Gadenin日本語Get the appQuick guide to leek problems
Leeks are tough plants, but they share most of their pests and diseases with onions and garlic. Recognizing the symptom early is the key, because several of these problems are easy to prevent and hard to fix once established.
| Problem | What you see | Main prevention |
|---|---|---|
| Leek rust | Bright orange pustules on leaves | Spacing, no excess nitrogen, resistant cultivars |
| Allium leaf miner | Rows of white dots; twisted leaves; brown pupae in stem | Insect mesh during adult flights |
| Leek moth | Tunnels and windows in leaves; caterpillars inside | Insect mesh; remove damaged leaves |
| Onion thrips | Silvery streaks and speckling on leaves | Steady watering; encourage predators |
| White rot | Yellowing, collapse, white fluffy growth at the base | Rotation; never import infected soil |
| Downy mildew | Pale patches with gray-violet fuzz in wet weather | Airflow, rotation, remove infected leaves |
Leek rust
Leek rust is a fungal disease that shows as raised orange spots, called pustules, on the leaves, usually from midsummer into autumn. Badly affected leaves yellow and die back. It rarely kills the plant, and the white stem underneath is still edible, but heavy infections reduce growth. It is encouraged by humid weather, crowded plants and soft growth from too much nitrogen.
- Give plants room Adequate spacing and weed control keep air moving and leaves drying quickly after rain.
- Avoid excess nitrogen late Keep feeding moderate and stop heavy feeding by late summer. Potash-rich feeding is often recommended to support firmer growth.
- Choose resistant cultivars Some cultivars are listed as showing good rust resistance. Check catalog descriptions if rust is a regular problem.
- Clean up afterward Remove badly affected leaves and clear old leek debris at the end of the season. New leaves produced in cooler weather are often clean.
Allium leaf miner and leek moth
Allium leaf miner is a small fly whose larvae tunnel down through the leaves into the stem, where they pupate as small brown cases. The first sign is a line of tiny white dots on the leaf where the adult female has fed. Infested leeks become twisted and distorted, and secondary rots often follow. It has two main flight periods, one in spring and one in autumn. It is established in the UK and Europe and has spread in the northeastern United States.
Leek moth caterpillars also mine the leaves, leaving pale windows and tunnels and eventually boring toward the center of the plant. It is a pest in Europe and the UK and has spread in parts of eastern Canada and the northeastern United States.
- Cover with insect mesh Fine insect mesh or row cover over the crop during the adult flight periods is the most reliable control for both pests. Secure the edges so insects cannot crawl underneath.
- Time plantings For leaf miner, transplanting after the spring flight period has passed reduces exposure, though autumn protection may still be needed.
- Remove damaged plants Pull and destroy heavily infested leeks, and do not leave old allium debris in the bed where pupae can survive.
- Rotate the onion family Move leeks, onions and garlic to a different bed each year so emerging adults do not find a host right where they hatch.
Onion thrips
Onion thrips are tiny insects that rasp the leaf surface, leaving silvery streaks and fine speckling. They are worst in hot, dry weather, and heavy infestations stunt growth. Keeping plants well watered helps them outgrow light damage, and a strong spray of water can knock numbers down. Encouraging natural predators by avoiding broad-spectrum insecticides is usually more useful than spraying.
- Water steadily in hot spells Drought-stressed leeks suffer most. Consistent moisture keeps them growing past light damage.
- Check the leaf bases Thrips hide in the tight folds where leaves meet. Look there for the insects themselves before deciding whether action is needed.
White rot and downy mildew
White rot is a serious soil-borne fungal disease of the onion family. Leaves yellow and wilt, plants loosen in the ground, and a white fluffy growth appears on the base and roots, later dotted with tiny black resting bodies. Those resting bodies survive in the soil for many years. There is no cure, so prevention is everything: rotate, buy clean transplants and sets, and never move soil, tools or plant debris from an infected bed to a clean one.
Downy mildew appears in cool, wet weather as pale, elongated patches on the leaves, which develop a gray-violet fuzz when humid. Leaves collapse from the tip. Good spacing, a sunny open site, watering at the base rather than overhead, and removing infected leaves all help.
- Remove white rot plants with soil Dig out affected leeks with the soil around the roots and dispose of them in the trash, not the compost.
- Retire infected ground Do not grow onion family crops in a bed that has had white rot. Grow them elsewhere instead.
- Keep foliage dry Water at soil level and early in the day so leaves dry quickly, which discourages downy mildew.
If you are unsure what you are seeing, your local extension service or gardening advisory service can usually identify leek problems from a photo or sample.