Calendula Pests and Diseases: Aphids, Powdery Mildew and Slugs

Aphids cluster on the buds and under the young leaves

Calendula is a magnet for aphids, particularly the black bean aphid, and they arrive early. They pack tightly around the flower buds and the top two inches of each shoot, where they are hidden from above by the leaves, so the first thing most gardeners notice is ants running up and down the stems or a sticky shine on the foliage underneath. Buds that are heavily colonised open deformed or fail to open at all.

This is not entirely a problem. Calendula is grown deliberately as a trap and nurse plant near beans and brassicas precisely because it draws the aphids in and then feeds the hoverfly larvae, ladybirds and parasitic wasps that go on to clear the crop. A moderate colony on calendula is doing useful work. Intervene only when the buds themselves are being destroyed, and never with an insecticide, because the flowers are worked constantly by bees and hoverflies and many of them are going to be eaten.

1. Check the bud clusters weekly Turn a shoot tip over and look at the base of the buds. The colony starts there days before it shows anywhere you would notice.
2. Squash or wash rather than spray Run a gloved finger and thumb up the shoot, or knock them off with a jet of water. Even soap sprays kill hoverfly larvae, which are the thing actually solving the problem.
3. Cut out the worst shoots Snap off the two or three most infested tips at a leaf pair and bin them. The plant branches from below and loses nothing.
4. Break up the ant traffic Ants farm and defend the aphids. Disturbing the nest or greasing a container leg lets the predators back in and the colony collapses within a week.

If you are growing calendula as a trap crop alongside beans, resist the urge to clean it thoroughly. A plant with no aphids on it is not doing the job it was sown for.

Powdery mildew is an end of season certainty

White powdery patches appear on the upper surface of the older, lower leaves from late summer and spread upwards until the bottom half of the plant is gray and crisp. On calendula this is not a matter of whether but of when. It is driven by crowded growth, still air and dry roots rather than by rain, and it always begins in the dense middle of a plant that has been left unpruned and is lying on itself.

It rarely kills the plant but it ends its usefulness, because mildewed leaves stop working and the flowering thins out fast once they do. Spraying achieves little at this stage. The reliable answer is the hard cut back that the plant wants anyway: take everything to eight or ten inches (20 to 25 cm), remove every scrap of infected material from the bed, water well, and the regrowth comes back clean and flowers into the frosts. Sowing at twelve inches (30 cm) rather than six, and watering at the root rather than over the top, delays the whole thing by weeks.

Do not put mildewed or rusted calendula foliage in a cold compost pile that goes back on the same bed. Heap it hot or put it out with the garden waste.

Slugs take the seedlings and nothing else does

The vulnerable stage is short: the fortnight between emergence and the fourth true leaf. In damp weather slugs and snails will clear an entire row of calendula seedlings overnight, and because the seedlings are soft and upright the damage is total rather than partial. Once the plant has four leaves and its stems have toughened, slugs mostly lose interest and it is untroubled for the rest of its life.

Protect that fortnight and the problem is over. Sow into soil that is warm enough for the row to be up in a week rather than three, clear boards, pots and thick mulch back a foot from the row so there is nowhere to shelter, and go out with a torch after dark on the first two damp evenings after emergence. Cut off bottle cloches over individual stations work well and can be lifted once the seedlings fill them.

1. Sow warm enough to grow fast A row up in six days passes through the risk quickly. A cold sowing sits at two leaves for a fortnight and gets eaten.
2. Clear the shelter from around the row Pull mulch, boards and pots back twelve inches (30 cm). Mulch right up against a new sowing is a slug refuge, not a benefit.
3. Patrol for one week after emergence Two torchlit evenings after damp weather catch most of the population. That is when the losses actually happen.
4. Sow spares at the row end A dozen extra seeds give you transplants. Calendula thinnings move well at three or four leaves, so gaps can be filled.

Damage that is not a pest at all

Several things that look like disease on calendula are growing problems. Yellowing lower leaves in midsummer on an otherwise healthy plant are simply age and shading, and stripping them improves airflow and does no harm. Pale bleached flowers in high summer are heat, not disease. A plant that splits open and lies flat after a storm has soft growth from rich ground, and no spray addresses that.

The one to take seriously is a plant that wilts as a whole while the soil is moist, with the stem dark and soft at ground level. That is stem or root rot, it does not recover, and the plant should be pulled and the ground left. Repeating calendula in the same bed year after year makes both rot and mildew worse, so move the sowing around the garden even though it is only an annual.

Note the dates the trouble arrived

Calendula problems come round on a fairly fixed schedule in any one garden, so a few dates turn most of them into a preventable calendar. Write down when you first saw aphids on the buds, when the first white mildew patch appeared, and when the plant flopped. Two seasons of those dates and you can time the hard cut back to just before the mildew rather than a month after it.

Record the position of each plant and what had been done to that ground the previous autumn. Bed by bed the pattern is consistent: the plants in freshly manured ground flop and mildew first, and the plants in the thin sunny corner do neither. Also mark which stations lost seedlings to slugs, because those damp corners repeat every year and are worth sowing later or protecting from the start.