White Rust

White rust? Rust isn’t white. Rust is orange!


The rust on an old car is orange. Fungal rust spores that grow on the underside of rose leaves are bright orange.


But white rust isn’t orange or fungal.

White blister rust on mizen (Scot Nelson) Public Domain

White rust, white blister rust, or blister rust, is a disease that attacks plants in the cabbage family, causing white blisters, deformed growth, and leaf loss. Wet leaves and cool temperatures are all that’s needed for white rust to take hold in broccoli, cauliflower, collards, radish, and turnips. Arugula and rapini are especially susceptible to white rust. Candytuft and shepherd's purse may also become infected.


White rust pathogen

White rust (Albugo candida) was thought to be a fungal disease until molecular research discovered that it is caused by tiny algae-like microbes, called oomycetes, that parasitize vascular plants. Downy mildews are also caused by oomycetes. The reproductive spores created by white rust can lay dormant in soil during the dry season, waiting for the rainy season.

White blister rust on brassica (Scot Nelson) Public Domain

White blister rust on shepherd’s purse stems (Michal Maňas) CC BY 2.5

White rust symptoms

As flowers and leaves are infected with white rust oomycetes, white blisters (pustules) form under the plant’s skin on the underside of leaves. These blisters contain baby oomycetes. When the blisters pop, oomycete spores take to the wind, infecting nearby plants. Spores are also transferred to other plants by splashing water from irrigation or rain. These pustules appear waxy and may be pinkish, at first. Upper leaf surfaces exhibit pale green or yellowish spots that can reach 1 inch in diameter. [This disease should not be confused with chrysanthemum white rust (Puccinia spp.), which is a fungal disease and appears as masses of black, brown, purple, orange, or yellow pustules. Chrysanthemum white rust is under a CA state eradication program.]


White rust controls
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As far as I know, there are no resistant plant varieties available (yet). Commercial growers rely on fungicides to control this disease, but you can reduce the likelihood of infection simply by decreasing the amount of time leaves stay wet. Rather than overhead watering, use drip irrigation or soaker hoses. These methods save water, too. In severe cases, an area may need to be left fallow, or you can use some serious crop rotation.