Coriander (Cilantro) Problems: Aphids, Damping Off and Leaf Spot

Aphids in the growing tips

Aphids are the pest you will actually meet. They gather in the crown and on the underside of the newest leaves, where the plant is softest, and because coriander leaf is eaten raw a colony makes the crop unusable rather than merely unsightly. Soft, overfed, crowded plants are found first.

Because the crop is short lived, the answer is almost never a spray. A firm jet of water into the crown every couple of days knocks the colony down enough that predators catch up, and hoverfly larvae work fast on coriander in particular. If a plant is badly colonised and already nearing its bolt, pull it and sow again rather than treating it.

1. Look into the center twice a week Part the outer leaves and check the crown. Fifty aphids caught early is a two minute job; a fortnight later the whole row is sticky and unusable.
2. Wash rather than spray A hard jet of water under the leaves, repeated every two or three days for a week, clears most infestations on a plant you are about to eat.
3. Do not overfeed Extra nitrogen produces exactly the soft growth aphids breed fastest on. On plants in the ground, feed only if the leaves are visibly pale.
4. Let one plant flower nearby Coriander umbels are among the best hoverfly and parasitic wasp plants in a vegetable garden. A flowering plant left at the end of the row pays for its space in aphid control.

Check any bought plant before it goes near your row. Greenhouse raised coriander frequently arrives with aphids already established deep in the crown.

Damping off and seedlings that vanish

In a cold, wet spring, coriander seedlings collapse at soil level with a pinched, dark stem, or fail to emerge at all after a rotten seed. Coriander is particularly vulnerable because germination is slow, and every extra day the seed sits in cold, saturated soil is another day for fungi to reach it.

The fix is patience rather than treatment. Wait until the soil is genuinely above 50F (10C), sow no deeper than half an inch (1 cm), and sow into soil that drains. A raised row three inches (8 cm) high in a wet spring loses far fewer seedlings than a flat bed.

Water seedlings in the morning and only when the surface has dried. Damp soil overnight in cool weather is the single condition damping off needs.

Bacterial leaf spot in wet spells

In prolonged wet, warm weather coriander develops angular brown or black spots on the leaves, often with a yellow halo, that spread until whole leaves go papery. It is bacterial, seed borne, and spread by splashing water, which is why it usually appears in a row that has been overhead watered or after a run of heavy rain.

There is no cure once it is in the crop. Cut and use anything clean immediately, pull the badly spotted plants, and do not compost them. The important part is not repeating it: water at soil level rather than over the leaves, space plants so air moves through the row, and do not save seed from an affected plant.

1. Water the soil, never the foliage Overhead watering splashes bacteria from the lower leaves onto clean ones. A can with the rose off, aimed at the base, breaks the cycle.
2. Thin for airflow A dense stand stays wet for hours after rain. Thinning to full spacing costs you a few plants and saves the rest.
3. Do not work in the row when it is wet Bacteria move on hands, clothes and tools. Cut and thin only once the foliage has dried.
4. Rotate away from carrot family beds Do not follow coriander with carrots, parsley, dill, parsnip or celery in the same ground, and do not sow coriander after them.

Powdery mildew, mites and other late season trouble

Late in a hot, dry summer you may see a dusty white bloom on the upper leaf surfaces, or fine stippling and webbing from spider mites on plants under cover. Both are diseases of stress and dry air rather than of neglect, and both arrive on plants that are close to bolting anyway.

Weigh the cost of treatment against the life left in the plant. A coriander plant with three weeks to run and mildew on the older leaves is worth stripping and using; a plant already throwing feathery growth is worth pulling. This is the practical discipline with a crop this short: replacing it is nearly always faster than saving it.

Keep a note of which bed and which week each problem appeared in. Coriander goes through five or six sowings a year, so a single season gives you enough repeats to see the pattern clearly.