Blackleg, caused by the fungus Leptosphaeria maculans, is the most serious disease of canola in Australia. Blackleg lesions are grey, circular spots with black fruiting bodies (pycnidia) forming in the centre (Figures 1A & 1C). Lesions can appear on the plants foliage at any growth stage, even on resistant varieties.
Traditionally, blackleg causes stem canker become infected. Once the lesion has formed on the plant, the fungus grows within the plant’s vascular system to the crown where it causes the crown of the plant to rot, resulting in a canker (Figure 1B). Severe canker will sever the roots from the stem, whereas a less severe infection will result in internal infection of the crown restricting water and nutrient flow within the plant.
Blackleg survives on canola stubble, producing fruiting bodies (pseudothecia) that contain large quantities of airborne spores that can travel long distances (>1 km). Secondary infection occurs when spores produced by pycnidia within the lesion are spread via rainsplash.
Management practices to control blackleg include choosing resistant varieties (see the latest Blackleg Management Guide), fungicides, crop rotation and sowing at least 500 metres away for last year’s canola stubble, if possible.
White leaf spot
White leaf spot is caused by the fungus Mycosphaerella capsellae (asexual stage Pseudocercosporella capsellae). Lesions are greyish white to brown (Figure 2A). As lesions mature, they develop brown margins (Figure 2B). Mature lesions may have a narrow brown line of zonation (Figure 2C). White leaf spot lesions do not contain pycnidia (black dots) that are a symptom of blackleg (Figure 2D).
The disease is most evident under cool moist winter conditions. White leaf spot does not usually cause of large yield losses as the crop grows away from the disease as the weather warms up in spring.
Downy mildew is caused by the fungus Peronospora parasitica. Symptoms include yellowish-brown angular lesions on the upper surfaces of leaves (Figure 3A). Lesions can be seen on young seedlings when cotyledons or first true leaves are present. Infected cotyledons tend to die prematurely. Downy mildew lesions often correspond with patches of a white mealy fungal growth on the under of the leaf (Figure 3B). As the disease develops, individual spots join to form large irregularly shaped blotches with areas of necrosis. Necrotic lesions may cause a large part of the leaf to dry out and the upper surface to develop a yellow-red colour (Figure 3C).
Infection occurs under cool moist conditions where leaves or cotyledons are in contact with the soil or other leaves. Although the disease can severely attack seedlings, significant yield loss does not usually occur. Downy mildew is rarely found beyond the rosette stage and crops normally grow away from it with the onset of warmer weather. Please note that Peronospora parasitica is a completely different fungus to Erysiphe polygoni which causes powdery mildew in canola
Alternaria leaf spot
Alternaria leaf and pod spot is caused by Alternaria brassicae and occasionally by Alternaria brasicicola. Alternaria lesions on leaves and pods have a concentric or target-like appearance, and are brown, black or greyish-white with a dark border (Figure 4A and 4B). Lesions on green leaves can be surrounded by a chlorotic (yellow) halo (Figure 4A). Stem spots are elongated and almost black. Pod infection may cause seed to shrivel and pods to ripen prematurely and shatter (Figure 4C). Alternaria lesions do not contain pycnidia (black dots) that are a characteristic of blackleg.
The fungus survives on infected canola stubble, on cruciferous weeds and on seed. Initial crop infections are caused by wind-blown spores and the disease spreads with wet humid weather during spring. Alternaria is most damaging when it occurs on canola pods, infected pods may result in fewer, shrivelled seed and can also cause premature pod shatter resulting in yield loss. Pod infections can also result in infected seed. If infected seed is used for sowing subsequent crops, seedling death can result. Alternaria is very common in canola crops, but is not usually severe enough to warrant control.