White Rust

Table of contents

    White Rust (Staghead)

    The fungus Albugo candida causes this disease. White rust is a common disease of B. rapa cultivars throughout western Canada. Some cultivars have resistance to one of two races that have been identified from B. rapa but are susceptible to the other. Cultivars with resistance to both races are being developed. Cultivars of B. napus are resistant to the current races of A. candida from B. rapa and from mustard. Yield losses in excess of 20% 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. rapa and vice versa).

    Symptoms

    Figure 41. Staghead Symptoms on Canola that is
    also Infected with Downy Mildew

    Figure 41.

    Photo by Beth Hoar

    White to cream-coloured 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 (Figure 41).

    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 spores 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, airborne 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.

    Management

    Grow resistant cultivars, use certified seed and use a crop rotation with at least three years between canola crops.