Gadenin日本語Get the appFlea beetles are the main event
Flea beetles are small dark beetles, an eighth of an inch (3 mm) long, that jump when disturbed and chew round shot holes through the leaf. They are drawn to mustard more strongly than to most brassicas and they arrive within days of emergence, particularly in warm dry weather. On a plant with four true leaves they can take out the whole stand; on a plant with ten they are cosmetic.
The answer is entirely preventive. Lay lightweight row cover or insect mesh over the bed on the day you sow, before anything is up, and bury or weight the edges completely. Once beetles are already on the crop, covering it seals them in and makes matters worse, so at that point remove the cover and deal with them directly.
Two other measures genuinely help. Keeping the soil surface damp discourages the adults, which prefer hot dry dust, so overhead watering in the afternoon during an outbreak has a real effect. And sowing thickly then thinning gives you enough surplus seedlings to absorb early damage rather than losing the row.
| Symptom | Likely cause | What to do |
|---|---|---|
| Round shot holes, seedlings stunted | Flea beetles | Cover the row; keep soil surface damp; thin later |
| Ragged holes, slime trails at dawn | Slugs and snails | Clear debris, trap or bait, water in the morning |
| Sticky curled growing points | Aphids | Strong water jet, remove worst leaves, check undersides |
| Large irregular holes, green droppings | Caterpillars | Hand pick, keep mesh on, check leaf undersides weekly |
| Whole plant wilts in sun, recovers at night | Root damage or cabbage root fly | Lift and check roots; use collars on the next sowing |
Slugs, aphids and caterpillars
Slugs do their worst damage on autumn and overwintering plants, working at night and sheltering under the outer leaves that lie on the soil. Keeping the base of each plant clear of fallen leaves, watering in the morning rather than the evening, and going out with a torch on the first mild damp night after a dry spell will deal with most of the population.
Aphids gather in the growing point and on leaf undersides, and they are worse on plants that have been fed heavily with nitrogen. A firm jet of water knocks them off and they largely do not climb back. If a growing point is badly colonised, take that leaf off entirely rather than trying to clean it, and check the crop weekly rather than monthly, because aphid numbers double quickly in warm weather.
Cabbage white caterpillars and other brassica caterpillars chew large ragged holes and leave dark green droppings that are usually easier to spot than the caterpillar itself. Hand picking is entirely adequate on a garden scale if you do it weekly and look on the undersides of leaves near the damage. Mesh left on for the whole crop prevents the butterflies laying in the first place, which is the simpler route.
Diseases and disorders
Most disease problems on leaf mustard trace back to crowding, poor airflow and wet foliage. Downy mildew shows as pale yellow patches on the upper leaf surface with a gray fuzz underneath and appears in cool damp spells on dense plantings. White rust produces raised white blisters on the undersides. Both are managed the same way: correct spacing, remove infected leaves as soon as you see them, water at the base rather than over the top, and do not work among wet plants.
Base and crown rot on mature plants generally follows mulch or fallen leaves held tight against the stem through a wet cold spell. Pull mulch back an inch (2.5 cm) from the stems going into winter and clear yellowing lower leaves as they appear.
Clubroot is the one soil-borne disease worth taking seriously because it persists for years. Plants stunt, wilt in the middle of the day and have swollen distorted roots when lifted. There is no cure for an infected bed, so the defense is rotation: keep at least three years between brassica crops on the same ground, and never move soil or compost from an affected bed to a clean one.
- Inspect twice a week while plants are small Look at the undersides of leaves and into the growing point. Almost every problem on this crop is cheap to fix in the first week and expensive to fix in the third.
- Keep the soil surface clear Remove fallen and yellowing leaves rather than leaving them as mulch. That single habit removes the shelter that slugs, rots and overwintering pests depend on.
- Rotate the whole brassica group together Mustard shares its pests and diseases with cabbage, kale, radish and turnip. Treat them as one block in your rotation plan and leave three years before returning to the same ground.
Log the date you first see flea beetle damage each year, along with the bed used. That first-sighting date shifts very little between seasons, and once you have two years of it you know exactly how early the row cover has to go on.
Knowing when to write off a stand
This crop is quick and cheap enough that persisting with a bad stand is usually a mistake. If a seedling block has lost more than half its plants to flea beetles before the four-leaf stage, clear it, wait two weeks, and re-sow under cover rather than nursing survivors that will stay stunted and bolt early.
The same logic applies to a bolting spring crop hit by aphids, or a mildewed block in a wet autumn. Pull it, take the debris out of the garden rather than composting it on site if disease is involved, and sow again. From sowing to first baby leaf cut is only three to four weeks, so a restart costs far less than the season it might otherwise waste.