White rust is caused by the fungus Albugo candida. The disease is uncommon on B. napus (Australian canola varieties) but does infect B. juncea (juncea canola).
White to cream coloured pustules form on the underside of leaves and on floral parts. These pustules rupture the host epidermis exposing a white chalky dust. On the upper surface of the leaves the infected areas are bleached and thickened. Systemic infections of the growing tips and flower heads give rise to stagheads. Stagheads are very conspicuous in the crop as swollen, twisted and distorted flower heads that produce little to no seed and become brown and hard as they mature, (Figure 11.22). Symptoms for white rust should not be confused with severe calcium deficiency symptoms that cause flowering stalks to collapse resulting in the withering death of the flower head.
Figure 11.22 Distinctive white rust staghead on B. juncea plant
Resting spores (oospores) of the fungus can survive in infected plant material or as a seed contaminant for many years when conditions remain dry.
When conditions become moist the resting spores are able to directly infect plants. However, they usually produce tiny motile spores that can swim in free water to infect seedlings, causing cream-white pustules to form.
Inside the pustules new swimming spores are formed and then distributed throughout the canopy by rain splash to form secondary infections. They do this by growing through stomata into adjacent cells, causing systemic infections and subsequent stagheads if the growing tips of plants become infected.
Resting spores can be formed in any infected tissues but are present in larger numbers in stagheads. When the crop is harvested, stagheads break releasing resting spores that contaminate harvested seed or blow out to contaminate the soil.
WHITE LEAF SPOT
CLUBROOT IN CANOLA AND JUNCEA CANOLA