Shab disease on lavender — symptoms and treatment

Severity: high

What is Shab disease

Shab disease is caused by Phomopsis lavandulae, a fungus that infects the woody stems of lavender. It is the most serious disease of lavender plantations. The fungus works inside the stem rather than on the leaf surface, blocking the water-conducting tissue so that shoots die of thirst while standing in moist soil. That is what makes it so damaging: there is nothing to spray, and by the time the shoots go brown the fungus is already in the wood. Severity is rated high — Shab kills bushes, and once in a plantation it works through it.

Symptoms

Shab announces itself one shoot at a time. A single stem wilts and browns while the rest of the bush looks healthy around it — the classic early picture, and the most useful clue you will get. Follow that dead shoot down and you find the reason: grayish-brown lesions along the stem, and cankers at the base where the fungus has girdled the wood. Above a girdling canker nothing survives. More shoots then go the same way, and progressive plant death follows as the cankers reach the crown.

Key signs:

• Early: wilting and browning of individual shoots on an otherwise healthy bush
• Early: grayish-brown lesions on stems
• Advanced: cankers at the stem base, girdling the wood
• Advanced: progressive plant death

Root rot also kills lavender, but takes the whole bush from the roots up. Shab kills shoot by shoot, with healthy roots and stem cankers.

Causes and conditions

The fungus survives in infected stems and dead lavender debris left in the plantation, and its spores are moved to healthy plants by rain splash and wind-driven rain. It infects through wounds — pruning cuts, harvest cuts, frost cracks, breakage from wind or machinery — which is why a plantation cut for flowers every year gives the fungus a fresh set of doors each season. Wet weather drives spore release, so damp spells after cutting are the risk periods. The other main route is propagation: cuttings from an infected mother plant carry the fungus into a new planting.

Treatment

There is no effective chemical treatment for Shab disease. Management is removal and clean propagation.

Remove infected plants — cultural

Upon detection. Remove and destroy infected plants. No effective chemical treatment. Use disease-free cuttings for propagation. Avoid wounding plants.

Act as soon as you see it: take out affected plants entirely, roots and all, and destroy them away from the plantation — that debris is where the fungus survives into the next season. Where one or two shoots are affected, cutting back into clean wood may buy time, but a bush with cankers at the stem base is a spore source and is better removed. Disinfect tools between plants.

Prevention

• Propagate only from disease-free cuttings, never from a plantation with Shab.
• Avoid wounding plants — trim and harvest cleanly, and keep machinery from knocking the bushes.
• Cut and prune in dry weather.
• Remove and destroy dead stems, prunings and dead bushes rather than leaving debris in the rows.
• Give bushes space and an airy site so stems dry quickly after rain.

Frequently asked questions

Is it contagious to my other lavender? Yes. Spores splash and blow from infected stems onto neighbouring bushes, and tools carry it from plant to plant — which is why affected bushes should be removed promptly.

Can I save a plant with Shab? Not once cankers reach the stem base — girdled wood does not recover. Cutting back into healthy wood may rescue a bush caught at the single-shoot stage, but removal is reliable.

Can I replant lavender where a bush died of Shab? Yes, provided you remove the infected plant and its debris first. Unlike a soil-borne rot, Shab lives in the stems — take those away and plant clean stock.