Brewster, 2008; Conn, 2012).

Sources of infection:

• Soil containing sclerotia from previous infected plants.
• Infected planting material (sets, mother bulbs, garlic cloves).
• Plant debris and weeds, though the main host is Alliums (Akhatov et al., 2013; Brewster, 2008).

What to do:

White rot is one of the most difficult diseases to eradicate. It is easier to prevent than to cure.

1. Quarantine measures. White rot is a quarantine object in many countries. If the disease is found on your plot:

• Do not bring soil, plants, or tools from the infected plot to a clean one.
• Remove diseased plants along with a clump of soil and burn them.
• After harvest, do not leave even small roots and scales in the field—they may all contain sclerotia (Akhatov et al., 2013; Conn, 2012).

2. Crop rotation and "starving" fallow. Since sclerotia live for decades, the usual 3–4-year rotation is ineffective. On infested soils, it is recommended to:

• Avoid Allium crops for a long period (6–10 years).
• Grow trap crops (e.g., cereals) that do not stimulate sclerotia germination (Brewster, 2008).

3. Provocative germination. This method is based on stimulating sclerotia to germinate in the absence of the host—then the fungus starves to death.

• Applying onion or garlic extracts, or preparations of diallyl disulfide (a component of onion essential oil) to the soil stimulates sclerotia germination. After germination, if no onion roots are nearby, the mycelium dies.
• The method is effective in warm regions but requires precise dosages and often gives inconsistent results. Rarely used on an industrial scale, and almost never in home gardens due to complexity (Brewster, 2008; Conn, 2012).

4. Soil solarization. Covering moist soil with clear plastic film for 4–8 weeks during summer can heat the top layer to 50–60 °C, killing sclerotia. This method is effective in regions with hot summers (Mediterranean, subtropical climates). In cooler climates, the effect is weaker (Akhatov et al., 2013; Brewster, 2008).

5. Biologicals. Some antagonistic fungi (e.g., Trichoderma viride, Sporidesmium sclerotivorum) can destroy sclerotia in soil. Products based on Trichoderma are available, but their field efficacy is often inconsistent and depends on soil and weather (Brewster, 2008; Conn, 2012).

6. Chemical fungicides. Currently, the most effective method is seed treatment and planting material treatment with tebuconazole (products based on it). For example, pre-planting soaking of sets in a tebuconazole solution reduces white rot incidence. However, for home gardeners, this chemical means are often unavailable, so the main focus is on prevention (Brewster, 2008; Akhatov et al., 2013).

Why it's so difficult. White rot sclerotia are among the most durable resting structures in the plant world. They do not germinate without a signal from onion roots, so they can wait for decades. Destroying them is difficult, and a single surviving individual can start a new population. This is why preventing the introduction of the pathogen onto a clean site is more important than controlling an existing one.

3.3. Other Basal Diseases (briefly)

• Pink root — caused by the fungus Pyrenochaeta terrestris. Affects roots, turning them pinkish-red, then roots die. The base is rarely affected, but severe root damage weakens the plant. The disease is widespread in warm regions (Brewster, 2008; Conn, 2012). More details on root diseases in Chapter 5.

• Southern blight (Sclerotium rolfsii) — found in subtropics and tropics, causes rot of the base and crown region, with white coating and small brown sclerotia (Brewster, 2008).

Comparative Table of Basal Diseases

Practical Recommendations for the Gardener

1. Golden rule: basal diseases are easier to prevent than to cure. When buying sets or garlic for planting, carefully inspect the base—it should be dry, clean, without dark spots or coating.

2. If white rot has been present on the plot, it is best to avoid planting onions there for 5–7 years. If unavoidable, use solarization, and dust bulbs at planting with Trichoderma or chalk (chalk creates an alkaline environment unfavorable for the fungus).

3. Avoid dense planting and overwatering—this reduces the risk of Fusarium.

4. After harvest, do not leave plant debris on the bed—remove and burn it, especially if there were suspicions of disease.

5. Remember: Fusarium more often affects plants weakened by drought or pests, so timely watering and protection against onion fly are also part of prevention.

4.1. Bacterial Rots

Bacterial rots are one of the most common causes of crop loss during storage and in the field. The pathogens are various species of bacteria, but all cause softening and decomposition of tissues, often with a characteristic odor. They differ from fungal diseases by their slimy consistency, wateriness, and sharp, sometimes sour or putrid smell (Conn, 2012; Brewster, 2008).

Let's consider the main types of bacterial rots found on onions.

4.2. White Rot (Sclerotial) — Bulb Infection

Although we discussed white rot in the basal diseases section, it is important to emphasize that in storage, it manifests as complete destruction of the bulb. If in the field the disease starts with roots and the base, during storage infected bulbs are covered with white, cottony mycelium, on which black sclerotia form. The bulb becomes soft, light, and then either mummifies or rots completely (Akhatov et al., 2013; Brewster, 2008).

Feature of white rot in storage:

• Infection almost always occurs in the field, while symptoms appear during storage.
• Sclerotia that enter the storage with diseased bulbs can remain on shelves, in boxes, and infect the next harvest.
• This is why thorough cleaning and disinfection of storage facilities are so important (Brewster, 2008).

Control measures during storage:

• Before storage, carefully sort onions, removing all bulbs with suspicious bases (soft, dark, with coating).
• Store at low temperatures (0–2 °C)—this slows, but does not stop, white rot development.
• Do not store onions for more than 1 season on shelves where diseased bulbs were previously kept without disinfection (treatment with formalin solution or sulfur bombs) (Akhatov et al., 2013; Brewster, 2008).

General Rules for Preventing Bacterial Rots and White Rot

1. Minimize damage — the main principle. Bacteria and fungi (except white rot, which enters through roots) use wounds as entry points.

2. Full maturity and proper drying. Unripe, moist onions are an ideal environment for any rot.

3. Storage cleanliness. Before storing the harvest, be sure to remove old onion remains, disinfect shelves and containers.

4. Cold storage (0–2 °C) and moderate humidity (75–80%) significantly reduce the activity of both bacteria and fungi.

5. Crop rotation and removal of plant debris in the field reduce the overall infection background.

Comparative Table of Bacterial Rots (how to distinguish them)

What to do if the disease appears in storage?

• Immediately sort all onions, remove all affected bulbs along with neighbors if suspected (they may already be infected but symptomless).
• Dry the remaining onions (increase ventilation, slightly raise temperature to 25–30 °C briefly to evaporate moisture, then cool down again).
• Ventilate the storage to reduce humidity.
• If damage is extensive — urgently sell or process the onions, as further storage will lead to total loss.

5.1. Fungal Root Diseases

5.2. Nematode Root Diseases

Nematodes are microscopic roundworms living in soil. Some species parasitize onion roots, causing deformation, death, and weakening of plants. Unlike fungal diseases, nematode damage often has characteristic external signs on roots (galls, swellings, shortening).

5.3. How to Distinguish Fungal Root Diseases from Nematode Diseases?

General Recommendations for Root Protection

1. Healthy planting material. Inspect sets before planting: there should be no dark spots, coating, or damage. If nematodes are suspected, heat-treat sets in hot water (45–50 °C, 10–15 minutes) (Conn, 2012).

2. Soil improvement. Pythium is more common on heavy, waterlogged soils. Adding sand, organic matter, and creating raised beds improves drainage and reduces risk (Brewster, 2008).

3. Crop rotation. This is a universal method against all soil-borne pathogens. Do not plant onions after onions, garlic, or other Alliums for at least 3–4 years, preferably 5–6 years (especially for pink root and nematodes) (Autko et al., 2012).

4. Weed control. Many nematodes and fungi can survive on weed roots. Regular weeding reduces the infection reservoir (Brewster, 2008).

5. Watering. Avoid both drought and overwatering. Stress from lack of moisture weakens roots and makes them more susceptible to all pathogens (Conn, 2012).

6. Liming. Pink root develops better on acidic soils (pH 5.5–6.5). Maintain pH at 6.5–7.0, apply lime if necessary (Conn, 2012; Autko et al., 2012).

Why prevention is important. Root diseases are almost always unnoticed until infestation becomes widespread. Treating them in the field is impossible — chemical products for soil-borne pathogens are either unavailable or ineffective for private gardens. Therefore, the main emphasis is on prevention: resistant varieties, proper agronomy, and crop rotation.

Brief Summary of Root Diseases

• Pink root — roots turn pink, then brown and die; main protection — resistant varieties and early sowing dates.
• Pythium — roots darken and rot in cold, wet weather; drainage and moderate watering help.
• Nematodes — cause galls, necrosis, or root shortening; control — crop rotation, solarization, healthy planting material.

6.1. Main Storage Diseases

Most storage diseases are caused by the same pathogens that infected onions in the field, but their activity manifests after harvest when conditions (temperature, humidity) change and weakened or damaged tissues become vulnerable.

6.2. Storage Conditions that Prevent Diseases

Proper storage is not just "putting onions in the cellar." It is a whole system of measures aimed at creating an environment unfavorable to pathogens and preserving onion quality.

6.3. What to do if the disease has already appeared in storage?

If you notice rot in storage, act quickly:

1. Sort all onions. Remove diseased bulbs and burn them (or bury deep away from the plot). Do not leave them even in compost — spores and sclerotia can survive.

2. Increase ventilation and, if necessary, dry the remaining onions (temporarily raise temperature to 25–30 °C with good ventilation to evaporate moisture, then cool again).

3. Ventilate the storage to reduce air humidity.

4. Treat the storage after unloading the harvest: whitewash walls and racks with lime milk (2 kg lime per 10 L water) or treat with sulfur bombs (for enclosed spaces). This reduces the number of spores and bacteria left from diseased bulbs (Akhatov et al., 2013; Brewster, 2008).

5. If rot is widespread — urgently sell or process the onions (drying, freezing, canning), as further storage will lead to total loss (Conn, 2012).

6.4. Physiological Disorders Resembling Diseases

Not all storage losses are caused by pathogens. Sometimes the cause is improper conditions:

• Sweating (overheating). If onions are stored warm and moist, temperatures rise inside the mass, tissues "sweat" — become watery, acquire an unpleasant odor, but without typical fungal or bacterial coating. This is often confused with bacterial rot, but the cause is improper drying and cooling (Conn, 2012; Brewster, 2008).

• Freezing. At temperatures below –2 °C, bulb tissues freeze, become glassy, and after thawing soften and rot. Note: such bulbs can be used immediately after thawing, but they are not suitable for storage (Akhatov et al., 2013).

• Sprouting. If storage temperature rises above 5 °C, onions break dormancy and begin to sprout. Sprouts weaken the bulb and make it vulnerable to pathogens. To suppress sprouting, commercial growers use the product Fazor (maleic hydrazide), but for home gardens, maintaining low temperature is sufficient (Brewster, 2008).

• Translucent scale. During storage at high humidity and temperature, some scales become grayish, watery, translucent. This is a physiological disorder, not a disease. It can be distinguished by the absence of coating, odor, and affected tissues do not soften to a slimy state (Conn, 2012).

6.5. Practical Recommendations for the Gardener (Summary)

1. Harvest onions only after full maturity — a dry neck is the key to successful storage.

2. Dry thoroughly — first in the field (in good weather), then in a ventilated room at 30–35 °C (you can use heaters with fans) until the neck is completely dry.

3. Sort and grade — remove all suspicious, damaged, and small bulbs (small ones store poorly).

4. Store in a cool (0–2 °C) and dry (75–80% humidity) place with continuous ventilation.

5. Check regularly — inspect every 2–3 weeks, and at the first sign of rot, remove diseased bulbs.

6. Disinfect storage after each season — whitewashing or sulfur fumigation.

7. Do not mix different batches of onions (especially from different fields) — this reduces the risk of cross-contamination.

Remember: storage diseases are easier to prevent than to cure. Proper preparation and storage conditions are 90% of success; the rest is vigilance and timely response.

7.1. General Criteria for Distinction

7.2. Main Physiological Disorders of Onions

Below are the most common non-infectious problems, their external appearance, and key differences from similar diseases.

7.3. Quick Diagnostic Table

7.4. Practical Recommendations

1. Keep a journal. Record when symptoms appeared, what the weather was, what treatments were done. This helps identify the cause.

2. If a disease is suspected, look for the pathogen. Use a magnifying glass: look for coating, spores, sclerotia, fluffy mycelium. If absent, it is likely physiological.

3. Check storage conditions. If rot appears in storage, measure temperature and humidity. Often, simply correcting them stops the problem (Conn, 2012).

4. Do not rush with chemicals. Before spraying fungicide, ensure symptoms are not caused by nutrient deficiency or sunburn. Useless treatment will not help and may harm.

5. Compare with neighboring plants. If the disease affects individual plants — likely infection. If all plants look similarly bad — look for a common factor (weather, soil, watering) (Brewster, 2008).

6. In doubtful cases, consult. There are many forums and groups where experienced gardeners help with diagnosis. Or contact your local plant protection service.

Final advice: Remember, healthy onions are the result of integrated care: proper crop rotation, balanced nutrition, moderate watering, pest protection, timely harvest, and proper storage. If you provide plants with comfortable conditions, many diseases and disorders will bypass them. And the ability to distinguish infection from physiology helps to act precisely and efficiently, without extra costs and worries.