Swiss Chard Pests and Diseases: Leaf Miner, Slugs and Leaf Spot

Leaf miner is the one that defines the crop

If you grow chard you will meet leaf miner. The adult is a small gray fly that lays eggs in neat rows on the underside of the leaf, and the larvae hatch inside the leaf itself and eat the tissue between the upper and lower surfaces. What you see is a pale winding tunnel that widens into a translucent blotch, and at that point the larva is already inside and no spray applied to the surface will reach it.

The damage is cosmetic rather than fatal. A mined leaf is still edible if you cut the blotch out, and a healthy plant outgrows moderate mining without any real loss. This matters, because people abandon chard over leaf miner damage that would have cost them almost nothing.

The reason it is worth controlling anyway is that populations build. Larvae drop to the soil to pupate, adults emerge and lay again, and by the third generation of a warm season a plant can be carrying more mined leaf than clean. Interrupting that cycle early is far easier than dealing with it in high summer.

Never compost mined leaves in an open heap next to the bed. Take them away or bury them deep, or the pupae simply return.

Keeping leaf miner down

The whole strategy is to stop the adult fly reaching the leaf and to break the cycle by removing larvae before they pupate. None of it involves spraying, which is fortunate, because contact sprays do not penetrate to a larva inside a leaf.

1. Cover from the day you sow Fine insect mesh over hoops, sealed at the edges with soil, keeps the adult off entirely. It has to go on before the first flight, not after you see the first tunnel.
2. Check leaf undersides weekly Turn over the outer leaves once a week and rub off any egg rows you find. Five minutes a week through spring is worth more than anything you can do once midsummer arrives.
3. Strip mined outer leaves Remove every heavily mined leaf at each pick and take it off the plot. Each one you leave is a batch of pupae going into your soil for the next generation.
4. Do not grow it after beetroot The same leaf miner attacks beetroot and spinach. Following one with another in the same bed hands the emerging adults their host plant. Rotate away from the whole family.

Overfed plants with soft dark leaves are laid on preferentially. Easing off the nitrogen genuinely reduces the egg count.

Slugs, snails and birds

Slugs are the more serious threat to young chard, and they take seedlings out completely rather than merely damaging them. A newly emerged row can vanish overnight, which people mistake for germination failure. The vulnerable window runs from emergence to about six true leaves; after that a chard plant is too vigorous to be checked by slug damage.

Birds strip young leaves in early spring when there is little else green, and pigeons in particular will reduce a bed to midribs. Netting through spring is the only reliable answer; scaring devices work for a few days.

Aphids colonise the leaf undersides in summer, usually on plants that have been fed hard. They cause little direct damage on a crop like this, but they carry virus and their honeydew brings sooty mold. A strong jet of water every few days clears a colony without any spray.

Leaf spot and downy mildew

Chard is notably less troubled by disease than spinach, and most of what appears is a consequence of crowding rather than bad luck. Cercospora leaf spot shows as round tan spots with a distinct dark purple margin, usually on the oldest leaves first, in warm humid weather. Downy mildew produces pale patches on the top of the leaf with gray growth on the underside in cool damp conditions.

Both spread from old leaves lying against the crown in still, damp air. Stripping the outer leaves regularly, spacing plants properly at thinning and clearing debris from the center of the plant deal with most of it without any intervention. Watering the soil rather than the foliage matters too: leaves that go into the night wet are the ones that get infected.

Neither is worth losing a crop over. Remove affected leaves, improve the airflow and keep picking. It is only worth pulling a plant when more than half of the standing leaf is spotted, because at that point it is a source of spores for its neighbors.

Take the affected leaves out of the plot rather than dropping them between the rows. Spores overwinter on debris and reinfect from the soil surface in spring.

Working out what you are looking at

Several chard problems produce marked or holed leaves and reaching for a treatment before identifying which is how time and money get wasted. Turn the leaf over first, because that answers most questions in one move.

Write down the date you first see leaf miner tunnels each year. That date is remarkably consistent in a given garden, and knowing it tells you when to have the mesh on next season.