Gadenin日本語Get the appSilver streaking on the leaves is thrips
Fine silver or whitish streaks running lengthways along the leaves, with tiny black specks of frass beside them, are onion thrips. Hold a leaf against white paper and tap it and you will see slivers about a sixteenth of an inch (1.5 mm) long move. They breed fastest in hot dry weather and multiply in the tight space where the leaves fold together, which is exactly where sprays do not reach.
The practical response is water and a hard cut. Spray the plants forcefully with plain water in the early morning two or three days running to knock populations down, and if the damage covers more than a third of the leaf area, cut the whole clump to the crown and let it regrow clean. Because the plant regrows in three weeks, sacrificing a cut is a cheap fix. Keep the bed watered in dry spells, since drought stressed plants get hit far harder.
Thrips overwinter in plant debris and in nearby grass. Clearing dead allium leaves off the bed in autumn removes a real part of next year's population.
Orange pustules are rust, and it spreads by nitrogen
Bright orange raised spots that smear rusty powder on your fingers are allium rust. It appears in mild damp weather, often in late spring and again in autumn, and it runs fastest through soft, over fed, crowded growth. Once it is on more than a few leaves it will not be cured on those leaves.
Cut affected leaves off and remove them from the garden rather than composting them, thin crowded clumps to get air moving, and stop feeding nitrogen for the rest of the season. Rust rarely kills an established Welsh onion clump, it just makes the leaves unpleasant, and a hard cut followed by clean regrowth usually resolves it. If the same bed rusts badly two years running, move the planting.
| What you see | Likely cause | First action |
|---|---|---|
| Silver streaks with black specks | Onion thrips | Forceful water spray, then cut hard if widespread |
| Orange powdery pustules | Rust | Remove leaves, thin the clump, stop nitrogen |
| Pale oval patches, gray fuzz in damp weather | Downy mildew | Cut and remove, improve air flow, avoid evening watering |
| White blotches turning to papery tips | Leaf blight or sun scorch | Cut back, check watering is deep not frequent |
| Pinholes in a row down a leaf | Allium leaf miner or leek moth | Net the bed, destroy affected leaves |
| Shank soft and slimy at soil level | Bacterial soft rot after earthing up | Lift and remove the plant, pull soil back from neighbors |
| Whole clump yellows and lifts easily | Onion fly maggots or white rot at the roots | Lift, inspect the base, do not replant alliums there |
Pinholes and tunnels: leaf miner and leek moth
A neat line of small round holes down a leaf, made before the leaf unrolled, points to allium leaf miner or leek moth. Later you find pale tunnels inside the leaf and small brown pupae tucked into the shank. Both have two generations a year, one in spring and one in late summer to autumn, and the autumn one does the worse damage because the larvae bore down into the shank.
Insect netting with a fine mesh, put on before the adults fly and sealed at the soil, is the only reliable control in a garden. Cut out and destroy any leaf with tunnels the moment you see it, and check the shank at the same time, because a plant that looks lightly damaged above ground can be hollow inside. Rotate the bed away from where you grew alliums last year.
Rot at the base, which is usually your own doing
A shank that has gone soft, slimy and sour smelling just at soil level is bacterial soft rot, and on this crop it nearly always follows earthing up with wet soil, or earthing up too high so that soil sat in the leaf joint. Lift the affected plants entirely and take them off the bed, then pull the ridge back from the neighboring plants and let the soil surface dry for a week.
White rot is a different and more serious thing. The plant yellows, lifts out with almost no roots, and the base carries a white fluffy growth with tiny black dots the size of a poppy seed. There is no cure. Remove every trace of the plant and the soil clinging to it, and do not plant any allium in that ground for many years, because the resting bodies survive in soil for a very long time. Never move soil or tools from that patch to a clean bed.
Yellow leaf tips alone, with a firm base and healthy roots, are almost always waterlogging or hunger rather than disease. Check drainage and give a nitrogen feed before you assume the worst.
The habits that prevent most of it
Welsh onion is genuinely one of the tougher vegetables and most of these problems cost you a cut rather than the plant. The judgement call to get right is the difference between something a hard cut fixes, which is nearly everything on the leaves, and something at the roots that means you clear the ground, which is white rot and nothing else.
- Divide crowded clumps every three years Dense clumps stay damp in the center and that is where rust, mildew and rot start. Dividing is a disease control measure as much as a propagation one.
- Water the soil, not the leaves Water at the base in the morning. Wet leaves overnight are the conditions downy mildew needs, and they are entirely under your control.
- Clear dead leaf off the bed Collapsed yellow leaves lying on the crown shelter thrips and slugs and hold moisture against the shank. Take them off every few weeks.
- Keep a written first sighting date Write down the date you first see thrips damage or rust each year, and where in the bed it started. These dates repeat within about a week annually, so after two seasons you can net or cut ahead of the problem instead of behind it.
- Move the bed after four years Even a healthy perennial planting accumulates soil borne trouble. Start a new bed from divisions on fresh ground before the old one declines, not after.