Shiso Pests and Diseases: Holes in Leaves and What Causes Them

Read the hole and you know the culprit

Almost everything that goes wrong with shiso shows up as damage to the leaf, and the shape of that damage identifies the cause more reliably than hunting for the animal itself, which is often nocturnal or hiding underneath. Turn a damaged leaf over and check the underside before you decide anything.

Flea beetles hit young plants hardest

Flea beetles are small dark beetles that jump when disturbed and chew neat round holes an eighth of an inch (2 to 3 mm) across. They are the most common shiso pest by a distance. A mature bush shrugs off the damage, which is cosmetic on a large leaf, but a seedling with four leaves can be killed outright in a few dry, warm days.

They are worst in dry weather and on plants growing in dusty, dry soil, so the strongest control is not a spray but keeping the young plants well watered and covered until they are big enough not to care.

1. Mesh from the day you sow Fine insect mesh over the bed or tray from sowing until the plants are eight inches (20 cm) tall keeps them off entirely. Putting it on after the beetles arrive traps them inside.
2. Keep the soil damp Flea beetles avoid damp surfaces. Watering the soil surface in the evening during a dry spell measurably reduces the damage.
3. Mulch around the plants An inch of mulch stops the adults reaching the soil to lay and cuts next season's numbers as well as this one's.
4. Accept the damage on big plants Once a plant is a foot across, shot-holes are cosmetic. Spraying a mature plant you are about to eat is not a good trade.

Pick the leaves you intend to serve whole from the middle of the bush, where the beetles do least damage. The outer leaves take the punishment and are still perfectly good chopped.

Caterpillars work overnight

Shiso attracts several leaf-eating caterpillars, and unlike flea beetle damage theirs escalates fast: whole leaves disappear rather than being perforated. If a plant looked fine on Sunday and is skeletal on Wednesday, it is caterpillars, and they are still on the plant.

Look on the undersides of the leaves, in the joints where a leaf stalk meets the stem, and in any rolled or stuck-together leaves. Green caterpillars on green shiso are hard to see, but the dark pellets they leave on lower leaves give away exactly which shoot to search.

1. Follow the droppings upward Find the dark pellets on a lower leaf and the caterpillar is almost always directly above them. This is faster than searching the whole plant.
2. Check morning and evening Most feed at dusk and dawn and hide low down in the heat of the day. Two short searches at those times beat one long one at noon.
3. Open rolled leaves A leaf folded over and stuck shut has a caterpillar inside. Unroll it rather than just removing the leaf, or you will not know if you got it.
4. Remove eggs while you are there Small clusters of pale eggs on the leaf undersides are next week's problem. Rubbing them off takes seconds.

Red-leaved shiso makes caterpillar hunting much easier, because a green caterpillar on a purple leaf is obvious from three feet away. It is a real advantage if you have had a bad year for them.

Mites and aphids in hot, dry spells

Spider mites thrive in hot, dry, still air, which is exactly the condition of a shiso in a sheltered corner in high summer. The first sign is a fine pale speckling across the leaf, as if it has been dusted, followed by faint webbing in the joints. Left alone the leaves go bronze and drop.

Aphids cluster on the growing tips and the undersides of the youngest leaves, leaving them sticky and curled. Neither pest needs a chemical answer on a plant you are picking weekly, but both need to be caught in the first few days.

Both mites and aphids do most damage on a plant that is short of water. A shiso watered properly through a heatwave rarely gets a serious infestation of either.

Fungal problems and when to give up

In a wet season shiso develops brown leaf spots with yellow edges, and in a badly crowded planting the lower leaves rot off entirely. This is almost always an airflow problem rather than an infection you could have prevented with a spray. Thinning the plants and lifting the lower leaves clear of the soil stops it spreading upward.

There is also a point at which a plant is not worth the effort. Judge it on the growing tips, not the old leaves: as long as the new growth at the tips is clean and normally sized, the plant is winning. When the new growth itself comes out spotted, distorted or half the size it should be, pull the plant, remove it from the garden, and do not compost it.

1. Take off the lowest leaves Remove leaves within four inches (10 cm) of the soil. They are the entry point for splash-borne fungus and they contribute little.
2. Water the soil, not the plant Overhead watering in the evening leaves the foliage wet all night, which is what leaf spot needs. Water the base in the morning.
3. Thin crowded plantings Sacrifice every other plant rather than lose all of them. Shiso planted at four inches apart in a wet summer will rot as a block.
4. Move the bed next year Do not grow shiso in the same ground two years running if you have had leaf spot. Self-sown seedlings will try to make that decision for you, so pull them out of the old bed.

Keep a short note of what appeared and roughly when, season by season. Pests arrive at much the same point in the season each year, and a two-line record turns next year's response from a reaction into a preparation.