Gadenin日本語Get the appFlea beetles are the main event
If your mustard leaves look as though someone has fired a shotgun at them, that is flea beetle. The adults are tiny, dark, shiny and jump like fleas when disturbed, and they chew round pits and holes a sixteenth to an eighth of an inch (1 to 3 mm) across in the leaf. On a mature plant the damage is cosmetic and the crop is still perfectly edible. On a seedling it is fatal, because a plant with half its first true leaves gone simply stops.
The beetles are most active in warm dry weather and they find this family by smell within days of germination. That means the damage happens in the window before you are paying attention. A row sown in warm weather and left uncovered for a week can be riddled before the seedlings have their second true leaf.
The answer is physical exclusion, applied at sowing rather than at the first sign of holes. Fine insect mesh or lightweight row cover laid over hoops, with the edges buried or weighted all the way round, keeps them off completely. Put it on the day you sow, and take it off only once the plants are established and you are ready to pick, or leave it on for the whole crop. Sprays are far less effective because the beetles arrive continuously from outside the bed.
Two cultural measures help alongside the mesh. Keep the surface damp, since flea beetles strongly prefer dry conditions and overhead watering in the afternoon discourages them. And sow in cooler weather where you can: a late-summer sowing goes through its vulnerable seedling stage as beetle activity is falling off.
| Problem | What you see | What to do |
|---|---|---|
| Flea beetle | Round shot holes; tiny jumping beetles | Insect mesh from the day of sowing; keep soil damp |
| Slugs and snails | Ragged holes with irregular edges, slime trails | Clear damp debris; trap or hand-pick at night |
| Aphids | Clusters on undersides and in the crown, sticky leaves | Strong water jet; squash colonies; remove worst leaves |
| Cabbage caterpillars | Large ragged holes, dark green droppings | Same mesh excludes the butterflies; pick off by hand |
| Downy mildew | Yellow blotches above, gray-purple felt beneath | Thin for airflow, water at the base, remove affected leaves |
| Clubroot | Stunting and wilting, swollen distorted roots | Do not replant this family in that soil; raise the pH |
Slugs, aphids and caterpillars
Slugs do the most damage in damp cool weather and in dense baby leaf beds where the canopy never dries out. Their holes are ragged and irregular rather than the neat pits of flea beetle, and there is usually slime somewhere on the plant. Keep the bed edges clear of boards and dead foliage, and go out with a torch an hour after dark on damp evenings, which removes more slugs in ten minutes than anything else.
Aphids gather in the crown and along the undersides of leaves, and on a crop you intend to eat raw they are more of a nuisance than a threat. A firm jet of water knocks colonies off, and removing the two or three worst leaves usually resolves a small infestation. Watch for ants farming them, which is often the first thing you notice.
Cabbage white caterpillars will strip a full-size plant. The same fine mesh that excludes flea beetles keeps the butterflies from laying, so one cover handles both. If plants are uncovered, check the undersides of leaves weekly for clusters of yellow eggs and rub them off before they hatch.
Diseases and disorders
Downy mildew is the common disease on this crop, appearing as pale yellow blotches on the upper surface with a grayish felt beneath, and it comes with cool damp still air. Crowding is the cause more often than the weather. Thin to the proper spacing, water at the base rather than over the leaves, and pick off affected leaves as soon as you see them.
Clubroot is the serious one. Plants stunt and wilt in warm afternoons despite damp soil, and if you lift one the roots are swollen and distorted. It lives in the soil for many years and there is no cure. If you find it, stop growing anything in this family in that ground, keep the pH above 7.0 by liming, and be careful not to move soil on tools and boots into clean beds.
Yellowing lower leaves on an otherwise healthy plant are usually just age or shortage of nitrogen. Yellowing across the whole plant with poor growth in wet weather is more likely to be waterlogged roots. Mustard tolerates a lot but will not sit in standing water, so if a bed puddles after rain, raise it before you sow into it again.
- Cover on the day of sowing Mesh applied after the first holes appear is already too late. Lay it over hoops with the seed, seal the edges completely, and lift it only to thin and water.
- Inspect the undersides weekly Aphid colonies, caterpillar eggs and downy mildew all show on the underside first. A two-minute look at half a dozen plants a week catches all three while they are still trivial.
- Do not follow one brassica with another Rotate this family off a bed for three years. Following mustard with cabbage or radish concentrates the pests and diseases they share.
Log the date you first see flea beetle damage each year, and where. That date is remarkably consistent from season to season, and once you know it you know exactly when the mesh has to be on.
Knowing when to give up on a stand
This crop is fast enough that resowing is almost always cheaper than rescuing. A seedling bed that has been badly hit by flea beetle before it has three true leaves will never make a decent crop, and the right move is to clear it, put mesh over the bed, and sow again the same day.
The judgement is different for mature plants. Shot-holed leaves on a full-size plant are ugly but the plant is productive and the leaves are fine to eat once washed. Keep those, pick around the worst damage, and save your effort for protecting the next sowing.