Gadenin日本語Get the appFlea beetle: shot holes in the seedlings
Flea beetle is the pest that ruins bok choy, and it does its damage in the first three weeks. The adults are tiny and dark and jump when disturbed, and they chew round pits and small holes through the leaf until a seedling looks as though it was hit with birdshot. On a plant with four leaves that is most of its leaf area gone, and it never gets its speed back.
They are worst in warm dry spells in spring and again in late summer, and they walk and jump more than they fly, which is why a mesh edge buried into the soil works so well. On a mature plant the same damage is cosmetic, and a full-size head shrugs it off entirely. The whole problem is a seedling problem.
| Situation | Risk | Action |
|---|---|---|
| Warm dry spring, no cover | Very high | Mesh from the day of sowing, edges buried |
| Bed next to last year's brassicas | High | Move the bed and clear the old stumps |
| Seedlings at one to four true leaves | High | Keep the soil damp, because they avoid humid beds |
| Full-size heads | Low | Ignore the holes, they cost you nothing |
Bare dusty soil suits flea beetle and damp mulched soil does not. Watering the seedling bed in the evening through a dry spell reduces the damage noticeably on its own.
Slugs and snails: whole seedlings gone overnight
The other big loss is slugs, and the signature is different. Look for irregular holes with ragged edges eaten inward from the leaf margin, silvery trails on the soil, and whole seedlings vanished leaving a bare stem. Bok choy is soft, low and dense, which is close to ideal slug habitat, and a full head with its outer leaves resting on damp soil hides them all day.
Search at dusk or after rain with a torch and pick them off, because that is what finds the large ones doing the real damage. Then take away the shelter: lift the outer leaves clear of the soil, keep bed edges free of long grass and old boards, and do not mulch heavily right up to the stem of a young plant.
Caterpillars, aphids and root fly
Cabbage white caterpillars strip leaves to the veins and are easy to find on the undersides once you know to turn the leaf over. Gray cabbage aphid forms waxy gray colonies deep in the crown and is far harder to wash out of a head than to prevent. Cabbage root fly lays at the stem base and its larvae eat the roots, so the plant wilts in sun, recovers at night, and eventually lifts out of the soil with almost no root left on it.
All three are excluded by the same mesh that stops flea beetle, which is why the cover pays for itself several times over. Where a plant is already infested, aphid colonies deep in the crown mean that head is not worth saving. Cut it out and take it off the plot rather than leaving it as a source for the rest of the bed.
| Pest | What you see | What to do |
|---|---|---|
| Cabbage white caterpillar | Leaves stripped to the veins, green droppings | Turn leaves over and pick off. Mesh prevents it |
| Gray cabbage aphid | Waxy gray colonies packed into the crown | Cut out bad heads. Stop overfeeding nitrogen |
| Cabbage root fly | Wilts in sun, lifts with almost no roots | Collar discs at the stem base, or mesh from sowing |
| Cutworm | Seedling cut clean through at soil level overnight | Search the soil an inch down around the stem |
Diseases: downy mildew and clubroot
Downy mildew shows as pale yellow angular patches on the upper leaf with a fine gray-purple bloom underneath, and it takes hold in cool wet weather with still air. Overcrowding and evening watering are what invite it. Thin on time, water in the morning, and ventilate any cloche on fine days.
Clubroot is the one that ends brassica growing in a bed. Plants are stunted and wilt in sun, and when you pull one the roots are swollen into distorted clubs. There is no cure, and the spores persist in soil for many years, so the response is rotation and lime. Keep bok choy off any bed that has grown brassicas in the last three years, and raise the pH towards 7.0, which slows the disease considerably.
Do not compost clubbed roots, and do not carry soil or tools between an affected bed and a clean one. That is how one bad bed becomes a whole garden.
Reading the damage
Most of the time you can name the culprit from the shape of the hole alone, which matters because the responses are entirely different. Check the leaf underside first, then the soil at the stem, then the roots, in that order.
| What you see | Most likely cause | How to check |
|---|---|---|
| Many small round pits and shot holes | Flea beetle | Tap the plant and watch for jumping beetles |
| Ragged holes from the leaf edge, slime trails | Slugs and snails | Look under leaves and boards at dusk |
| Leaf stripped to the veins, droppings present | Caterpillars | Turn every leaf over and search the crown |
| Yellow angular patches with bloom underneath | Downy mildew | Look at the leaf underside in good light |
| Wilts in sun and recovers at night | Root fly or clubroot | Pull one plant and look at the roots |
| Brown papery edges on inner leaves only | Tip burn, not a pest at all | Check watering and recent nitrogen |
Prevention is a rotation and a cover
Almost everything above is prevented by two habits rather than treated by any product. First, do not grow bok choy where any brassica grew in the last three years, which breaks flea beetle, root fly and clubroot at once. Second, cover from the day of sowing and open the bed only for as long as it takes to thin or weed.
Keep a note of which bed each sowing went into and what appeared in it. Brassica problems are soil problems on a multi-year cycle, and the record of where the crop stood is the only thing that lets you plan a rotation honestly two seasons later, when you can no longer remember.