Yellow onion (Allium cepa) is the most widely cultivated species of the genus Allium and the world's most economically significant bulb vegetable. It produces papery-skinned bulbs dense with sulfur compounds that drive its pungency raw and its sweetness when cooked. Its close relatives include garlic, leek, shallot, and chives.
Bulb formation is controlled by day length, not temperature. Long-day varieties (14–16 hours) suit zones 3–6; short-day types (10–12 hours) perform in zones 7–10; intermediate-day types cover the middle latitudes. Choosing the correct type for a given latitude is the single most consequential decision at planting — the wrong choice yields small bulbs or causes premature bolting. Onions are heavy nitrogen feeders with shallow roots that compete poorly with weeds; consistent mulching conserves moisture and suppresses competition. Soil pH between 6.0 and 7.0 is optimal. After harvest, cure bulbs in a dry, ventilated location at 70–80°F for two to three weeks before long-term storage.
Onions are rich in quercetin, a flavonoid with documented antioxidant and anti-inflammatory activity. Regular consumption has been associated in epidemiological studies with modest improvements in blood lipid profiles and blood glucose regulation. Traditional medicine across many cultures has used raw onion topically for minor skin irritation and internally as a remedy for cough and congestion.
Onion White Rot
Yellowing and dieback of leaves, soft rotting at the base, and a fluffy white fungal growth with small black sclerotia on roots and lower stems.
Remove and destroy infected plants. Do not compost. Avoid replanting alliums in affected soil for several years and rotate crops. There is no chemical cure once established.
Downy mildew
Yellowing on top of leaves with grey-purple fuzz underneath. Spreads rapidly in humid conditions.
Remove affected leaves. Improve air circulation. Avoid overhead watering. Copper spray as preventive.
Botrytis (grey mould)
Grey fuzzy mould on petals and stems, worst in cool wet conditions.
Remove affected parts immediately. Improve air circulation. Avoid overhead watering. Apply copper fungicide if severe.
Alternaria blight
Brown circular spots with yellow halos on leaves; spreads rapidly in wet weather.
Remove affected leaves immediately. Water at the base only. A copper-based fungicide can help if caught early.
Bacterial Soft Rot
Rhizomes turn mushy and foul-smelling; foliage yellows and pulls away easily at the base. Often follows borer damage or overly wet soil.
Cut away all soft tissue back to firm rhizome, let it dry, and dust the cut with sulfur. Improve drainage and avoid burying rhizomes. Destroy badly infected plants.
Damping Off
Seedlings collapse at the soil line with thin, water-soaked stems; seeds may fail to emerge or rot before sprouting.
Sow in well-draining mix, avoid overwatering, and ensure good airflow. Remove affected seedlings and let the surface dry between waterings.
Root-Knot Nematode
Swollen galls on roots, poor vigor, wilting in heat, and reduced fruiting. Containers and sandy soils are more prone.
Use clean potting mix in containers. Avoid replanting in infested soil. Maintain plant vigor with mulch and steady watering. Solarize affected garden soil.
Garlic Basal Rot
Yellowing and dieback of leaves from the tips, stunted plants, and soft brown decay at the base plate of the bulb, often with pink-white fungal growth on roots.
Plant disease-free cloves in well-draining soil, rotate alliums on a 4-year cycle, avoid overwatering, and discard infected bulbs rather than composting them.