Rosemary is relatively pest and disease free, which makes it an easy plant to grow. However, sometimes problems do occur and the common pests and how to deal with them are detailed below.
Frothy white foam around the stem or leaves of the plant. This foam is produced by the larva of a froghoppers to protect it from predators.
Until recently this was not considered a serious pest. However, these insects are the main vector of Xylella fastidiosa, the number one plant pathogen in the world. Fortunately, Xylella is still absent from the UK.
Simply wash off the foam and bug with a strong jet of water.
There is no effective control
Leafhoppers are small (2-3mm long) torpedo-shaped, sap-sucking insects. The adults and larvae feed on the underside of the leaves and cause small spots that go right through the leaf. They affect a large number of plants besides rosemary.
In protected environments it can be a serious pest. As well as disfiguring leaves, leafhoppers can transmit plant viruses.
This pest is rarely a serious problem with rosemaries grown outdoors.
Move the plant outdoors - this allows natural predators a chance to attack the leafhoppers. Do not place the infected plant near any other members of the mint family (e.g., basil, lavender, mint or sage) to avoid cross-infection.
If you cannot move the plant then suspend a yellow sticky trap above the plant and brush the foliage with your hand. The disturbed leafhoppers should land on the traps. Some people report that using a vacuum on low-power suction works, too!
Repeat regularly to trap subsequent generations.
There is no effective biological control for leafhopper in rosemary.
As organic growers we never use, or recommend the use of, systemic insecticidal sprays - especially on a plant you might eat!
Sometimes it is necessary to resort to some form of spraying, in which case:
Do not spray when bees, wasps or hoverflies could be affected. Spray in the late evening if possible.
A native of southern Europe, this beetle was first discovered in the UK in 1994 at RHS Wisley. Since then, it has established throughout southern England and is now reported from most English counties.
Rosemary Beetle is, at present, more or less absent from northern and south west England, Northern Ireland, Scotland and Wales. The adults are not very good fliers, so the movement of infected plant material is the main way this pest spreads.
The 6-8mm long adult beetle is unmistakable! The larvae are like small greyish-white slugs with darker stripes and are 8mm in length. Both the the larvae and adults eat the leaves and flowers of members of the mint family such as rosemary, lavender, sage and thyme. A severe infestation can strip a plant of all its leaves.
Give the plant a vigorous shake to dislodge the beetles into a suitable container (some recommend an upturned umbrella). Please dispose of the beetles humanely. Squash any grubs. Prune out badly damaged areas of the rosemary and feed the plant to promote regrowth.
The only effective control is Homo hortus - the gardener!
In our opinion, there are no totally effective (or safe) sprays. Manual control is best. If a plant is badly damaged the cost of a replacement plant may be less expensive than buying a chemical spray.
If you must spray, then do so only when there is no risk of the spray affecting beneficial or pollinating insects. For example, late evening or when the plant is not in flower (ideally both).
In some years this can be quite a nuisance. The caterpillar of the moth uses silk to "stitch" leaves together to make a protective enclosure. The caterpillar then eats the leaves (and sometimes the stem), which leads to unsightly brown patches or even dead sections. The dead sections can look a little like rosemary beetle damage.
The moth usually has two generations each year in June/July and later in August/September. In a mild year overwintering eggs can produce an April/May flush of caterpillars, too.
Although a plant can be made to look really tatty by this caterpillar it rarely kills the plant and the enforced pruning out of affected parts often results in a bushier plant.
Remove affected parts, squash grubs or leave them on the bird table! If the infestation was severe feed the plant to help it recover.
There is no effective control.
There are many pest species of tortrix moth and they are all resistant to insecticides. Manual intervention is the only solution for the gardener.
This is a common fungal disease of rosemary. Powdery mildew usually occurs on plants kept indoors. Although it makes the plant unsightly it rarely kills it. If left untreated leaves or stems will die.
Prune out the affected parts. If the plant is indoors move it outside to location in full sun. Check the compost, if it is dry water just the compost taking care not to wet the leaves.
The main cause of powdery mildew is a poor watering regime where the leaves get wet but the roots remain dry. The other cause is lack of ventilation or too much humidity. Correct these and the rosemary should thrive!
Powdery mildew is often worse after an extended spell of dull but warm and humid weather.
We do not advise using commercial fungicides. Two home brew sprays that can work are based on sodium bicarbonate or dilute milk.
Although the RHS recommends Ecofective Bug and Mildew Spray we have not tried this ourselves.