Pests and Diseases

Lavender is renowned for its hardiness, ease of growing and attracting beneficial creatures to your garden, but alas, it can also suffer with disease and pests.


Sprawling and wild Lavender growth
When lavender sprawls and becomes wild due to the weight of the yearly growth and blooms, causes the shrub to fall open and break in the middle and this common problem, is lack of pruning. A sprawling lavender over three years old isn’t worth saving. Pruning is essential to keep the shape, health and
an abundance of blooms. You have two choices:

  • Prune in spring by cutting the foliage by one third, to encourage new growth. This may also stimulate new growth at the base of the plant. If new growth comes up at the base, prune the plant back to just above the new growth. If not, when new foliage grows and after flowering, cut it by one third again. This will take a growing season, but it may well be worth the time and attention.
  • Take Cuttings from your lavender plant and pot them up with a rooting compound. Dig out the mother lavender plant in the autumn and prepare the soil  for your plant cuttings, which have been overwintered indoors or a green house.

Black Mold on Lavender
Usually caused by wet or damp conditions around the plant stem and under foliage, poor drainage in beds or containers and lack of sunlight.

Clear away any mulch and dead leaves surrounding the base stem, allowing for good drainage, plenty
of space and adequate air flow around your plant.

Make sure your Lavender plant is getting full sunlight, if there is indeed too little light, consider transplanting to a sunny position in spring or autumn. If you do move your plants, take into account the size of the lavender when it is full grown. Preparation of the soil prior to transplanting and aggressive pruning (leaving about 2″ of last year’s growth), will help the lavender to re-establish itself.

 

Root Rot
Symptoms of root rot is the bark comes away from the plant easily, wilting leaves, yellow leaves,
dying leaves and discolored root tissue. You may even be able to pull the stalk away from the root with little or no effort.

This disease favours a cold and wet soil. If a plant has a contained root system, it will be prone to this problem.  If your plant is in a pot, consider using porous clay pots to provide additional air to the root system. Mixing pumice into flowerbeds or containers helps with soil drainage and will help to prevent root rot.

 

Leaf Wilt
Lavenders that are grown in partial shade will experience leaf wilt.

Consider transplanting your Lavender to a sunnier position. If you have indoor lavender plants, they will benefit a sunnier position, or additional artificial grow lights.

 

Brown Leaf
Caused by bugs feeding on the chlorophyll in plants. Large numbers of bugs may kill a plant.

A treatment of an organic insecticide, or washing the lavender plants with liquid soap, is recommended
to prevent further damage to the lavender plants from these aggressive chlorophyll feeders.

 

Spittle Bug or Cuckoo spit
Spittle bugs are common on lavender plants as well as many other plants. Noticeable in the spring by a spit like substance on the  stems of the lavender. The spittle bug actually lives inside its own spit. These little bugs rarely reach a level of infestation that will harm the health of the Lavender, although, sometimes the stems with spittle bugs on them can die.

Spittle bugs are usually easily controlled without the use of pesticides or other chemical treatments by giving your plants a powerful spray of water, which usually removes both the bugs and the spittle.

 

Whitefly
Lavender is one of many host plants that attract whiteflies. These tiny pests feed on plant sap and if there are vast populations on your lavender plant’s foliage, it may become yellowed and mottled. This could be a honeydew left by whiteflies and could also lead to sooty mould.

While their presence may not kill your lavender plant, controlling a large population of whiteflies can be difficult, as these pests are not always effectively controlled with any available insecticides. Although, there are several natural enemies of whiteflies such as lady birds that feed on all stages of whiteflies, hand-removal of the whiteflies, or a powerful stream of water on the plant can also help to reduce populations.

 

Cicadelle (leaf hopper)
The cicadelle lays its eggs several centimetres below the plants. Not only do the young insects feed on the roots; the adult insects that emerge feed on the plants, both transmitting what scientists know as the stolbur phytoplasma.
A Plague of cicada insects in southern France, is devastating Provence's famous flower crops. It will not make itself known until spring and summer, when the cicadas are ready to emerge from the sun-warmed earth. Hundreds of thousands of cicada larvae will not only have devastated the plants' roots, but the adult insects will also transmit a fatal micro-bacterium that will make the plants slowly wither and die.
Producers say that between 2007 and 2010 the region's production of the most popular plant, lavandin, used in soaps, perfumes, insect repellents, essential oils and the celebrated herbes de Provence
has halved.
Thankfully the last sighting of the cicada leaf hopper in the UK was 15 years ago in the New Forest. BEWARE of imported lavender plants.

 

Aphids
Aphids themselves are not usually directly harmful to lavender, but they do spread alfalfa mosaic virus, a common disease of lavender.

 

Alfalfa Mosaic Virus
Easy to spot, as it effects the leaves and shoots with bright yellow patches that sometimes curl the leaf. While this virus doesn’t usually kill lavender plants, it can reduce overall health, production of new growth and quantity of blooms.

Alfa Mosaic is incredibly infectious and can be spread by touch from one plant to another, by way of your hands or clothing. Dig out and burn any plant that has this virus, useing gloves, a newspaper or a towel
to prevent the disease from spreading. If you are unable to burn the plant, tie it tightly in a plastic bag and throw it away. Do not compost any plant that you think may have this virus.

 

Lavender Shab Disease
This is a fungus that kills the stems of the lavender. The clearest sign, is when all the shoots wilt suddenly, even if there has been plenty of rain or watering. Close inspection of the affected plants (you may need
a magnifying glass), will reveal very small black shapes, called pycnidia emerging from the bark.
Under a powerful microscope they are a cupped shape and full of spores.

 

About 40 years ago, a lavender shab epidemic killed vast amounts of commercially grown lavender. Since that time, no further outbreaks have been recorded and anyone who grows, sells or maintains large amounts of lavender, should keep a vigil eye for this and any other disease, pest or aphid that lavender
is prone to.

 

Xylella Fastidiosa
A bacterial pathogen which causes multiple symptoms including wilts, diebacks, stunts and leaf scorches that could have a wide and damaging impact on urban landscapes and our countryside. The host list includes trees, shrubs and herbaceous plants including Lavender.

There are known outbreaks of Xylella fastidiosa in Italy, Corsica and mainland France. When buying any imported plant, BE AWARE, an outbreak in the UK could lead to destruction of host plants within 100 m, and a 10 km movement ban for host plants for five years.

The Department of Environment for Food and Rural Affairs is on a high alert for this very destructive disease. For more information go to their web site: www.gov.uk/guidance/protecting-plant-health-topical-issues

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