White Rust (Staghead)

This disease is caused by the fungus Albugo candida. Yield losses in excess of twenty percent have been recorded on susceptible cultivars when severely infected. White rust can cause disease problems on mustard (B.juncea) and occurs on cruciferous weeds. Races are generally fairly host or species specific (for example white rust from the weed shepherd's purse does not infect B. campestris and vice versa).

1. Appearance

White to cream-colored masses, or pustules of "white rust", appear on the underside of leaves from the seedling stage onward. Following infection of the stems and pods, raised green blisters form that turn white during wet weather. The most conspicuous symptom is the presence of swollen, twisted and distorted inflorescences called "stagheads" that become brown, hard and dry as they mature.

2. Disease cycle

The fungus overwinters as resting spores in decaying infected plant tissues (mainly stagheads) or as a seed contaminant. These spores may remain dormant in soil or on seed for a number of years. In the spring some of the sports germinate and infect the cotyledons and leaves of young susceptible plants. These infections develop and white pustules are formed on the underside of leaves or on stems. The pustules release chalk-like, air-borne spores that can spread the disease to other parts of the plant or to nearby plants to cause secondary infections on leaves, stems or flower buds. Stagheads develop from infected flower buds. At harvest, stagheads may be broken during threshing resulting in contamination of the seed with resting spores.

3. Control

Grow resistant cultivars. Polish varieties (B. rapa) are susceptible, except for Reward, which is resistant. Argentine varieties (B. napus) are resistant. Use certified seed. Use a crop rotation with at least three years between canola crops.