Gadenin日本語Get the appFlea beetles: the damage everyone gets
Small round holes peppered across the leaf, as though someone fired birdshot at it, are flea beetles. They are tiny dark beetles, an eighth of an inch (3 mm) long, that jump when disturbed, and they are the single most common problem on mizuna. Warm dry weather brings them out; they are far less trouble on an autumn crop grown into cooling weather, which is one more reason to favor autumn sowings.
The damage is cosmetic on an established plant and fatal on a seedling. A mizuna at four true leaves can lose a fifth of its leaf area and grow on without complaint. A plant at the seed leaf stage that gets the same treatment simply stops. So the protection that matters is protection in the first three weeks.
Fine insect mesh laid over the row from the day you sow is the answer, and it is close to complete. Bury or weight the edges so the beetles cannot walk in underneath, and keep it on until the plants are well past the seedling stage. Because mizuna does not need insects to set a crop, there is no cost to leaving the mesh on all season.
What to look for, and what it means
Most mizuna problems announce themselves clearly if you turn a few leaves over each time you pick. Checking the underside and the crown takes seconds and catches nearly everything at the stage where it is still easy to deal with.
| Symptom | Cause | First response |
|---|---|---|
| Small round holes across the leaf | Flea beetles | Cover with fine mesh; keep the plants watered |
| Sticky leaves, curled tips, colonies in the crown | Aphids | Blast off with water, or squash by hand; feed less |
| Large ragged holes, slime trails | Slugs and snails | Clear hiding places; hand-pick after dark |
| Pale yellow patches on top, gray fuzz beneath | Downy mildew | Remove affected leaves; thin for airflow |
| Leaves mined with pale winding trails | Leaf miner | Pick off and bin the affected leaves |
| Whole plant collapses at soil level | Soft rot or root problems in wet soil | Pull and remove it; improve drainage |
| Leaves stripped to the veins | Caterpillars | Search the undersides and remove by hand |
Aphids and downy mildew both come from crowding
Aphids settle deep in the crown among the stem bases where they are hard to see and hard to reach, and a soft, overfed, crowded plant is exactly what they want. So is downy mildew, which shows as pale patches on the upper leaf surface with a gray or violet fuzz on the underside, and which needs still, humid air at the base of the plant to establish.
Both are far more about growing conditions than about treatment. A plant at proper spacing, fed moderately, with the old lower leaves stripped away and the soil beneath it clear, rarely gets a serious infestation of either. The same plant crowded at two inches and pushed hard with high-nitrogen feed will get both.
When aphids do appear, a firm jet of water into the crown removes most of them and can be repeated every few days. Check the crown specifically, not just the leaves; a plant that looks clean from above can be carrying a colony at the base.
Mizuna is a brassica. Do not follow other brassicas in the same ground, and leave two or three years before mizuna returns to a bed that has grown any of them. Clubroot, once it is in a bed, stays for many years.
When to give up on a plant
Knowing when to pull is worth more than any spray. Mizuna is fast enough to replace that nursing a struggling plant is almost never the right call: a fresh sowing will be cropping in three to four weeks, which is often sooner than a badly damaged plant would recover.
- Pull immediately Any plant collapsed at the crown, any plant with a distorted swollen root, and any plant where more than half the leaves show mildew. Remove them from the garden entirely rather than composting them.
- Treat and watch for a week Flea beetle damage on established plants, scattered aphids, a few holes from caterpillars. Deal with the cause, then look again in seven days. If it has spread, pull.
- Ignore it Light shot-holing on outer leaves of a plant that is growing strongly. It costs you nothing and the inner leaves you actually pick are usually untouched.
Keeping a record that pays off
Note the date you first see each problem each year, and which bed it was in. Flea beetles and aphids arrive within a fairly narrow window in a given garden, and once you have two seasons of first-sighting dates you know exactly when the mesh has to go on. That single date, written down, prevents more damage than anything you can buy.