Gadenin日本語Get the appFlea beetles are the problem you will actually have
Small dark beetles, a couple of millimeters long, that jump when you disturb them. They chew shot holes through the leaf blade, and on a seedling with two seed leaves that is fatal. On an established plant it is cosmetic, but a heavily peppered leaf is not something you want in a salad bowl.
They are worst in warm dry weather and they find new brassica seedlings within a day or two of emergence. Once they are in a row, spraying does very little because the population keeps arriving from elsewhere. The answer is exclusion, and the timing of it is everything: fine insect mesh laid over hoops before the seed germinates, edges pinned down to bare soil all the way round with no gaps.
If a row is already under attack, watering the bed keeps the surface damp, which they dislike, and a sheet of yellow sticky trap laid flat between the rows takes a surprising number. But neither of those substitutes for the cover. Treat a bad flea beetle attack on unprotected seedlings as a lost sowing, clear it, and sow again under mesh.
The rest of the list, and how to spot each one early
Everything below is a standard brassica problem and none of them are specific to komatsuna. What is specific is the speed of the crop: at four to six weeks from sowing to cutting, a pest that takes a fortnight to build up may never do serious damage if you catch it at the start.
| Problem | First sign | When it appears | What to do |
|---|---|---|---|
| Flea beetle | Shot holes across the blade | Warm dry spells, worst on seedlings | Fine mesh from sowing; keep soil damp |
| Cabbage caterpillars | Ragged holes from the leaf edge, dark pellets | Warm season, after white or brown butterflies visit | Mesh; pick off by hand and check leaf undersides |
| Aphids | Clusters on new leaves, sticky sheen, curled tips | Mild spring and autumn weather | Rub off or hose off early; do not overfeed nitrogen |
| Slugs and snails | Whole seedlings gone overnight, slime trails | Wet spells, worst in autumn | Clear debris, trap or hand collect after dark |
| Cabbage root maggot | Plants wilt in the sun and pull up rootless | Cool wet conditions after sowing | Mesh to stop the fly laying; do not use fresh manure |
| Downy mildew | Pale yellow patches on top, gray mold beneath | Cool damp weather, crowded stands | Thin for airflow, water at the base, remove leaves |
| White rust | Raised white blisters on the leaf underside | Prolonged wet, cool conditions | Remove affected plants; do not resow brassicas there |
| Clubroot | Stunted plants, swollen distorted roots | Acid, wet soil where brassicas have grown | Lime to pH 7, rotate at least three years, use pots |
Clubroot is the only one on this list that changes what you can grow in a bed for years. If you pull a stunted plant and the roots are swollen and knobbly, do not compost it and do not sow any brassica in that ground again for at least three seasons.
Setting up the cover so it actually works
Insect mesh fails for the same three reasons every time: it goes on too late, the edges are not sealed, or the leaves press against it. Ten minutes of care at the start makes it a solution rather than a decoration.
- Put it on before the seed comes up The window between emergence and the first flea beetle is about a day. A cover laid over seedlings that already have beetles on them protects the beetles from birds and does more harm than good.
- Use a mesh fine enough for the target A mesh of about 0.8 mm excludes flea beetle as well as caterpillars and root fly. Ordinary garden netting sold for birds lets every one of them straight through. Check the stated aperture before buying.
- Seal every edge to bare soil Bury the edges in a shallow trench, or weight them continuously with boards or bricks. A single gap at a corner is enough for the row to be found. Do not simply drape the mesh over the plants.
- Hold it clear of the leaves with hoops Leaves touching the mesh get chewed through it and get scorched against it in sun. Wire or plastic hoops every three feet (1 m) keep a hand's width of air above the crop.
- Lift it only to weed and harvest, and reseal Water straight through the mesh; it lets water through perfectly well. Every time you open it, check the leaf undersides while you are in there, then close it properly rather than tucking it back roughly.
The wet weather diseases
Downy mildew is the one you are most likely to see: yellow angular patches on the upper surface of the leaf with a fine gray bloom underneath, appearing on the older lower leaves first. It runs in cool damp conditions and it runs fastest where the plants are crowded and the air is still. It will not kill a fast crop, but it makes the leaves unpleasant to eat.
The three practical measures are all cultural. Thin properly so air moves between the plants, water the soil rather than the foliage, and water in the morning so anything that does get wet dries before nightfall. Strip off and remove affected lower leaves as soon as you see them rather than leaving them to shed spores onto everything above.
White rust shows as small raised white blisters on the underside of the leaf, and is more serious because it disfigures the whole plant. There is no useful home treatment. Pull affected plants, take them out of the garden, and avoid sowing brassicas in that spot for a season.
Knowing when to give up on a row, and keeping the record
This is a five-week crop. That changes the calculation: nursing a badly damaged row through another three weeks costs you the same time as clearing it and sowing a clean one, and the clean one will be better. If more than about half the leaf area is chewed or spotted at the seedling stage, take it out. Sentiment is expensive here in a way it is not with a crop that takes six months.
Write down what appeared, on what date, and at what stage the plants were, along with whether the row was covered. The first appearance date of flea beetle or of the cabbage white butterfly in your garden is remarkably consistent from year to year, and once you have it written down for two seasons you know exactly when the cover has to be on rather than guessing.