Gadenin日本語Get the appThe first two weeks belong to flea beetle
Flea beetles are at their worst in warm dry weather, which is exactly the weather your late-summer sowing comes up in. They shot-hole the cotyledons and first true leaves, and a seedling that loses half its leaf area in its first week is permanently checked. A checked plant runs to bud early on a short stem with few joints, so this is not a cosmetic problem; it costs you the whole shape of the crop.
The answer is a physical barrier put on at sowing, not after you see damage. Fine insect mesh over hoops, sealed at the edges with soil, keeps them out completely. Keep it on until the plants have six or seven true leaves, at which point they are growing faster than the beetles can eat and the mesh can come off for good.
Keeping the surface damp helps as well, because flea beetles dislike humidity. A daily light watering over the seedling row through the first fortnight costs nothing and visibly reduces the damage.
| Problem | First sign | When it hits | What to do |
|---|---|---|---|
| Flea beetle | Tiny round shot holes in seedling leaves | First two weeks, warm and dry | Mesh from sowing; keep the surface damp |
| Cabbage caterpillars | Ragged holes, dark green pellets in the crown | Late summer into early autumn | Mesh, or pick off by hand every two days |
| Aphids | Gray or green colonies deep in the crown and on buds | Autumn, and again under winter cover | Strong water jet into the crown; cut infested shoots out |
| Slugs and snails | Rasped holes, silvered trails on leaf undersides | Wet spells, worst on young plants | Clear mulch back from the crown; trap and check nightly |
| Cabbage root fly | Plant wilts in sun with no dry soil to explain it | Weeks two to six after planting out | Collars at the stem base; pull and destroy affected plants |
| Downy mildew | Yellow patches above, gray furry growth beneath | Cool damp weather, worst under cover | Increase spacing and ventilation; remove worst leaves |
| Crown rot | Soft brown collapse at soil level | Cold wet spells | Stop watering; pull affected plants; improve drainage |
Aphids hide exactly where you harvest
The bud cluster at the top of a flowering stem is the perfect aphid habitat: sheltered, soft, and constantly producing new growth. Colonies build inside the buds where you cannot see them from above, and you find them when you are washing the stems. Once they are established in the buds there is no way to clean them out of a harvested stem to anyone's satisfaction, so the answer is to check before cutting, not after.
Turn a stem over and look into the underside of the bud cluster and at the joints where shoots emerge. If you find a colony, cut that shoot off and remove it from the bed entirely rather than dropping it between the rows. A hard jet of water directed up into the crown knocks most of a light infestation out and does no harm to the plant.
Aphids get much worse under winter row cover or a low tunnel, because the cover excludes their predators as well as the cold. If you are covering the bed, lift the cover on mild days and look into two or three crowns. Finding the first colony in week one is a two-minute job; finding it in week four means cutting out a third of the shoots on the bed.
Wet cold weather and the crown
The crown of a purple choy sum plant is a shallow cup made of overlapping leaf bases, and in cold wet weather it holds water. That standing water is where soft rot starts, and once the base of the plant goes soft and brown at soil level there is no recovery. Pull the plant, take it out of the bed, and do not compost it in a heap you intend to spread on brassica ground.
Prevention is mostly drainage and airflow. Plant into ground that does not puddle, keep mulch a thumb's width clear of the stem, earth up the base so water runs away from the crown rather than into it, and resist the temptation to plant closer than the spacing you decided on. A crowded bed in a wet winter stays wet all day, and that is when rot moves from one plant to its neighbors.
Downy mildew follows the same conditions. It shows as yellow blotches on the upper leaf surface with a gray bloom underneath, and it is far worse under cover than in the open. Take the worst leaves off, prop the cover open on any mild dry day for a few hours, and space plants further apart next season rather than reaching for a spray.
- Mesh goes on at sowing Not when you see damage. Fine insect mesh over hoops, edges buried, from sowing until six or seven true leaves. This single step removes flea beetle and caterpillars together.
- Check inside two crowns every week Part the leaves and look at the base and at the bud clusters. Aphids, caterpillar pellets and the first softening at soil level are all found here and nowhere else.
- Cut out and remove, do not drop Anything infested leaves the bed. Shoots dropped between the rows re-seed the problem within days, especially with aphids and slugs.
- Pull rotting plants immediately A plant that has gone soft at the base will not recover and is a source of infection for its neighbors in wet weather. Take it out the day you find it.
Write down what appeared and when
Record the date you first saw each problem and where in the bed it started. Pests arrive at close to the same point in the season every year, and knowing that flea beetle showed up nine days after sowing last time means the mesh goes on at sowing rather than on day ten. Note also which end of the bed was affected first, because that is usually a clue about shelter, drainage or a neighboring crop.
Keep a note of what you did and whether it worked. A bare record of an infestation is much less useful a year later than a line saying you jetted the crowns twice and it held them for a fortnight.
Do not grow this in ground that carried any other brassica in the previous year. Root fly, clubroot and several rots all build in the soil, and a three or four year gap between brassica crops on the same ground prevents more trouble than any treatment cures.