Why Does My Lavender Have Shab Disease Die-Back?

Shab is a distinctive fungal disease specific to lavender, caused by the pathogen Phomopsis lavandulae. Unlike root rot, which kills the whole plant from below, shab attacks individual branches or sections of the plant, causing them to grey out and die back while the rest of the plant continues to look healthy. It can appear to come and go as conditions change, making it harder to recognise than straightforward root problems. Understanding how shab spreads and how to manage it prevents it from gradually destroying an otherwise healthy plant.

How to recognise shab disease

The most characteristic symptom is one or more branches turning grey-brown and dying, while adjacent branches remain green. The infected branch withers from the tip downward, and the foliage takes on a dried, straw-like appearance before falling. If you cut through an infected stem, you will see dark brown discolouration in the wood rather than the clean, pale green interior of healthy tissue. In advanced cases, multiple branches die in sequence and the whole plant is progressively lost.

How shab enters the plant

Phomopsis spores are always present in the environment and enter lavender specifically through wounds — most commonly pruning cuts made with dirty secateurs, or damage from frost cracking the stems. Wet conditions after pruning accelerate infection. This is why pruning hygiene and timing matter so much: always prune with sterilised blades (wipe with methylated spirit or diluted bleach between plants), and avoid pruning when wet or cold weather is forecast.

Removing infected wood

There is no fungicide treatment for shab that is reliably effective in garden conditions. The management approach is purely physical: cut out every infected branch, cutting back well into healthy wood where the interior is clearly pale and clean. Immediately dispose of infected material in the bin, not on the compost heap where spores can persist. Sterilise your tools after each cut to avoid spreading the fungus to the next cut surface.

Improving air circulation to reduce spread

Shab spreads more rapidly in humid, still conditions where spore dispersal and wet wood surfaces are common. Opening up the centre of the plant with the annual summer prune, and ensuring lavender is not planted so close together that air movement is restricted, helps slow the progress of the disease in a partially infected plant.

When to replace the plant

If more than half the branches are infected and the removal of dead wood has left a skeletal plant with little healthy growth remaining, replacement is the practical choice. Take cuttings from any clearly healthy shoot tips before removing the plant. Do not replant lavender in the same spot without replacing or substantially improving the soil.