Marigold Pests and Diseases: Slugs, Nematodes and Flower Rot

Slugs and snails are the real threat to young plants

Marigold seedlings and freshly planted transplants are among the most attractive things in the whole garden to slugs. A plant that was healthy at dusk can be a chewed stub by morning, and because the whole seedling goes, gardeners often assume it was never planted properly or that something dug it up. The giveaway is the clean angled cut at soil level and the silvery trail on the surrounding soil in early light.

The risk window is the first two or three weeks after planting out, and it is worst in mild damp weather. Once a plant has a woody base and eight or more leaves it is far too tough to be destroyed in one night, though slugs will still graze the lower leaves.

Check with a torch an hour after dark rather than looking for the culprit in daylight. That single habit tells you whether you have a slug problem or something else, and it takes five minutes.

1. Plant out larger Hold plants in three inch (7.5 cm) pots until they have a firm woody base and are six inches tall. A larger plant simply outgrows the vulnerable stage before the slugs find it.
2. Clear the ground around each plant Pull mulch, fallen petals and weeds back a hand's width from the stem. Slugs need damp cover in daylight, and denying it within reach of the plant cuts damage far more than anything you sprinkle.
3. Water in the morning Evening watering leaves damp soil through the hours when slugs feed. Shifting to a morning routine for the first fortnight after planting makes a visible difference.
4. Go out with a torch after dark Two nights of hand collecting an hour after dusk removes most of the local population from a small bed, and it is the only method that works on the actual individuals eating your plants.

There is a persistent belief that marigolds are planted as a sacrificial trap crop to draw slugs away from vegetables. In practice the slugs eat the marigolds and then eat the vegetables too, so treat this as a way to spot slug pressure early, not as protection.

The nematode reputation: what is real and what is not

The claim that marigolds control root knot nematodes is genuine, well documented and much narrower than its garden reputation. The roots of Tagetes species release thiophene compounds that suppress several root knot and lesion nematodes in the soil immediately around them. This is a measured effect, not folklore.

The catch is how the effect has to be delivered. It works when marigolds are grown as a dense cover over the whole affected area for two to three months of the growing season, and then the ground is used for the susceptible crop afterwards. A dozen marigolds dotted between tomatoes do essentially nothing, because the roots only affect the soil they actually occupy and the effect does not travel.

It also only touches nematodes in the soil. It does nothing to whitefly, aphids, cabbage white butterflies or any other flying pest, despite marigolds being sold for exactly that purpose. The strong scent of the foliage is often given as the reason for repelling insects, but trials show no consistent effect on pests above ground.

Grow them anyway. They flower for months, they feed hoverflies whose larvae eat aphids, and they are honest about what they do. Just do not plant three of them and expect a pest free bed.

Flower rot and gray mold on the big heads

The large packed double heads of African types trap rain and dew inside the petals, and after a wet humid spell the center turns brown and slimy and then furs over with gray mold. It spreads from head to head fast and can move down into the stem. This is the commonest disease problem on marigolds and it is almost entirely about air movement and water.

Remove affected heads the day you see them and take them away rather than dropping them on the soil. Cut back to a leaf joint on sound tissue. Then look at why it happened: plants too close together, watering over the top, or a bed that gets no morning sun to dry the foliage.

The same fungus attacks stems where a spent flower stalk was left as a stub. That is one more reason to deadhead back to a leaf joint rather than just pulling the petals off.

Sap suckers and leaf spotting

Spider mites are the pest to watch in hot dry weather, especially on plants under an eave or against a warm wall. The first sign is a fine pale stippling on the upper leaf surface, then bronzing, then faint webbing in the leaf joints. Marigolds are notably prone to them. Hosing the undersides of the leaves hard, twice a week, breaks the cycle in the early stages.

Aphids cluster on the soft tips and around the buds and distort opening flowers. On a marigold they rarely reach damaging numbers before hoverflies and ladybirds find them, so squash the first colonies by hand and leave the rest a week before doing anything more.

Brown or gray angular spots on the lower leaves that spread upwards in wet weather are leaf spot fungi. Strip the affected lower leaves, improve the spacing, and stop watering over the plant. Marigolds tolerate losing their bottom leaves without much loss of flowering.

Write down the date you first see each problem and the bed it happened in. Slug damage, mite outbreaks and flower rot all recur in the same places at roughly the same point in the season, and one year of dates lets you act a week before it starts.