Onion

Pale yellow-green oval patches on leaves with a downy gray-purple fuzz in humid mornings; leaves yellowing and dying back from the tips.

Remove affected leaves, improve airflow, and apply copper protectively in a wet season. Harvest and cure carefully - infected necks invite storage rot.

Space for airflow, water at the base in the morning, rotate alliums, avoid low wet spots, and grow resistant varieties (Red Wing) where it recurs.

A cool, damp-weather disease (Peronospora destructor) that produces pale patches and fuzzy gray-violet growth on onion leaves, collapsing them and shrinking bulbs; also causes purple blotch to follow in the dead tissue.

Rust costs bulb size, since leaf area during bulbing is what sets how big the onion gets. Strip the worst leaves early and harvest on schedule.

Orange-red raised dots and streaks on the leaf blades, worst in humid spells and on crowded, poorly spaced or congested plantings.

Strip or shear off the worst-affected leaves early and let clean growth come back, improving airflow at the same time. A bulb crop usually still matures, so harvest on schedule rather than writing it off - rust rarely reaches the bulb itself.

Give wide spacing and full sun so the leaves dry quickly, avoid overhead watering and excess nitrogen, divide congested perennial clumps, and rotate alliums on a three-year cycle. Rust-tolerant varieties are available in some allium crops.

A fungal disease that raises bright orange pustules on allium leaves. The pustules destroy green tissue, and since those leaves are what feed the bulb, a heavy infection shrinks the crop or weakens the clump. It builds in humid weather, in crowded plantings and in high-nitrogen soil, and it rarely reaches the bulb itself.

Stunted, swollen or twisted plants; soft, spongy bulbs with cracked or discolored basal tissue; bloated leaf bases.

None in-season. Destroy infected crops, hot-compost nothing from them, and rest that bed from alliums for 4+ years.

The whole defense is clean stock: certified seed, sets, and transplants, and never replant from a suspect crop.

Microscopic worms that live inside onion tissue, causing bloated, twisted, stunted growth and soft, cracked bulbs - spread almost entirely through infected planting stock and seed.

Onion thrips reduce bulb size when populations run high, so this is a yield question rather than a cosmetic one. Keep the crop watered, since thrips explode on drought-stressed onions, and use reflective mulch where pressure is chronic.

Silvery-white streaks and fine speckling on leaves and petals, brown discolored patches, distorted buds and blooms that fail to open properly, and tiny sliver-like insects that drop out when you shake foliage or a flower over a sheet of white paper. Worst in hot, dry spells and on drought-stressed plants.

A strong water spray dislodges light infestations, and on a plant that can take it a hard shear and clean regrowth clears them outright. For heavier pressure use insecticidal soap, neem or spinosad, aimed down into the leaf sheaths and buds where thrips actually shelter, and repeat at intervals since eggs and pupae escape a single spray. Spinosad is a registered insecticide: use only a product labeled for the crop you are treating, follow the label exactly, and observe its pre-harvest interval.

Scout early, before the damage reaches the part of the plant you care about. Start with clean planting stock, keep plants well watered since drought-stressed plants suffer worst, avoid heavy nitrogen, clear weedy hosts nearby, and hang blue or yellow sticky traps to catch the first arrivals. Reflective mulch deters them, and predatory mites, lacewings and minute pirate bugs do real work outdoors.

Tiny slender sap-sucking insects that rasp at leaves, buds and petals and drink what runs out, leaving silvery-white streaking and fine flecking, brown scorched patches, and new growth that comes out twisted or fails to open. They breed fast in hot, dry weather, shelter deep in leaf sheaths, folds and unopened buds where sprays struggle to reach, and several species also transmit plant viruses - which is often more damaging than the feeding itself.