Kohlrabi Pests and Problems: Flea Beetle, Root Fly and Splitting

Different problems at different stages

Kohlrabi is in the ground for two to three months, and each part of that window has its own threat. The two that actually lose crops are flea beetle in the first fortnight and cabbage root fly on young transplants. Everything else on this list is either cosmetic or slow enough to catch.

If plants are stunted and pale across a whole patch and nothing is visible on the leaves, dig one up and look at the roots. Club root and root fly both show there and nowhere else.

Flea beetle: the one that decides the crop

Tiny black beetles that jump when disturbed chew round pinprick holes through the leaves. On a mature plant it is cosmetic. On seed leaves it is fatal, because a seedling with half its leaf area gone in its first week never catches up. This is why kohlrabi sowings fail more often than they should.

They are worst in warm dry weather and on bare, dusty soil. There is no useful cure once they are on the crop, so all of the effort goes into keeping them off it.

1. Cover the row the day you sow Fine insect mesh over hoops, with the edges buried in soil all the way round, keeps them off completely. Put it on before emergence, not after, or you seal the beetles in with the seedlings.
2. Keep the soil surface damp Flea beetle activity drops sharply on moist ground. Watering the row in the evening through the seedling stage measurably reduces the damage.
3. Sow slightly thicker and thin later If you cannot cover the row, sow four or five seeds per station instead of three. You will lose some and can still thin to a full stand.
4. Push the plants through the risk window Once a plant has four or five true leaves the damage stops mattering. A liquid feed and steady water at the two leaf stage shortens the time spent vulnerable.

Do not leave the mesh on all season. Once the plants are established, take it off so you can see the crop, hoe it, and spot caterpillars early.

Cabbage root fly: prevent it or lose the plant

The fly lays eggs at the base of the stem and the maggots eat the roots. The plant wilts in the sun, perks up overnight, wilts worse the next day, and then dies. By the time you see the wilting the roots are gone, so there is no treatment. It is entirely preventable, and the prevention costs almost nothing on transplants.

1. Fit a collar around each transplant A disc of cardboard or old carpet underlay about five inches (13 cm) across, slit to the center and fitted flat around the stem, stops the fly reaching the soil at the stem base.
2. Or keep the mesh on through planting The same insect mesh that excludes flea beetle excludes root fly. If you are covering anyway, you do not need collars as well.
3. Firm transplants in hard The fly is drawn to loose, freshly disturbed soil. Plant firmly and water in so the surface settles rather than staying open and crumbly.
4. Pull and destroy affected plants A wilting plant will not recover. Lift it with the surrounding soil and remove it from the bed rather than leaving the maggots to pupate where the next crop goes.

Direct sown kohlrabi is less exposed than transplants, because there is no lifted, loose soil around the stem to attract the fly. If root fly is a regular problem in your garden, sow in place.

Caterpillars, aphids and slugs

These three are all manageable by eye and by hand, which is why they belong together. None of them will lose you the crop if you look at the plants twice a week.

1. Turn the leaves over for caterpillar eggs Cabbage white butterflies lay yellow eggs in clusters on the undersides. Rubbing off the clusters takes seconds and saves you from finding thirty caterpillars a fortnight later.
2. Squash aphid colonies where they start Gray cabbage aphids gather at the growing point and on the undersides of new leaves. Squash the first colony between finger and thumb, or blast it off with water, before it spreads down the plant.
3. Check under the bulb for slugs Slugs graze the skin of the bulb where it meets the soil, leaving shallow pale scars that then rot. Clear fallen leaves and any mulch touching the bulb, which is where they shelter by day.
4. Remove only the leaves that carry a colony Losing leaves costs bulb size, so pick individual caterpillars off the rest rather than stripping the plant.

Splitting and woodiness are water problems

Cracks running out from the leaf scars are not a disease. They mean the bulb grew faster than its skin could stretch, which happens when heavy rain or a heavy watering follows a dry spell. A split bulb is edible but will not store, and the crack is where rot gets in.

Woodiness is the same story stretched over longer. A bulb that has been short of water, or has simply been left too long, builds fiber through the flesh. Neither is reversible, so both are handled by watering evenly and picking on time.

1. Water little and often once the bulb starts swelling Aim for about an inch (2.5 cm) a week, delivered in two or three goes rather than one flood. This is the fortnight when unevenness shows up as splits.
2. Mulch, but keep it off the bulb A mulch between the plants evens out the swings in soil moisture. Leave a clear ring of a couple of inches (5 cm) around each stem so the bulb itself stays dry.
3. Harvest before a forecast heavy rain If the bed is at size and a wet spell is coming after a dry one, cut them now. Splitting happens in the day or two after the rain arrives.
4. Use split bulbs immediately Cut away the cracked section and use the rest the same week. Do not try to store anything with a crack in it.

Purple or reddish skin on some plants is not damage. It is normal in cold weather and in some strains, and the flesh underneath is pale and unaffected.

Telling similar symptoms apart

Several problems look like the same thing from a standing position: a plant that is not growing. Working through them in order takes five minutes and stops you treating the wrong cause.

Club root is the only one on this list that stays in the soil for years. If you find it, note which bed it was in and keep brassicas off that ground for at least three seasons.