Gadenin日本語Get the appFlea beetles: the damage everyone gets
If your leaves are peppered with pinhole shot-holes, that is flea beetle, and every brassica grower has it. The beetles are tiny, dark and jump when disturbed, and they feed hardest in warm dry weather on young plants. They rarely kill anything established, but a seedling bed can be shredded to lace in three days, and a plant that spends its early life defending itself never makes good leaves.
The only reliable answer is exclusion. A fine insect mesh laid over hoops from the day you sow, with the edges properly buried, keeps them off entirely. It has to go on before the beetles arrive, not after you see holes, because once they are under the mesh you have built them a shelter. The other lever is timing: sow after the summer heat breaks and the pressure drops away sharply on its own. Keeping the bed damp helps too, since flea beetles prefer dusty dry conditions.
What to look for, and what it means
Most problems announce themselves on the leaves before the plant is in trouble. Walk the row once a week, turn a few leaves over and look at the crown, and you will catch nearly everything early enough to act.
| What you see | Likely cause | What to do |
|---|---|---|
| Pinhole shot-holes over the leaf | Flea beetle | Mesh the bed; keep soil damp; crop will outgrow it |
| Ragged holes with slime trails | Slugs and snails | Lift leaves off soil; trap or pick after dusk |
| Sticky curled young leaves | Aphids | Blast with water; strip the worst leaves; thin crowding |
| Pale yellow blotches, gray felt beneath | Downy mildew | Improve airflow; remove leaves; do not water overhead |
| Crown brown and soft, leaves collapse | Soft rot in a wet crown | Pull the plant; do not replant brassicas there this season |
| Wide pale tunnels inside the leaf | Leaf miner | Pick off the mined leaves and bin them |
Slugs and wet crowns are the winter problem
The flat, spreading habit that makes mibuna productive also makes it the best slug hotel in the winter garden. The outer straps lie on damp soil, and the space under a mature rosette stays sheltered and moist all day. Slugs graze from underneath, so the damage is often well advanced before you see anything from above. Check by lifting the skirt of the plant rather than looking down at it.
The same geometry causes rot. Water sits in the center of the rosette, and in a mild wet winter without a hard freeze to dry things out, that center can turn brown and soft while the outer leaves still look fine. Once the crown goes, the plant is finished, and pulling it promptly stops the rot spreading to neighbors. Spacing, morning watering at the root rather than the crown, and stripping the flat bottom leaves are what prevent it. Overhead watering in the evening is the single habit most likely to cause it.
Crowding is behind most aphid and mildew outbreaks. If a section of row is repeatedly worse than the rest, look at the spacing before you look for a spray. Ten inches (25 cm) between full plants prevents more disease than any product will cure.
When to give up on a plant
Salad crops are cheap to replace and slow to recover, so the threshold for pulling one should be low. Trying to nurse a badly damaged mibuna through means you are also keeping a reservoir of pests and spores in the middle of a healthy row.
- Pull it if the crown is soft A crown that is brown, mushy or smells sour will not recover. Lift the whole plant with its root, take it out of the bed and do not leave it lying on the soil nearby.
- Strip it if more than half the leaves are damaged If the crown is still firm, cut everything off at two inches (5 cm), clear away all the cut material, feed and water. A clean restart from the growing point usually beats picking over damaged leaves for weeks.
- Move the crop if it happens twice in the same spot Repeated rot or clubroot-like stunting in the same ground means the soil is the problem. Grow brassicas somewhere else for at least three years and put the mibuna in containers or a different bed meanwhile.
Keeping a record that pays off
Write down the date you first see flea beetle damage and the date you first see slug damage, every year, along with which end of the bed it started at. Two seasons of those dates tell you precisely when your mesh needs to go on and when the evening slug patrol is worth doing, and that is far more useful than a general rule. Note which bed each batch grew in as well, because brassica soil problems are cumulative and easy to forget between seasons.