Primula - Pests & diseases

General comment

If your primulas are planted in the right kind of soil, and in the right place, regular inspection will prevent most problems. Healthy, well-grown plants are less susceptible to attack by pests or disease and, in general, primulas are relatively trouble free. Nevertheless, it is necessary to be alert to the early signs of any possible problems, and to tackle them speedily if they arise. Particular care is required under glass, as problems can multiply with alarming rapidity, once they present themselves.

Vine Weevils

Probably the most important pest to look out for. The juvenile of the vine weevil is a cream comma shaped grub with a brown head about 1 cm long, which lives in the soil of your potted plant and has a particular liking for Primula roots. It is usually only a problem in pots. Normally, the first sign of their presence is the collapse of a plant which was perfectly healthy until then.

If you see any signs, take the plant out of the pot, squash any bugs you see and discard all the compost in the pot. Get rid of the infected soil (do not put it on your compost heap).If the plant still has some roots you can try and save it by washing the roots under the hose and replanting in new, clean compost.

The adult is a brown colour, - about 2 cm long and can be active from spring to autumn. They appear like a beetle, but with the 'snout' of a weevil. If you detect irregular notches eaten out of the sides of leaves, they may be present. The adult is nocturnal, and can be trapped in rolls of corrugated paper or flowerpots stuffed with fresh grass. Such traps must be inspected every day.

Mulching potted plants with buckwheat hulls or gravel around the neck of plants may deter the adult from laying, and anything which confuses their sense of smell may also help such as growing chives or garlic among your plants. If it becomes a serious problem we would suggest using an application of nematodes (Steinernema kraussei) in late summer.

Slugs

Slugs can devour a tray of young Primula seedlings and will do enormous damage to new growth in spring. Try sharp gravel, beer traps, wood ash or other appropriate slug control. Once plants are well established slugs rarely destroy them completely but do certainly make them look less attractive.

If you grow your Primula in pots, we have found that the most effective solution is to put a line of Vaseline around the top of the pot which creates a natural barrier. It will have to be reapplied at frequent intervals.

Greenfly and Aphids

Greenfly, whitefly and other aphids can prey on Primula though they are not usually too much of a problem in the garden. If they get out of control, they can weaken the plants and introduce viruses. Preventative measures include keeping the primulas weed-free, as companion weeds also act as hosts to the aphids and not allowing the plants to dry out. If facing a bad infestation, use a spray of black soap (linseed oil soap) mixed with green clay, and lavender oil or peppermint oil and repeat every few days.

Signs of green aphid.

Woolly Root Aphids

Woolly root aphids can be a problem when growing auriculas or other alpine Primula in pots under glass. If you take the plant out of the pot you will notice a white powder like substance around the roots or sometimes around the neck of the plant. If you look carefully you will see aphid like insects moving about. They can weaken the plant and eventually kill it if left to get out of control.

If you see any signs make sure you treat the plant quickly. Remove the plant from the pot and take off all the compost making sure you discard the infected soil. Wash the roots off with water and leave them to soak in a mixture of diluted black soap (linseed oil) and vegetable oil such as rapeseed oil. Rinse the roots of the plant again in water and repot in a clean pot.

Make sure you inspect your plants regularly. We would recommend repotting auriculas and alpine plants once a year to make sure no problems build up.

Red Spider Mite

This problem usually occurs in warm and dry conditions under glass. Barely visible to the naked eye, they are a sap-feeding mite that can cause mottled leaves and leaf loss and in severe cases death. We mainly see this on auriculas, especially on the powdery forms which will lose colour and you may see spider like fine webbing on the underside of the leaves. Prevention is the best cure by having very good ventilation in the greenhouse. Misting the leaves regularly to increase humidity can help.

If you have a severe infestation, we would suggest using a biological control which means importing predatory mites that will eat the other ones. There are several available but the one we use is Phytoseiulus persimilis.

Mice

In cold spells mice can eat the crowns of plants. Mice and voles will also eat developing seed pods and the fleshy roots of certain of the Asiatic primulas.

Birds

Wondering why your primroses lose their petals as soon as they open? The culprits could be birds that like the sweet petals, especially the yellow varieties. If this is a serious problem such as for pots on a balcony, some black cotton thread stretched over the area will keep them off.

Caterpillars

Caterpillars cause leaf and petal damage and this will be apparent on regular inspection, they will rarely kill the plants even after a bad infestation though.

Pick them off by hand or try spraying the plants with a garlic or peppermint oil mixture.

Fungus

Botrytis or grey mould is the major fungal enemy of Primulas grown under glass. It is unusual in the garden unless the plants become waterlogged. You will see furry grey mould which can kill the plants fairly rapidly.

In the greenhouse, make sure there is good air circulation by keeping plants well-spaced and well ventilated. Avoid over watering and remove dead or dying leaves which may harbour fungus spores. To avoid the problem on auriculas, make sure you are using a well-drained compost and water the pots from underneath.

Once it has taken hold there is not much you can apart from removing the infected leaves being careful not to spread the spores around and keep the plants quite dry.

Damping off can affect young seedlings. They will suddenly collapse and die spreading to other seedlings in proximity. To prevent this avoid overcrowding, overwatering and poor ventilation.

Other fungal diseases: Primulas can get other fungal and bacterial diseases that cause yellow or black spots on the leaves. They will appear during wet weather from spring until winter. They may eventually cause the leaf to go brown and drop off. Although they are unsightly they do not usually cause major damage to the plant. Remove the affected leaves by hand and if grown in pots, keep the leaves dry. If grown outside, the plants will usually recover by themselves.