Gadenin日本語Get the appLook underneath, because that is where the trouble is
Tatsoi has one unusual weakness among salad greens, and it follows directly from its shape. A flat rosette pressed against the soil creates a dark, permanently humid, sheltered space underneath itself, and that space is ideal for slugs, for woodlice, and for the fungal rots that take hold in a wet autumn. From above the plant can look immaculate while the bottom third of it is being eaten or is quietly turning to slime.
So the routine inspection for tatsoi is not turning leaves over to check the undersides, which is what you do for aphids on an upright green. It is lifting the whole skirt of the plant once a week and looking at the ground it is lying on. Ten seconds per plant catches almost everything early.
Anything you find there is easier to deal with the week you find it. Slugs sheltering under one rosette will be under all of them within a fortnight, and one leaf turning to slime against wet soil becomes a rotten crown in a wet spell.
Flea beetles are a seedling problem, not a plant problem
Peppered round holes across the leaves, as though the plant had been hit with birdshot, are flea beetles. They are dark beetles about an eighth of an inch (3 mm) long that jump when disturbed, they are worst in warm dry weather, and they are the most common visible damage on tatsoi.
On an established rosette the damage is cosmetic and the plant grows straight through it. On a seedling it is fatal, because a plant at the seed leaf stage that loses half its leaf area simply stops and never restarts. Everything therefore depends on protection during the first three weeks and almost nothing depends on it afterwards.
Fine insect mesh laid over the row from the day of sowing is close to complete protection. Weight or bury the edges so the beetles cannot walk underneath, and leave it in place until the plants have six or seven true leaves. Tatsoi needs no insects to make a crop, so there is no cost to leaving the mesh on longer, and in autumn it doubles as protection from cabbage caterpillars and from the worst of the rain splash that drives grit into the rosette.
What you see, and what to do about it
Most tatsoi problems are recognizable at a glance once you know where to look. Work through the row weekly, lifting each plant's outer leaves, and deal with what you find rather than making a note to come back.
| Symptom | Cause | First response |
|---|---|---|
| Small round holes across the leaves | Flea beetles | Cover with fine mesh; keep plants watered |
| Large ragged holes, slime trails, damage from below | Slugs and snails sheltering under the rosette | Lift the plant, clear them by hand after dark |
| Bottom leaves slimy and brown against the soil | Soft rot from leaves lying on wet ground | Strip them off; slide straw or a tile underneath |
| Sticky leaves, colonies at the center | Aphids | Wash out with a jet of water; feed less |
| Pale yellow patches above, gray fuzz beneath | Downy mildew | Remove affected leaves; thin for airflow |
| Leaves stripped to the veins in autumn | Cabbage caterpillars | Search under the leaves and remove by hand |
| Plant wilts in mild weather, root swollen and distorted | Clubroot | Lift and remove entirely; do not grow brassicas there |
| Whole rosette collapses at the crown | Crown rot in waterlogged soil | Pull and remove; improve drainage before resowing |
Slugs, and the winter arrangement
Slugs are the pest that matters most for a winter tatsoi crop, because unlike flea beetles they are active in cool damp weather, which is exactly the weather the crop is grown in. Under a cloche the problem intensifies: you have created a warm, sheltered, humid space and a flat plant lying on the ground inside it.
The answer is mostly mechanical and mostly about denying shelter rather than about killing what is already there. Keep the ground under and around the plants clear of dead leaves, weeds, boards and pots. Go out after dark with a torch two or three nights running when you first see damage, which removes the resident population far more effectively than anything spread on the surface. Check under every cloche each time you open it.
- Clear shelter before you cover the crop Before a cloche or row cover goes on, weed thoroughly, remove all fallen leaves and any loose debris, and pick off the slugs you find. Whatever is under there when you cover it stays there all winter in perfect conditions.
- Raise the leaves off wet ground A handful of straw, a slate, or a piece of tile slipped under each rosette keeps the bottom leaves off the mud. It reduces both slug damage and rot, and it makes the plants far cleaner at harvest.
- Ventilate on mild days Open the cloche whenever the weather is mild and dry. Still, saturated air under cover is what downy mildew and soft rot need; an hour of moving air on a mild afternoon prevents more disease than any treatment.
Tatsoi is a brassica. Do not follow other brassicas in the same ground, and leave two or three years before it returns to any bed that has grown them. Clubroot, once established in a bed, stays for many years and there is no cure.
Knowing when to pull, and what to record
Tatsoi is fast enough that replacing a badly damaged plant usually beats nursing it, at least while there is still time to sow. Pull immediately any plant with a swollen distorted root, any plant collapsed at the crown, and any plant where more than half the leaves show mildew, and take them off the plot rather than composting them. Treat and watch for a week where you have flea beetle damage on established plants, scattered aphids, or a few caterpillar holes. Ignore light shot-holing on the outer leaves of a plant that is growing well.
The record that pays for itself here is a first-sighting date. Write down the date you first see flea beetles, the date you first find slug damage, and the date you first see mildew, along with which bed it was in. Pests arrive within a fairly narrow window in a given garden, and after two seasons those dates tell you exactly when the mesh has to go on and when the straw has to go under the plants, which is before the damage rather than after it.