Gadenin日本語Get the appTwo pests do most of the damage
Turnips are a brassica and they inherit the whole brassica pest list, but in practice two problems account for most of the crop that gets lost. Flea beetle destroys seedlings in the first fortnight, and cabbage root fly hollows out the root in the second half of the crop. Everything else is secondary.
Both are prevented rather than cured. By the time you can see flea beetle damage the seedling has already lost the leaf area, and by the time a root fly maggot is visible in a turnip that turnip is finished. The decisions that determine whether you lose the crop are taken on the day you sow.
The good news is that a single insect mesh over the bed, put on at sowing and left until harvest, deals with both of them and with cabbage white caterpillars as well. Nothing else in growing turnips gives that much return for that little effort.
Flea beetle: shot-holes in the seedlings
Flea beetles are small dark beetles, a couple of millimeters long, that jump when disturbed. They chew round pits and holes through the leaves, so a badly attacked seedling looks as though it has been fired at with shot. Damage is worst in warm dry weather and on plants at the seed leaf stage.
A mature plant shrugs it off. A seedling with two round seed leaves does not, because it has no spare leaf area, and losing half of it means the plant stalls or dies outright. This is why an attack in the first ten days after emergence is far more serious than the same attack a month later.
- Cover the bed at sowing, not later Lay fine insect mesh over the bed the day you sow and bury or weight the edges all the way round. Putting it on after the seedlings appear is often too late, because the beetles arrive with the first leaves.
- Keep the surface damp Flea beetle activity drops sharply in damp conditions. Watering the row in the evening through the seedling stage measurably reduces the damage, and it helps the plants outgrow what damage they take.
- Get them through the seedling stage fast Anything that speeds early growth reduces the losses. Sow into warm soil rather than cold, do not sow too deep, and thin promptly so the survivors are not competing.
- Sow a wider row and accept losses Where you cannot cover the bed, sow more thickly than you need and thin later. It is a crude answer but turnip seed is cheap and it works.
Do not lay the mesh directly on the seedlings in hot weather. Support it on hoops so there is an air gap, and check underneath weekly. A mesh with a gap at one corner concentrates every beetle in the garden into the one place they can get in.
Cabbage root fly: the damage you find too late
The adult is a small gray fly that lays its eggs on the soil against the stem. The maggots hatch and burrow down into the root, leaving brown tunnels through the flesh and often hollowing the base entirely. The first sign above ground is a plant that wilts on a hot day and does not recover overnight, and by then the root is not salvageable.
There are usually two or three generations through the season, the first as the soil warms in spring and later ones through summer. A crop sown in the hottest part of the year and harvested as the weather cools frequently escapes the worst of it, which is one reason turnips sown for autumn are so much cleaner than spring sowings.
| Control | How it works | When to put it in place |
|---|---|---|
| Insect mesh over the bed | The fly never reaches the soil | At sowing, edges sealed |
| Collars around the stem | The fly cannot lay against the stem | As soon as the seedlings are thinned |
| Crop rotation | Reduces the pupae carried over in the bed | Three years between brassica crops |
| Firming the soil around plants | Makes it harder for maggots to move down | After every thinning and weeding |
| Timing the sowing | Misses the peak egg laying periods | Decide before you sow |
Lift and remove any plant that wilts and does not recover, and do not leave it on the bed. The maggots inside will complete their development in the soil and give you a worse problem in the same bed next season.
Clubroot, mildew and the rest
Clubroot is the disease that ends brassica growing in a bed. The roots swell into distorted galls, the plant wilts in warm weather and never fills a usable root, and the spores stay viable in the soil for many years. There is no cure once it is present, so everything rests on not bringing it in and on keeping the conditions it dislikes.
The other diseases are milder. Downy mildew shows as pale yellow patches on the upper leaf surface with gray growth underneath, mostly on crowded seedlings in damp still weather. White blister raises glossy white pustules on the leaves. Neither usually costs you the root if the plants are growing fast and have air moving through them.
| Problem | What you see | What to do |
|---|---|---|
| Clubroot | Swollen distorted roots, wilting in the heat | Lime to pH 7, rotate for many years, never move soil off the bed |
| Downy mildew | Yellow blotches above, gray fuzz below | Thin harder, water the soil not the leaves |
| White blister | Glossy white raised spots on leaves | Remove affected leaves, improve airflow |
| Mealy cabbage aphid | Gray waxy colonies in the leaf crown | Squash colonies early, avoid overfeeding nitrogen |
| Cabbage white caterpillars | Leaves stripped to the veins | Mesh, or pick off the eggs under the leaves |
| Slugs | Whole seedlings gone overnight | Clear cover from the bed edges, patrol after rain |
Deciding what to pull and when to give up
Turnips grow fast enough that resowing is nearly always a better answer than nursing a damaged crop. Where a slower vegetable would justify weeks of effort trying to save it, a turnip row can be cleared and sown again with only a month lost, and the second sowing usually beats the rescued first one.
- Seedlings shredded by flea beetle If more than half the seedlings have lost most of their leaf area, clear the row and sow again under mesh. Survivors of a heavy attack rarely make good roots, and they hold the bed for weeks doing it.
- Scattered wilting plants Lift the wilting ones and check the roots for maggot tunnels. If it is root fly and it is only a few plants, remove those and put collars or mesh on the rest. Firm the soil around every remaining plant.
- Distorted swollen roots Suspect clubroot. Lift and dispose of the plants away from the plot, do not compost them, and clean tools and boots before working elsewhere in the garden. Do not grow brassicas in that bed again for several years.
- Record the date of the first damage Write down when you first saw flea beetle holes or the first wilting plant. Those dates repeat closely from year to year, and knowing them lets you have the mesh on the bed before the pest arrives rather than a week after.
Every one of these controls is cheaper and easier than the alternative, and the alternative is spraying a crop you eat within weeks of treating it. Mesh at sowing, rotation and prompt removal of damaged plants deal with the whole list.