White leaf spot is caused by the fungus Mycosphaerella capsellae (also called Pseudocercosporella capsellae ). The disease has a worldwide distribution and a wide host range among cruciferous weeds. In Australia, white leaf spot commonly infects canola seedlings. It is not usually severe enough to cause yield loss.
Leaf, stem and pod lesions are greyish-white to light brown, (Figure 11.20). Unlike blackleg lesions, white leaf spot lesions do not contain pycnidial fruiting bodies (black dots) and usually have a more granular surface compared with the smooth surface of blackleg lesions, (Figure 11.21). Leaf lesions often have a brown margin when they mature; they can be up to one centimetre in diameter and often join to form large irregular shaped lesions. Nutrient deficient crops have been reported to be more severely affected by the disease. In severe epidemics, infections can defoliate susceptible varieties.
Figure 11.20 White leaf spot infection on canola leaves Photos: S. Marcroft
Figure 11.21 Typical white leaf spot lesion (red circles) and blackleg leaf lesions (blue circle). Note the white leaf spot have no small black spots (fruiting bodies). Photo: S. Marcroft
The fungus survives on canola stubble as thick- walled mycelium. When prolonged wet weather conditions prevail autumn/winter wind-borne spores are produced that cause primary leaf lesions on canola. These initial lesions go on to produce new wind-borne spores that cause the rapid spread of the disease throughout the crop. The disease is not usually seed-borne but can be spread by infected seeds or infected debris with the seed.
WHITE RUST OR STAGHEAD