White rust (staghead)

White rust is a canola disease caused by the pathogen Albugo candida that affects mainly Brassica rapa and B. juncea cultivars. B. napus cultivars are resistant. This disease was a common problem for canola growers when B. rapa acres were high (many decades ago), but since B. napus cultivars currently make up the majority of canola acres in Western Canada, it is a minor disease.

Overview

The pathogen Albugo candida causes this plant disease. White rust (which is also sometimes referred to as staghead) is a common disease of Brassica rapa and B. juncea cultivars throughout western Canada. Some cultivars have resistance to one of the two races that have been identified from B. rapa, but are susceptible to the other. Efforts have been put towards developing cultivars with resistance to both races. Cultivars of B. napus are resistant to the current races of A. candida. Yield losses in excess of 20 per cent have been recorded on susceptible B. rapa cultivars, when severely infected. White rust can cause disease problems 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

Staghead symptoms on canola infected with downy mildew
Staghead symptoms on canola that is also infected with downy mildew; Photo credit Beth Hoar

White to cream-coloured masses or pustules 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.

Disease cycle

The pathogen 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 physical 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.

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