alternaria dark leaf spot

Symptoms

Dark-brown angular leaf spots with concentric rings (target-pattern), spots later coalesce and leaves drop, dark stem and pod lesions late in season.

Plants affected

• oilseed rape / canola

Easily confused with

• phoma stem canker / blackleg

  How to tell them apart: Leptosphaeria maculans (phoma leaf spot/blackleg) shows pale grey-tan, dry, irregular leaf lesions, usually a few mm to a couple of cm across, peppered with tiny jet-black pinhead dots (pycnidia) scattered across the spot surface — this dotting is the decisive sign. Spots have a diffuse edge with no concentric rings, and the fungus later moves down the petiole to cause a sunken stem canker at the base. Alternaria brassicae (dark leaf spot) instead makes darker brown to charcoal-grey round spots with clear concentric target-like rings, often surrounded by a yellow halo, and the spot centre may turn papery and fall out leaving a shot-hole. Look for the black pycnidial dots and stem-base canker for phoma, and the bullseye rings plus yellow halo for Alternaria.

Common questions

How do I recognise alternaria dark leaf spot on oilseed rape / canola?

Dark-brown angular leaf spots with concentric rings (target-pattern), spots later coalesce and leaves drop, dark stem and pod lesions late in season.

Do I have to report alternaria dark leaf spot?

No. alternaria dark leaf spot is not a notifiable organism, so there is nothing to report to an authority. Treat it as a normal garden problem.

How is alternaria dark leaf spot treated?

Mainly with biological and cultural measures — removing affected material, changing the conditions that favour it, and treating with agents approved in your country. The exact steps and their timing depend on the plant, the season and where you live: Dr. Phyto works them out in a free diagnosis.