During 2017 there is an increased incidence of canola white leaf spot in many regions of southern Australia. The disease is distributed worldwide, but in Australia, it is not usually severe enough to cause yield loss. However, if environmental conditions are favourable, it can result in significant defoliation causing reduced plant vigour and subsequent yield loss.
Leaf and stem lesions are greyish white to light brown (Figure 1). Leaf lesions can be up to one centimetre in diameter and coalesce to form large irregular shaped lesions (Figure 2). Mature lesions often have a brown margin. White leaf spot lesions do not contain pycnidia (black dots) that are a characteristic of blackleg.
White leaf spot is very common but normally only occurs on oldest leaves (near the soil surface where constantwetness occurs). If conditions are conducive it can move up the canopy infecting younger leaves reducing the leaf area significantly (Figure 3). This is when yield loss may occur.
White leaf spot is caused by the fungus Mycosphaerella capsellae (asexual stage Pseudocercosporella capsellae). The fungus survives on canola stubble as thick-walled mycelium. Under favourable autumn and winter conditions, it produces ascospores which cause leaf lesions. These initial lesions go on to produce new wind-borne spores that cause the rapid spread of disease throughout the crop. The disease is not usually seed-borne, but it can be spread by infected seeds or infected debris with the seed.
Use an integrated approach to managing white leaf spot:
- control cruciferous weeds and volunteer canola,
- use crop rotations and try not to sow near the previous year’s canola stubble to reduce infection from wind-borne spores.
White leaf spot is not usually severe enough to warrant fungicide control.
For a comprehensive guide to canola diseases see the GRDC publication: Diseases of canola and their management: The Back Pocket Guide