Gadenin日本語Get the appRust: the orange pustules you can cut off
Rust shows as raised orange or rusty-brown pustules scattered along the tops, most often in mild damp weather with crowded plants. Rub one and orange powder comes off on your finger, which distinguishes it from simple leaf scorch. It spreads readily through a bed but it is a leaf disease, not a bulb disease, and the plant survives it.
Treat it by cutting. Shear off the affected tops at the usual inch (2.5 cm) above soil, take the cut leaves right out of the garden rather than composting them, and let the clump regrow clean. Then fix the conditions that let it in: split overcrowded clumps, stop overhead watering, cut back on nitrogen feeding and open up the spacing so air moves through the row.
Cut leaves showing rust go in the household waste, not the compost pile. Rust spores survive composting at garden temperatures and come straight back with the next mulch.
Onion thrips and allium leaf miner
Onion thrips produce silvery mottled streaks along the leaves, usually starting in hot dry weather. Look right down into the crowded neck of the clump where the leaves meet; the insects themselves are tiny yellow-brown slivers that run when disturbed. A heavy attack leaves the tops silvered, twisted and barely worth cutting.
Allium leaf miner leaves a neat row of small white dots punched down the leaf, and later the larvae tunnel down into the neck and bulb. The dots are unmistakable once you have seen them and are the point at which to act, because after the larvae reach the bulb nothing helps.
For both, a fine insect mesh over the row during the flight periods is the only genuinely reliable control. A hard spray of water knocks thrips numbers down, and keeping the bed weed-free removes the grassy hosts thrips breed on. Cut and remove badly damaged tops so the clump regrows under the mesh.
| Symptom | Likely cause | Action |
|---|---|---|
| Orange powdery pustules on tops | Rust | Cut tops off and remove; improve airflow |
| Silver mottled streaks, hot dry spell | Onion thrips | Mesh the row, spray with water, weed the bed |
| Row of white dots down the leaf | Allium leaf miner | Mesh immediately; remove tunneled shoots |
| Pale oval patches, gray fuzz in damp weather | Downy mildew | Cut and remove; stop overhead watering |
| Yellowing, wilting, white fluff and black grains at base | White rot | Remove and destroy; do not grow alliums there again |
| Soft brown neck, sour smell after lifting | Neck rot from poor curing | Discard the bulb; cure longer next time |
| Maggots in the base, plant collapses | Onion fly | Lift and destroy affected plants; mesh next season |
White rot is the one that ends the bed
White rot is the serious one. Plants yellow and wilt for no obvious reason, and when you lift one the base is covered in white cottony fungus studded with tiny black grains the size of poppy seeds. Those grains are resting bodies that survive in soil for fifteen years or more, waiting for the next allium root to come near. There is no cure and no treatment.
If you find it, lift the affected plants with the soil around them, bag them and dispose of them away from the garden. Do not compost anything, do not move soil or tools from that bed to another without cleaning them, and do not plant onions, leeks, garlic, chives or bunching shallot in that ground again. Start a new bed as far away as you can, from clean bulbs, and never buy in bulbs of unknown origin to plant near your good stock.
The single most effective prevention is rotation. Give any allium bed a three-year gap before alliums return, and keep your own replanting stock rather than repeatedly buying in bulbs that may bring soil disease with them.
Rot during dormancy and curing
The riskiest weeks of the year are the dormant ones. The bulbs are sitting underground with no leaves drawing water, and if the summer is wet or the bed is irrigated out of habit, they rot. Turn off any watering to the row as soon as the tops start yellowing, and if your summers are reliably wet, lift the bulbs promptly rather than leaving them in the ground.
Neck rot appears in storage as a soft brown patch at the neck, spreading down, often with a gray mold and a sour smell. It almost always traces back to curing that was cut short or to bulbs that were washed. Check the store every couple of weeks and pull anything soft immediately, because one rotting bulb takes the ones touching it.
- Walk the row weekly and look down into the necks Most problems are visible in the crowded base of the clump a week before they show on the tops. Part the leaves and look at where they meet the bulb.
- Record first sightings by date and clump Write down what you saw, where in the row, and on what date. Rust and thrips arrive at roughly the same point each year, and one season of records tells you when to have the mesh ready rather than reacting after the damage.
- Remove rather than treat leaf problems This crop regrows from the bulb in two to three weeks, so cutting infected tops away costs you one harvest and solves the problem. That is a better trade than any spray.
- Keep clean stock separate If one part of the bed has trouble, split and cure those clumps separately and do not mix their bulbs into your main replanting stock.