Gadenin日本語Get the appFlea beetles decide whether the crop starts
Small round holes a sixteenth to an eighth of an inch (1 to 3 mm) across, scattered across the leaf like shot, are flea beetle. The adults are tiny, dark, shiny and spring away when you lean over them. They are at their most numerous in the warm dry weather of late summer, which is exactly when this crop is sown, so the sowing and the peak of the pest coincide by definition.
The damage matters entirely according to plant size. On a seedling with only its seed leaves and a first true leaf, heavy feeding is fatal, and a row sown uncovered in warm weather can be destroyed in under a week. On a plant a foot (30 cm) across, the same beetles are cosmetic and the crop is unaffected.
The answer is exclusion applied on the day of sowing, not at the first sign of holes. Lay fine insect mesh over hoops and bury or weight the edges the whole way round so there is no gap at soil level, since the beetles walk in as readily as they fly. Keep it on until the plants are established and press against it. Sprays achieve little because fresh beetles arrive from outside the bed continuously.
Two cultural measures support the mesh. Flea beetles strongly prefer dry surfaces, so watering overhead in the afternoon during the seedling stage genuinely reduces feeding. And do not sow immediately after another brassica in the same ground, because the population is already sitting there.
| Problem | What you see | What to do |
|---|---|---|
| Flea beetle | Round shot holes; tiny jumping beetles | Fine mesh from the day of sowing; keep the surface damp |
| Cabbage caterpillars | Large ragged holes, dark green droppings in the crown | The same mesh excludes the butterflies; pick off by hand |
| Slugs and snails | Ragged holes with irregular edges, slime, damage at the base | Clear debris and dead leaves; hunt after dark on damp evenings |
| Aphids | Gray or green clusters in the crown and under leaves | Strong water jet; remove the worst leaves; watch for ants |
| Downy mildew | Yellow blotches above, gray felt beneath | Widen spacing, water at the base, strip affected leaves |
| Crown and base rot | Slimy brown collapse where leaves meet the root | Improve drainage, stop late feeding, cut affected plants at once |
| Clubroot | Wilting in sun despite damp soil; swollen distorted roots | Stop growing this family in that ground; lime to raise the pH |
Caterpillars, slugs and aphids
Cabbage white caterpillars will reduce a large plant to midribs in a fortnight if the butterflies get access while the weather is still warm. Their damage is unmistakably different from flea beetle: big irregular holes, and dark green droppings collecting where the leaf stalks meet the crown. If you find droppings, the caterpillar is nearby, usually pressed along a midrib on the underside. The mesh you put on against flea beetle keeps the butterflies out entirely, which is the strongest argument for leaving it on.
Slugs become the main pest later, once the canopy has closed and the ground beneath the plants stays permanently damp and shaded. They chew ragged holes and, worse, work into the base of the plant where the leaf stalks are packed together, which spoils the part of the crop you most want. Keep the lower leaves cleared and the bed edges free of boards and debris, and go out with a torch an hour after dark on damp evenings. Ten minutes of hand collecting does more than anything else available.
Aphids cluster in the crown and on the undersides of leaves and are more a nuisance than a threat on a vigorous plant, but they are hard to wash out of a densely packed base at harvest. Knock colonies off with a firm jet of water, remove the two or three worst leaves, and look for ants running up the stalks, which is usually the first sign that a colony has established.
Rots, mildew and the soil-borne problem
Crown rot is the disorder that costs whole plants. It appears where the leaf stalks meet the root as a slimy brown softening, and the plant collapses from the middle outwards. Three things cause it: soil that stays waterlogged, late nitrogen feeding that leaves soft growth going into cold weather, and mulch or fallen leaves packed tight against the crown. Fix all three preventively, because there is no treatment once it has started. Cut and remove an affected plant immediately rather than letting it turn to slime in the row.
Downy mildew shows as pale yellow blotches on the upper leaf surface with a grayish felt on the underside, and it arrives with cool damp still air, which is the autumn weather this crop grows in. Crowding is more often the real cause than the weather. Proper spacing, watering at the base rather than over the foliage, and prompt removal of affected leaves keep it to a level that does not matter.
Clubroot is the one that ends a bed. Plants wilt in afternoon sun despite damp soil, stay stunted, and when lifted show swollen, distorted, club-shaped roots. It persists in soil for up to twenty years and there is no cure. If you find it, stop growing anything in this family in that ground, lime to bring the pH above 7.0, and be careful about carrying soil on boots and tools into clean beds. Raising future plants in large pots of clean compost is the only practical way to keep growing brassicas on infected land.
- Seal the mesh edges completely Bury the edges in a shallow trench or weight them with boards along their full length. A mesh cover with a two-inch gap at one corner protects nothing, since both flea beetles and egg-laying butterflies will find it.
- Look under the outer leaves every fortnight Lift the skirt of each plant and check the crown and the soil beneath. Caterpillar droppings, slug damage, aphid colonies and the start of crown rot are all visible there weeks before they are visible from above.
- Rotate this family off the bed for three years Do not follow this crop with cabbage, radish, turnip or any other brassica. Repeating the family in one bed concentrates the shared pests and is how clubroot establishes in a garden.
- Lift and remove the stumps at harvest Cut roots and stumps left in the ground over winter carry pests and disease into the next season. Pull them when you cut the plants, check the roots for swellings, and put them out with the rubbish rather than on the compost pile if anything looks wrong.
Log the date you first see flea beetle damage and the date you first see cabbage white butterflies over the bed each year. Both are remarkably consistent from season to season, and once you know them you know exactly how long the mesh has to stay on.
When to give up on a plant or a bed
The judgement differs completely between seedlings and mature plants. A seedling bed badly hit before the plants have three true leaves will never make full-size plants, and the correct response is to clear it, put mesh over the ground, and sow again the same day if the sowing window is still open. Trying to nurse damaged seedlings through wastes the rest of the season.
A large plant with shot-holed or caterpillar-chewed outer leaves is a different matter and is almost always worth keeping. Strip the damaged outer ring at harvest and the packed inner leaves and stalks, which are the part of the crop you actually want, are usually untouched. Save your effort for protecting the next sowing rather than rescuing the current one.