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 set the stage for white rust to strike broccoli, cauliflower, collards, radish, and turnips. Arugula and rapini are especially susceptible to white rust.

White rust pathogen

Scientists thought white rust (Albugo candida) was a fungal disease until molecular research discovered that it is an infection 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 the soil during the dry season, waiting for the rainy season.

White blister rust on brassica (Scot Nelson)

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

White rust symptoms

As flowers and leaves become infected with white rust oomycetes, white blisters (pustules) form under the plant’s skin on the underside of leaves. The blisters appear waxy and may be pinkish at first. Upper leaf surfaces exhibit pale green or yellowish spots that can reach 1 inch in diameter.

These blisters contain baby oomycetes. When the blisters pop, oomycete spores take to the wind, infecting nearby plants. Spores can also move to other plants by splashing water from irrigation or rain. 

[White rust should not be confused with Chrysanthemum white rust (Puccinia spp.), a fungal disease with masses of blisters that may be black, brown, purple, orange, or yellow.]

White rust controls

There are currently no resistant plant varieties available. Commercial growers rely on fungicides to control this disease. You can reduce the likelihood of infection simply by decreasing how much 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 crop rotation..